Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.

Free fatty acids (FFA) cause apoptosis of pancreatic β-cells and might contribute to β-cell loss in type 2 diabetes via the induction of endoplasmic reticulum (ER) stress. We studied here the molecular mechanisms implicated in FFA-induced ER stress initiation and apoptosis in INS-1E cells, FACS-purified primary β-cells and human islets exposed to oleate and/or palmitate. Treatment with saturated and/or unsaturated FFA led to differential ER stress signaling. Palmitate induced more apoptosis and markedly activated the IRE1, PERK and ATF6 pathways, owing to a sustained depletion of ER Ca²⁺ stores, whereas the unsaturated FFA oleate led to milder PERK and IRE1 activation and comparable ATF6 signaling. Non-metabolizable methyl-FFA analogs induced neither ER stress nor β-cell apoptosis. The FFA-induced ER stress response was not modified by high glucose concentrations, suggesting that ER stress in primary β-cells is primarily lipotoxic, and not glucolipotoxic. Palmitate, but not oleate, activated JNK. JNK inhibitors reduced palmitate-mediated AP-1 activation and apoptosis. Blocking the transcription factor CHOP delayed palmitate-induced β-cell apoptosis. In conclusion, saturated FFA induce ER stress via ER Ca²⁺ depletion. The IRE1 and resulting JNK activation contribute to β-cell apoptosis. PERK activation by palmitate also contributes to β-cell apoptosis via CHOP.

Apoptosis is probably the main form of β-cell death in both type 1 diabetes mellitus (T1DM) and T2DM. In T1DM, cytokines contribute to β-cell destruction through nuclear factor-κB (NF-κB) activation. Previous studies suggested that in T2DM high glucose and free fatty acids (FFAs) are β-cell toxic also via NF-κB activation. The aims of this study were to clarify whether common mechanisms are involved in FFA- and cytokine-induced β-cell apoptosis and determine whether TNFα, an adipocyte-derived cytokine, potentiates FFA toxicity through enhanced NF-κB activation. Apoptosis was induced in insulinoma (INS)-1E cells, rat islets, and fluorescence-activated cell sorting-purified β-cells by oleate, palmitate, and/or cytokines (IL-1β, interferon-γ, TNFα). Palmitate and IL-1β induced a similar percentage of apoptosis in INS-1E cells, whereas oleate was less toxic. TNFα did not potentiate FFA toxicity in primary β-cells. The NF-κB-dependent genes inducible nitric oxide synthase and monocyte chemoattractant protein-1 were induced by IL-1β but not by FFAs. Cytokines activated NF-κB in INS-1E and β-cells, but FFAs did not. Moreover, FFAs did not enhance NF-κB activation by TNFα. Palmitate and oleate induced C/EBP homologous protein, activating transcription factor-4, and immunoglobulin heavy chain binding protein mRNAs, X-box binding protein-1 alternative splicing, and activation of the activating transcription factor-6 promoter in INS-1E cells, suggesting that FFAs trigger an endoplasmic reticulum (ER) stress response. We conclude that apoptosis is the main mode of FFA- and cytokine-induced β-cell death but the mechanisms involved are different. Whereas cytokines induce NF-κB activation and ER stress (secondary to nitric oxide formation), FFAs activate an ER stress response via an NF-κB- and nitric oxide-independent mechanism. Our results argue against a unifying hypothesis for the mechanisms of β-cell death in T1DM and T2DM.

Cytokines and free radicals are mediators of β-cell death in type 1 diabetes. Under in vitro conditions, interleukin-1β (IL-1β) + γ-interferon (IFN-γ) induce nitric oxide (NO) production and apoptosis in rodent and human pancreatic β-cells. We have previously shown, by microarray analysis of primary β-cells, that IL-1β + IFN-γ decrease expression of the mRNA encoding for the sarcoendoplasmic reticulum pump Ca²⁺ ATPase 2b (SERCA2b) while inducing expression of the endoplasmic reticulum stress–related and proapoptotic gene CHOP (C/EBP [CCAAT/enhancer binding protein] homologous protein). In the present study we show that cytokine-induced apoptosis and necrosis in primary rat β-cells and INS-1E cells largely depends on NO production. IL-1β + IFN-γ, via NO synthesis, markedly decreased SERCA2b protein expression and depleted ER Ca²⁺ stores. Of note, β-cells showed marked sensitivity to apoptosis induced by SERCA blockers, as compared with fibroblasts. Cytokine-induced ER Ca²⁺ depletion was paralleled by an NO-dependent induction of CHOP protein and activation of diverse components of the ER stress response, including activation of inositol-requiring ER-to-nucleus signal kinase 1α (IRE1α) and PRK (RNA-dependent protein kinase)-like ER kinase (PERK)/activating transcription factor 4 (ATF4), but not ATF6. In contrast, the ER stress–inducing agent thapsigargin triggered these four pathways in parallel. In conclusion, our results suggest that the IL-1β + IFN-γ–induced decrease in SERCA2b expression, with subsequent depletion of ER Ca²⁺ and activation of the ER stress pathway, is a potential contributory mechanism to β-cell death.

Type 1 diabetes mