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Endocrinology Vol. 145, No. 10 4522-4531
Copyright © 2004 by The Endocrine Society

Cross-Talk between Phosphatidylinositol 3-Kinase/AKT and c-Jun NH2-Terminal Kinase Mediates Survival of Isolated Human Islets

Reid Aikin, Dusica Maysinger and Lawrence Rosenberg

Department of Surgery (R.A., L.R.), McGill University, Montréal, Québec, Canada H3G 1A4.; and Department of Pharmacology and Therapeutics (R.A., D.M.), McGill University, Montréal, Québec, Canada H3G 1Y6

Address all correspondence and requests for reprints to: Dr. Lawrence Rosenberg, Department of Surgery, Montréal General Hospital, Room C9-128, 1650 Cedar Avenue, Montréal, Québec, Canada H3G 1A4. E-mail: lawrence.rosenberg{at}mcgill.ca.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Therapeutic strategies aimed at the inhibition of specific cell death mechanisms may increase islet yield and improve cell viability and function after routine isolation. The aim of the current study was to explore the possibility of AKT-JNK cross-talk in islets after isolation and the relevance of c-jun NH2-terminal kinases (JNK) suppression on islet survival. After routine isolation, increased AKT activity correlated with suppression of JNK activation, suggesting that they may be related events. Indeed, the increase in AKT activation after isolation correlated with suppression of apoptosis signal-regulating kinase 1 (ASK1), a kinase acting upstream of JNK, by phosphorylation at Ser83. We therefore examined whether modulators of phosphatidylinositol 3-kinase (PI3K)/AKT signaling affected JNK activation. PI3K inhibition led to increased JNK phosphorylation and islet cell death, which could be reversed by the specific JNK inhibitor SP600125. In addition, IGF-I suppressed cytokine-mediated JNK activation in a PI3K-dependent manner. We also demonstrate that inhibition of PI3K rendered islets more susceptible to cytokine-mediated cell death. SP600125 transiently protected islets from cytokine-mediated cell death, suggesting that JNK may not be necessary for cytokine-induced cell death. When administered immediately after isolation, SP600125 improved islet survival and function, even 48 h after removal of SP600125, suggesting that JNK inhibition by SP600125 may be a viable strategy for improving isolated islet survival. Taken together, these results demonstrate that PI3K/AKT suppresses the JNK pathway in islets, and this cross-talk represents an important antiapoptotic consequence of PI3K/AKT activation.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
THE ABILITY OF cells to react to environmental changes is dependent on the cooperation of intracellular signal transduction pathways to coordinate the cellular response. The integration of various external cues leads to regulation of physiological processes such as proliferation, differentiation, and cell death. The MAPKs have been shown to play an important regulatory role in a variety of cellular processes (1). MAPKs are activated by a sequential cascade of protein phosphorylation in which MAPKs are phosphorylated by a MAPK kinase (MAPKK), which is itself activated by a MAPKK kinase (MAPKKK). This three-tiered kinase module is a common characteristic of all MAPK pathways (2).

The c-jun NH2-terminal kinases (JNK) are a group of MAPKs that play a role in apoptosis, proliferation, survival signaling, and embryonic morphogenesis (reviewed in Ref.3). JNK is activated by dual phosphorylation at Tyr185 and Thr183 by two dual-specificity MAPKKs, MAPK kinase 4 (MKK4) and MAPK kinase 7 (MKK7). These two MAPKKs are activated by a large group of MAPKKKs including the mitogen-activated protein/ERK kinase kinase (MEKK) family, the apoptosis signaling-regulating kinases (ASK), the mixed-lineage protein kinases (MLK), TGFß-activated kinase 1 (TAK1), and tumor progression locus 2 (TPL2) (4). Once activated, JNK can phosphorylate proteins in the nucleus and cytoplasm. Stress-induced JNK activation leads to phosphorylation of transcription factors, including c-jun and activating transcription factor 2, which heterodimerize to bind divergent activator protein-1 sites in the c-jun promoter (5). However, despite the fact that c-jun expression is required in some cases for apoptosis, little is known about the genes that are involved. JNK-mediated induction of the Fas ligand gene was proposed as a possible mechanism of JNK-induced apoptosis (6, 7); however, blockage of Fas signaling does not prevent all forms of stress-induced apoptosis (8). A more direct link between JNK and the apoptotic machinery was observed in JNK-deficient cells that failed to release cytochrome C from the intermembrane space of the mitochondria in response to UV radiation (9). This effect is due to the ability of JNK to directly phosphorylate members of the BH3-only group of the Bcl2 family (Bim and Bmf), leading to Bax-dependent apoptosis (10, 11). Therefore, JNK can regulate apoptosis by both transcription-dependent and -independent mechanisms that are only now beginning to be elucidated. In addition, the duration of JNK activation appears to be important in determining the physiological outcome. Sustained JNK activation, in contrast to transient activation, appears to promote apoptosis in a variety of cell types (12, 13, 14, 15, 16). However, the consequences of JNK activation greatly depend on the type of stimulus and the cellular context (17).

ASK1 is a MAPKKK that activates the JNK and p38 MAPK pathways in response to proinflammatory cytokines, oxidative stress, and endoplasmic reticulum stress (18). ASK1 can be activated by interaction with the TNF receptor-associated factor (TRAF) domain of TRAF-2 and TRAF-6 and is required for TNF{alpha}-induced JNK activation (19). ASK1-deficient cells are resistant to apoptosis induced by TNF{alpha} or H2O2 and do not exhibit sustained JNK activation in response to these insults (16). Overexpression of ASK1 induces apoptosis through induction of the mitochondrial pathway (20, 21, 22, 23). The NH2-terminal of ASK1 contains an inhibitory domain that can interact with other proteins such as thioredoxin and 14-3-3, which prevent ASK1 activation (24, 25, 26). ASK1 is also inhibited by interaction with the HIV-1 protein Nef, providing infected T cells with protection from TNF{alpha} and Fas, demonstrating the importance of ASK1 in cytokine-mediated cell death (27).

AKT (protein kinase B) is a Ser/Thr kinase that has been shown to be a critical mediator of cell survival in response to growth factor stimuli (28, 29). AKT is recruited to the plasma membrane after activation of phosphatidylinositol 3-kinase (PI3K) and is activated through phosphorylation by phosphoinositide-dependent kinases 1 and 2 (30, 31). AKT can directly phosphorylate several proapoptotic proteins leading to suppression of apoptotic signals. We and others (32, 33) have observed strong activation of AKT within 12–24 h after islet isolation. In contrast, strong JNK activity is observed immediately after islet isolation and decreases over the next 24 h (34). The temporal correlation between these events suggested that AKT could be acting to suppress the JNK pathway in islets after isolation. Indeed, it has been demonstrated that AKT can regulate several proteins to suppress the JNK pathway, such as MLK3 (35), JNK-interacting protein (JIP) 1 (36), and MKK4 (37). In particular, AKT was demonstrated to directly phosphorylate ASK1, leading to suppression of JNK signaling (38). We therefore explored the possibility of AKT-JNK cross-talk in islets after isolation and the impact of JNK inhibition on islet survival. In the present study, we demonstrate that PI3K/AKT signaling suppresses JNK activation and inhibits JNK-mediated islet cell death.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Human islet isolation
Pancreata were retrieved from heart-beating cadaveric donors at the time of multiorgan harvest for transplantation. Consent for donation of tissues for research was obtained by the local organ procurement organization. Warm ischemia time was approximately 5 min and cold ischemia time, using University of Wisconsin (UW) solution perfusion, was between 30 min and 1 h. The main pancreatic duct was cannulated and a cold (6–8 C) solution of 1.43 mg/ml Liberase HI enzyme blend (Roche, Montréal, Canada) in perfusion solution (Mediatech, Herndon, VA) supplemented with 0.2 mg/ml DNase I (Roche) was infused through the cannula into the pancreas using a syringe. The distended pancreas was placed in a sterilized aluminum digestion chamber (Bio-Rep, Miami, FL) through which the perfusion solution was recirculated at 37 C. The extent of tissue digestion was assessed by staining aliquots of digestate with dithizone (Sigma-Aldrich Canada Ltd., Oakville, Canada) and visualizing the islets under an inverted light microscope (Nikon, Montréal, Canada). The digestion process was terminated by cooling the circuit to between 5 and 10 C when the majority of the islets were seen to be free of surrounding acinar tissue. The digestate was collected, centrifuged, and washed three times with wash solution (Mediatech), and islets were purified on a continuous density gradient (Biocoll separating solution; Biochrom AG, Berlin, Germany) using a COBE 2991 cell processor (COBE BCT, Denver, CO). The number of islet equivalents (IEQs) and islet purity were assessed using dithizone (Sigma), a zinc chelator that stains the zinc-rich ß-cells. Glucose-stimulated insulin secretion was routinely assessed to ensure islet functionality.

Islet cell culture
Isolated islets (>90% purity) were cultured in CMRL 1066 media (Life Technologies, Inc., Burlington, Canada) containing 10% fetal bovine serum (Wisent, St. Bruno, Canada) at 37 C in a humidified atmosphere of 5% CO2. Medium was changed every other day. For acute cytokine stimulation experiments, islets were treated with 100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, and 100 ng/ml interferon (IFN)-{gamma} (R&D Systems Inc., Minneapolis, MN). The biological activities of the cytokines are 10 U/ng (TNF{alpha}), 50 U/ng (IL-1ß), 10 U/ng (IFN{gamma}). IGF-I (Upstate Biotechnology, Lake Placid, NY) was used at a concentration of 100 ng/ml. The PI3K inhibitor wortmannin (Sigma) was used at 100 nM. The inhibitors of JNK (SP600125) and p38 (SB203580) (both from Calbiochem, San Diego, CA) were used at a final concentration of 20 µM. The general caspase inhibitor Z-VAD-fmk (Calbiochem) was added after isolation to the culture media at a final concentration of 50 µM. For all compounds prepared in dimethylsulfoxide (DMSO), the final concentration of DMSO in the culture media was kept less than 0.1%. Vehicle controls were prepared for all treatments.

Caspase-3 assay
The islet samples (2000 IEQ) were spun down for 2 min at 900 rpm and 4 C. The pellet was washed twice with ice-cold PBS and then dissolved in lysis buffer [50 mM Tris-HCl (pH 8.0), 1.37 mM NaCl, 1% (vol/vol) Nonidet P-40, 10% (vol/vol) glycerol]. The samples were sonicated and spun down for 20 min at 14,000 rpm, 4 C, and the supernatant was kept for analysis. The protein content was determined using a protein assay dye reagent (Bio-Rad, Mississauga, Canada). In a 96-well plate, lysate containing 100 µg protein was added to caspase buffer [50 mM HEPES, 1 mM EDTA, 0.1% [(3-cholamidopropyl) dimethyl-ammonio]-1-propane-sulfonate, 10% sucrose, 10 mM dithiothreitol, 100 mM NaCl (pH 7.2)] (CHAPS) containing 200 µM Ac-DEVD-pNA. Samples were incubated at 37 C for 90 min, and the release of pNA was analyzed by measuring the absorbance at 405 nm using a Benchmark microplate reader (Bio-Rad).

MTT assay
Aliquots containing 500 IEQ in 500 µl media were placed in sterile Eppendorf tubes and 50 µl of stock 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) (5 mg/ml) (Sigma) was added to each sample. The samples were incubated at 37 C for 2 h, washed twice with cold PBS, and lysed with 200 µl DMSO (Sigma). Two 100-µl aliquots from each sample were loaded onto a 96-well plate, and the absorbance was measured at 595 nm using a Benchmark microplate reader (Bio-Rad). Four independent samples were analyzed per experiment, and each experiment was performed at least three times.

Mitochondrial depolarization
Islets were dispersed by first washing with dispersion solution (Life Technologies) followed by a 10-min incubation at 37 C with trypsin/EDTA (Life Technologies). Trypsinized islets were washed with cold CMRL 1066 with 10% fetal bovine serum, pipetted gently, and resuspended in PBS. JC1 (Molecular Probes Inc., Eugene, OR) was used according to the manufacturer’s directions to identify cells with depolarized mitochondria. The stained islets were placed on glass slides with coverslips and incubated for 15 min at room temperature in a foil-covered container with a moist paper towel. Slides were visualized under a fluorescent BX60 microscope (Olympus, Tokyo, Japan) connected via a digital video camera to a personal computer. Five hundred cells were counted per slide, with three slides per group for at least three independent experiments. Images were analyzed using Image Pro Plus 4.0 software (Media Cybernetics Inc., Silver Spring, MD).

Glucose-stimulated insulin release
Cultured islets (100 IEQ per group in duplicate) were washed with CMRL 1066 and incubated in Hanks’ buffered saline containing 2.2 mM glucose for two consecutive periods of 60 min at 37 C. Next, islets were incubated for 30 min with 22 mM glucose and then another 30 min with 22 mM glucose with 50 µM 3-isobutil-1-methylxanthine (IBMX), an inhibitor of cAMP phosphodiesterase. Finally, islets were washed with Hanks’ buffered saline and incubated for 1 h in 2.2 mM glucose. The supernatants were kept after each incubation and analyzed for insulin content using a commercially available insulin ELISA kit (ALPCO Diagnostics, Windham, NH). Insulin release was normalized to the protein content of the pellet determined using a protein assay dye reagent (Bio-Rad).

Western blotting
The islet samples (2500 IEQ) were spun down for 2 min at 900 rpm and 4 C. The pellet was washed twice with ice-cold PBS and then dissolved in lysis buffer [50 mM Tris-HCl (pH 8.0), 1.37 mM NaCl, 1% (vol/vol) Nonidet P-40, 10% (vol/vol) glycerol, 0.1 mM sodium orthovanadate, complete protease inhibitor cocktail tablet (Roche)]. The samples were sonicated and spun down for 20 min at 14,000 rpm, 4 C, and the supernatant was kept for analysis. The protein content was determined using a protein assay dye reagent (Bio-Rad). Lysates were diluted 6:1 with 6x Laemmli sample buffer [0.375 M Tris-HCl (pH 6.8), 12% (wt/vol) sodium dodecyl sulfate, 3% (vol/vol) glycerol, 0.2% (wt/vol) bromophenol blue, 12% (vol/vol) ß-mercaptoethanol in doubly distilled water] and boiled for 5 min. An equal amount of protein (75 µg) was loaded for each sample into a 12% polyacrylamide gel run at 100 V for 90 min. Transfer onto nitrocellulose was conducted at 250 mA for 90 min. Membranes were blocked with 2% BSA in washing buffer (25 mM Tris, 150 mM NaCl, 0.05% Tween 20 in doubly distilled water). Blocked membranes were then probed with primary antibodies. Anti-phospho-JNK (Promega Corp., Madison, WI) was used at a 1:5000 dilution. Anti-JNK1 and anti-JIP1 (Santa Cruz Biotechnology, Santa Cruz, CA) were used at a dilution of 1:1000. Anti-AKT, anti-phospho-AKT (Ser-473), anti-c-jun, anti-phospho-c-jun (Ser-73), anti-ASK1, and anti-phospho-ASK1 (Ser-83) (all from Cell Signaling Technology, Beverly, MA) were used at a dilution of 1:1000. After primary antibody incubation, blots were washed for 1 h in washing buffer then incubated for 1 h in antirabbit horseradish peroxidase-linked antibody (1:4000) (Amersham Life Sciences Inc., Buckinghamshire, UK). After another 1 h washing, the blots were developed using the enhanced chemiluminescence system (Amersham) and X-OMAT film (Kodak, Rochester, NY). Membranes were stripped by incubating at 65 C for half an hour in stripping buffer [100 mM ß-mercaptoethanol, 2% (wt/vol) sodium dodecyl sulfate and 62.5 mM Tris-HCl (pH 6.7)] and reprobed with primary antibody.

Statistical analysis
All results are expressed as mean ± SEM. Statistical significance was determined using a one-way ANOVA with a post hoc Bonferroni’s test as well as the Student’s t test. Differences were considered significant when P < 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
After routine isolation decreased JNK activation correlates with increased AKT activation
Using Western blot analysis, the levels of JNK and AKT phosphorylation were examined during the first 72 h after isolation (Fig. 1AGo). We observed that JNK phosphorylation levels decreased concomitantly with a rise in AKT phosphorylation (Fig. 1BGo). By 16 h after isolation, AKT is highly activated, whereas JNK activation begins to decrease. The tight temporal correlation between these two events supported the notion that they might be related.



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  		FIG. 1. Increased AKT phosphorylation correlates with decreased JNK phosphorylation. A, Whole-cell lysates were taken at the indicated times after isolation and analyzed by Western blotting. The blots are representative of six independent experiments. B, Semiquantification of representative blots using densitometric analysis. Open circles represent AKT phosphorylation, and closed circles denote JNK phosphorylation.

 
We examined the possibility that an upstream kinase of JNK was inhibited, causing the observed decrease in JNK phosphorylation. ASK1 is a key regulator of the JNK pathway amenable to inhibition by AKT-mediated phosphorylation at Ser83 (38). We therefore examined the levels of ASK1 phosphorylation at Ser83 over the first 72 h after isolation (Fig. 1AGo). ASK1 phosphorylation was evident by 16 h and remained elevated, consistent with the pattern of AKT activation. Expression of JIP1, a scaffold protein that can regulate JNK activity in islets (39), remained unchanged (Fig. 1AGo).

PI3K inhibition leads to increased JNK-mediated apoptosis
To test whether increased AKT-mediated suppression of ASK1 is responsible for the observed decrease in JNK activity, we employed the PI3K inhibitor wortmannin. Islets were cultured for 48 h, at which point AKT is highly active and JNK activity is low, and then treated with wortmannin. An increase in JNK phosphorylation was detectable after 12 h of wortmannin treatment and increased over the ensuing 24 h (Fig. 2AGo). Wortmannin treatment completely blocked AKT phosphorylation and consequently ASK1 phosphorylation at Ser83. These results suggest that PI3K/AKT activation in isolated islets is responsible for the suppression of JNK via inactivation of ASK1.



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  		FIG. 2. PI3K inhibition leads to increased JNK-mediated cell death. A, Freshly isolated islets were cultured for 48 h before the addition of 100 nM wortmannin. Whole-cell lysates were taken after 12, 24, and 36 h of treatment and analyzed by Western blotting. The blot is representative of three independent experiments. B, Isolated islets were cultured for 48 h and pretreated with SP600125 (20 µM) for 30 min before the addition of wortmannin (100 nM). Whole-cell lysates were taken after 24 h of treatment and analyzed by Western blotting. The blot is representative of three independent experiments. C, Isolated islets were cultured for 48 h and pretreated with SP600125 (20 µM) for 30 min before the addition of wortmannin (100 nM). Islet viability was assessed by MTT assay after 24 and 72 h of treatment. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from five independent experiments. *, P < 0.05 compared with the control.

 
We previously demonstrated that wortmannin treatment induced cell death in isolated canine islets (32). We thus examined the possibility that PI3K inhibition leads to JNK-mediated cell death. Human islets were pretreated for 30 min with a selective JNK inhibitor (SP600125) before wortmannin treatment, and JNK phosphorylation was assessed after 24 h in culture. SP600125 was able to completely block wortmannin-induced JNK phosphorylation (Fig. 2BGo). In addition, SP600125 was able to prevent decreased MTT reduction after 24 and 72 h of wortmannin treatment.

IGF-I suppresses cytokine-mediated JNK activation in a PI3K-dependent manner
To study whether activators of AKT can lead to suppression of JNK signaling, we examined the effects of IGF-I on cytokine-mediated JNK activation. Isolated islets were cultured for 48 h before treatment, at which time JNK activity is reduced (Fig. 1AGo). Cytokine treatment led to increased JNK phosphorylation and a 30% decrease in MTT reduction after 24 h in culture (Fig. 3Go). Pretreatment with IGF-I abolished cytokine-mediated JNK activation and protected islets against the cytokine-induced decrease in MTT reduction. Wortmannin pretreatment abolished the protective effect of IGF-I and was able to restore cytokine-mediated activation of JNK, suggesting that the protective effect of IGF-I is mediated by PI3K-dependent suppression of JNK. Indeed, JNK inhibition by SP600125 blocked JNK phosphorylation and partially restored islet survival against cytokine-induced cell death (Fig. 3Go). These findings suggest that the protective effect of IGF-I against cytokine-induced cell death is partially mediated by PI3K-mediated suppression of JNK signaling.



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  		FIG. 3. IGF-I suppresses cytokine-mediated JNK activation in a PI3K-dependent manner. Freshly isolated islets were cultured for 48 h and then treated with or without cytokines (Cyto, 100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, 100 ng/ml IFN{gamma}), IGF-I (100 ng/ml), wortmannin (Wort, 100 nM), and SP600125 (SP, 20 µM) for 24 h. A, Whole-cell lysates were taken and JNK phosphorylation was analyzed by Western blotting. The blot is representative of three independent experiments. B, Islet viability was assessed by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from three independent experiments. *, P < 0.05; **, P < 0.01 compared with the control (Ctrl).

 
Inhibition of PI3K/AKT renders islets more susceptible to cytokine-mediated cell death
To examine the possibility that the rise in AKT activation in untreated isolated islets reduces their susceptibility to cytokine-mediated cell death, we treated islets with cytokines in the presence of wortmannin. Treatment with either wortmannin alone or cytokines alone led to increased JNK phosphorylation, increased caspase-3 activity, and reduced viability (Fig. 4Go). However, cotreatment of islets with both wortmannin and cytokines led to increased levels of JNK phosphorylation when compared with islets treated with only cytokines (Fig. 4AGo). Inhibition of PI3K also increased cytokine-induced caspase-3 activation and caused a significant decrease in MTT reduction when compared with cytokine-treated islets (Fig. 4Go, B and C). These findings indicate that the elevated levels of AKT activity occurring in cultured islets could render these cells less susceptible to cytokine-mediated cell death.



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  		FIG. 4. Inhibition of PI3K/AKT renders islets more susceptible to cytokine-mediated cell death. Islets were cultured for 48 h before the addition of wortmannin (Wort, 100 nM) and/or cytokines (Cyto, 100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, 100 ng/ml IFN{gamma}). The islets were then cultured for an additional 24 h. A, Lysates were taken and JNK phosphorylation was analyzed by Western blotting. The blot shown is representative of three independent experiments. B, Caspase-3 activity of islet cell lysates was assessed by measuring the cleavage of DEVD-pNA. The amount of pNA released was measured by analyzing the absorption at 405 nm. For each graph, bars represent the mean values ± SEM for three independent experiments. C, Islet viability was assessed by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from four independent experiments. *, P < 0.05; **, P < 0.01 compared with the control (Ctrl).

 
The JNK inhibitor SP600125 transiently protects islets from cytokine-mediated cell death
JNK has previously been implicated in cytokine-mediated ß-cell death (40, 41, 42). Therefore, we examined whether JNK inhibition by pretreatment with SP600125 is sufficient to block cytokine-mediated cell death. As expected, SP600125 was able to completely block cytokine-induced JNK and c-jun phosphorylation after 24 h treatment (Fig. 5AGo). SP600125 also suppressed the cytokine-mediated decrease in islet viability after 24 h in culture, as measured by MTT assay (Fig. 5BGo). However, by 72 h the protective effect of SP600125 against cytokines was diminished. These data indicate that SP600125 can delay, but not prevent, cytokine-induced cell death.



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  		FIG. 5. The JNK inhibitor SP600125 transiently protects islets from cytokine-mediated cell death. Freshly isolated islets were cultured for 48 h and then treated with cytokines (100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, 100 ng/ml IFN{gamma}) with or without SP600125 (20 µM). A, Lysates were taken after 24 h and JNK and c-jun phosphorylation were analyzed by Western blotting. The blots shown are representative of three independent experiments. B, Islet viability was assessed after 24 and 72 h by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from four independent experiments. *, P < 0.05; **, P < 0.01 compared with the control.

 
SP600125 improves isolated islet survival and function after routine isolation
We have previously shown that freshly isolated islets display high JNK activity immediately after isolation, which diminishes over the first 48 h after isolation (34, 43). This early activation of JNK coincides with the peak of islet apoptosis, which occurs 24 h after isolation (34, 43, 44). We therefore examined the effects of SP600125 treatment immediately after isolation on islet survival and function after 48 h in culture. SP600125 treatment led to a 37% increase in islet viability after 48 h in culture (Fig. 6AGo). In addition, SP600125 reduced caspase-3 activity by 47% (Fig. 6BGo).



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  		FIG. 6. SP600125 improves isolated islet survival and function after routine isolation. Immediately after isolation, islets were cultured with or without the addition of SP600125 (SP, 20 µM) to the culture media. A, After 48 h in culture, islet viability was assessed by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from five independent experiments. B, Caspase-3 activity was assessed by measuring the cleavage of DEVD-pNA by islet cell lysates taken after 48 h in culture. The amount of pNA released was measured by analyzing the absorption at 405 nm. Bars represent the mean values ± SEM for three independent experiments. C, Mitochondrial depolarization was assessed after 48 h in culture by JC1 staining. The number of cells with depolarized mitochondria was expressed as a percentage of the total cells counted. Bars represent the mean values ± SEM for three independent experiments. D, Glucose-stimulated insulin release was examined after 48 h in culture. Bars represent mean values ± SEM for three independent experiments. E, Islets were cultured with or without the addition of SP600125 (20 µM) to the culture media for the indicated periods. After 96 h in culture, islet viability was assessed by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from five independent experiments. *, P < 0.05; **, P < 0.01 compared with the control (Ctrl).

 
We previously demonstrated increased mitochondrial depolarization in islets after 72 h in culture (44), which could account for increased necrotic-like cell death in isolated islets. Because JNK has been shown to be required for cytochrome C release (9), we examined whether SP600125 could diminish mitochondrial depolarization in isolated islets. Indeed, SP600125 treatment caused a 62% decrease in the number of cells with depolarized mitochondria (11.4 ± 1.2 vs. 4.3 ± 0.7) as assessed by JC1 staining (Fig. 6CGo).

To examine the effect of SP600125 on islet function, static glucose-stimulated insulin release was examined in islets after 48 h in culture with or without SP600125. Insulin secretion by SP600125-treated islets was not significantly different from that of untreated islets at both low and high glucose levels (Fig. 6DGo). However, the addition of IBMX resulted in increased insulin secretion by the SP600125-treated islets, indicating that either these islets released more insulin per ß-cell or contained a higher percentage of ß-cells. Both SP600125-treated and control islets were able to shut off insulin secretion in response to low glucose (Fig. 6DGo).

To ascertain whether islets treated with SP600125 maintained their increased viability when SP600125 was withdrawn, we cultured islets for 96 h during which SP600125 was administered during the first 48 h and then withdrawn, during the last 48 h, or during the whole 96 h incubation. Islets that received SP600125 during the first 48 h only (approximate culture period before transplantation) showed a 32% increase in MTT reduction despite the removal of SP600125 after 48 h (Fig. 6EGo). Islets that received SP600125 throughout the 96-h culture period were equally viable, displaying a 28% increase in MTT reduction. Surprisingly, islets treated with SP600125 during only the last 48 h displayed a 21% increase in MTT reduction, compared with the untreated control, despite the fact that JNK activation has decreased significantly at this point.

Sustained JNK activation caused by wortmannin or cytokines does not require caspase activation
Caspase activity is required in some cases for sustained JNK activation (45). We therefore examined the effects of general caspase inhibition on JNK activation by wortmannin. Pretreatment of islets with 50 µM Z-VAD-fmk had no effect on JNK phosphorylation induced by 24 h treatment with wortmannin (Fig. 7AGo). Similarly, Z-VAD-fmk had no effect on JNK phosphorylation after 24 h of cytokine treatment (Fig. 7BGo).



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  		FIG. 7. Sustained JNK activation caused by wortmannin or cytokines does not require caspase activation. A, Freshly isolated islets were cultured for 48 h before the addition of 100 nM wortmannin with or without 50 µM Z-VAD-fmk or 20 µM SP600125. Whole-cell lysates were taken after 24 h of treatment and JNK phosphorylation was analyzed by Western blotting. The blot is representative of three independent experiments. B, Freshly isolated islets were cultured for 48 h and then treated with cytokines (100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, 100 ng/ml IFN{gamma}) with or without 50 µM Z-VAD-fmk or 20 µM SP600125. Whole-cell lysates were taken after 24 h of treatment and JNK phosphorylation was analyzed by Western blotting. The blot is representative of three independent experiments.

 
Inhibition of p38 with SB203580 has a negative effect on isolated islet survival
Many stimuli that cause JNK activation also activate the p38 pathway because these two kinases share many common upstream regulators. In particular ASK1, which in addition to acting upstream of JNK, also leads to p38 activation (20). Furthermore, just as AKT can have a suppressive effect on JNK, AKT has also been demonstrated to suppress the p38 pathway (46). We therefore explored the possibility that p38 was also contributing to cytokine- and wortmannin-induced cell death using the p38 inhibitor SB203580. Pretreatment of islets with SB203580 had no effect on cytokine-induced cell death (Fig. 8AGo). Similarly, SB203580 had no protective effect against the wortmannin-induced decrease in islet viability (Fig. 8AGo). When administered immediately after isolation, SB203580 led to decreased viability, suggesting that p38 may act as a survival pathway in isolated islets (Fig. 8BGo).



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  		FIG. 8. Inhibition of p38 with SB203580 has a negative effect on isolated islet survival. A, Freshly isolated islets were cultured for 48 h and then treated with or without cytokines (Cyto, 100 ng/ml TNF{alpha}, 2 ng/ml IL-1ß, 100 ng/ml IFN{gamma}), wortmannin (Wort, 100 nM), SB203580 (SB, 20 µM), and SP600125 (20 µM) for 24 h. Islet viability was assessed by MTT assay. Bars represent the mean MTT reduction expressed as a percentage of the control ± SEM for quadruplicate samples from three independent experiments. B, Immediately after isolation, islets were treated with increasing concentrations of SB203580 (1–100 µM). After 48 h in culture, islet viability was assessed by MTT assay. Bars represent relative MTT reduction values expressed as a percentage of the control ± SEM for quadruplicate samples from four independent experiments. *, P < 0.05; **, P < 0.01 compared with the control (Ctrl).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our results suggest that there is a cross-talk between PI3K/AKT and JNK in human islets via negative regulation of ASK1. The ability of AKT to suppress the JNK pathway has been observed in a variety of cell systems (47, 48, 49). AKT has been shown to suppress the JNK pathway by phosphorylating and negatively regulating ASK1 (38), MLK3 (35), and MKK4 (SEK1) (37). In addition, AKT has also been suggested to prevent JNK activation by directly interacting with JIP1 and preventing the recruitment of upstream kinases to JNK (36). It has also been suggested that AKT may suppress JNK through up-regulation of JIP1 (48), but we observed no changes in JIP1 expression after isolation (Fig. 1Go). Another mechanism of suppressing JNK signaling is through the up-regulation of MAPK phosphatase (MKP)-1, an inducible phosphatase that is thought to preferentially dephosphorylate p38 and JNK (50), yet in some cases it can also inactivate ERK (51). We have shown that after islet isolation, MKP-1 expression increases over the first 72 h after isolation (34). This increase in MKP-1 expression coincides with the decrease in JNK phosphorylation and could represent another mechanism of JNK suppression in isolated islets. Therefore, there are several mechanisms by which AKT can suppress JNK activation, indicating that JNK suppression is an important consequence of AKT-mediated survival. However, it is plausible that the particular mechanisms of JNK suppression will depend on the nature of the stimuli and the cellular context. It is interesting that cytokine treatment caused JNK activation in islets despite high AKT activity, suggesting that AKT may suppress JNK activation only by particular stimuli. Indeed, there is an assortment of kinases acting upstream of JNK, not all of which may be suppressed by AKT. In addition, the subcellular distribution of particular kinases may play an important role in permitting particular interactions between signaling molecules.

Our results demonstrate that inhibition of PI3K/AKT signaling in human islets results in a time-dependent increase in JNK activation (Fig. 2Go). A similar result was obtained in cerebellar granule neurons in which inhibition of PI3K led to an increase in JNK activation (52). In addition, expression of dominant-negative (DN) AKT in human vascular smooth muscle cells led to increased JNK activation and Fas ligand induction (53). These findings suggest that either the inhibition of PI3K/AKT itself stimulates the JNK pathway or that in the absence of PI3K/AKT signaling, an external activator of the JNK pathway becomes apparent. The stimuli responsible for the activation of JNK in the absence of PI3K/AKT signaling could be any number of insults to which islets are subjected to during the isolation procedure. Indeed, the same stimuli that are responsible for JNK activation immediately after isolation could still be present and are uncovered through derepression of the JNK pathways by inhibition of PI3K/AKT.

JNK activation can be induced by growth factor withdrawal (54), detachment from the extracellular matrix (55), ischemia (56), osmotic stress (57), reactive oxygen species (58), and cytokines (59), all of which are possible consequences of islet isolation. We have demonstrated that freshly isolated islets display high JNK activity, which diminishes over the first 48 h after isolation (34, 43). This early activation of JNK coincides with the peak of islet apoptosis, which occurs 24 h after isolation (34, 43, 44). Addition of exogenous insulin to the culture media immediately after isolation reduced JNK activity and led to a decrease in DNA fragmentation after 24 h in culture (43). Preservation of rat pancreata for 24 h by the two-layer method, which uses oxygenated perfluorohydrocarbons to increase oxygen delivery to the organ, resulted in decreased apoptosis and lower JNK activity after isolation when compared with islets from pancreata stored in UW solution (60). In addition, the same study showed that islets from pancreata that were processed immediately after resection had significantly lower JNK activation than islet from pancreata stored for 24 h by the two-layer method or in UW solution. The most convincing evidence implicating JNK in transplanted islet survival comes from transplantation of rat islets overexpressing DN JNK into streptozotocin-induced diabetic nude mice (61). Mice receiving DN-JNK islets displayed lower blood glucose levels, which could be attributed to maintenance of insulin gene transcription despite the presence of oxidative stress at the graft site as well as increased survival of islets due to impaired apoptotic signaling by JNK. Taken together, these findings point toward a role for JNK in mediating isolated islet apoptosis. The ability of SP600125 to improve isolated islet survival and function supports the notion that JNK is an important regulator of islet death.

When administered immediately after isolation, SP600125 improved islet survival and function, even 48 h after removal of SP600125 (Fig. 6EGo). This finding indicates that islets rescued by JNK inhibition remain viable despite the removal of SP600125. Therefore, SP600125 could be a beneficial constituent of islet culture media and perhaps other solutions during the isolation. JNK inhibition was also able to improve isolated islet survival when treatment was commenced 48 h after isolation, at a point when JNK activation is significantly reduced (Fig. 6EGo). The beneficial effects of SP600125 even when JNK activity is reduced could be due to the fact that JNK is required for mitochondria-mediated cell death (9, 10, 11), which appears to be occurring at later time points in islets (44). We previously suggested that the inability of caspase inhibition to prevent islet cell death may be due to the fact that the caspases lie downstream of any mitochondrial dysfunction, and thus their inhibition has no effect on mitochondria-mediated cell death (44). In this case, caspase inhibition led to a more necrotic-like mode of islet cell death. In contrast, SP600125 significantly reduced mitochondrial depolarization, suggesting that JNK regulates events upstream of mitochondrial dysfunction in islets. Recently SP600125 (20 µM) has been shown to result in rapid activation of cAMP-response element binding protein (CREB) in MIN6 cells under basal conditions (62), which could also explain the positive effect of SP600125 at later time points after isolation because CREB is implicated in islet survival (63, 64). However, we did not observe an increase in p38 or CREB phosphorylation after 30 min of SP600125 treatment in isolated human islets (data not shown). Similarly, SP600125 did not result in increased p38 phosphorylation in stimulated Jurkat T cells (65). Therefore, it is plausible that SP600125-induced CREB activation is observable only under nonstimulated conditions, as suggested by Vaishnav et al. (62).

There is considerable evidence supporting a role for JNK in IL-1ß-mediated ß-cell apoptosis (40, 41, 42, 66). However, JNK inhibition using SP600125 afforded only transient protection of human islets against a combination of IL-1ß, TNF{alpha}, and IFN{gamma} (Fig. 5Go). This may be due to the ability of these cytokines to activate several pathways, in addition to JNK, known to regulate islet apoptosis, including signal transducer and activator of transcription (67, 68), production of reactive oxygen species (69), increased cytosolic Ca2+ levels (70), and decreased Bcl-2 (64). In addition, the ability of cytokines to induce apoptosis through Fas-associated death domain-mediated recruitment of caspase-8 (extrinsic pathway) is also possible, although perhaps not likely due to the high levels of expression of the endogenous caspase-8 inhibitor Fas-associated death domain-like IL-1ß converting enzyme inhibitory protein (FLIP) in human islets (71). Our results suggest that JNK is necessary for proper execution of cytokine-stimulated cell death in human islets, but the precise role of JNK in cytokine-mediated islet cell death remains unclear.

In addition to increased islet viability, we observed increased insulin secretion by human islets treated with SP600125 (Fig. 6DGo). Previously SP600125 has been shown to increase insulin release in MIN6 cells, and overexpression of DN JNK led to increased insulin gene transcription (72). A role for JNK in stress-mediated suppression of insulin gene transcription has also been demonstrated in rat islets (61). These effects could be explained by the suppressive effect of c-jun on insulin gene transcription (73). Therefore, in addition to promoting isolated islet survival, JNK inhibition can also improve insulin secretion.

Many stressors that activate JNK also activate p38. However, as with JNK, the exact role of p38 in regulation of cell survival remains unclear because p38 appears to mediate both survival and apoptotic signals (4). Our current results suggest that p38 activity may represent a survival signal in isolated human islets because inhibition of p38 increased isolated islet cell death (Fig. 8Go). Similarly, p38 inhibition has previously been demonstrated to aggravate cytokine-mediated human islet cell death (74). In addition, inhibition of p38 potentiated IL-1ß-induced cell death in a ß-cell line (40). These findings are in contrast to those in rat islets, in which p38 inhibition reduced apoptosis induced by IL-1ß and IFN{gamma} (75). We previously observed an increase in p38 phosphorylation in canine islets treated with insulin, which can act as a survival stimulus in many cell systems (43). This effect, however, is cell type dependent because insulin has been shown to inhibit p38 activation in primary neuronal cultures (76). IGF-I-mediated survival has been shown to involve p38-dependent phosphorylation of CREB and induction of Bcl-2 expression (77, 78). Taken together, these data suggest that p38 could play a role in regulating islet survival, possibly by mediating growth factor signaling. It is also plausible that activation of p38 in response to insulin represents an inhibitory feedback pathway to suppress insulin transcription (79).

In summary, our results demonstrate that PI3K/AKT suppresses the JNK pathway in human islets and that this cross-talk represents an important antiapoptotic consequence of PI3K/AKT activation. In addition, due to its beneficial effects on islet survival and function, JNK inhibition by SP600125 may be a viable strategy for improving isolated islet survival.


   Acknowledgments
 
The authors thank M. Lipsett, S. Hanley, and M. Castellarin for technical assistance with the islet isolation procedure, as well as J. Tam and R. Savic for critically reviewing the manuscript.


   Footnotes
 
This work was supported by the Juvenile Diabetes Research Foundation, Canadian Institutes for Health Research, and Canadian Diabetes Association. L.R. is a National Scientist supported by the Fonds de la Recherche en Santé du Québec.

Abbreviations: ASK, Apoptosis signaling-regulating kinase; CREB, cAMP-response element binding protein; DMSO, dimethylsulfoxide; DN, dominant-negative; IBMX, 3-isobutil-1-methylxanthine; IEQ, islet equivalent; IFN, interferon; JIP, JNK-interacting protein; JNK, c-jun NH2-terminal kinase; MAPKK, MAPK kinase; MAPKKK, MAPKK kinase; MKP, MAPK phosphatase; MLK, mixed-lineage protein kinase; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PI3K, phosphatidylinositol 3-kinase; TRAF, TNF receptor-associated factor.

Received April 16, 2004.

Accepted for publication June 28, 2004.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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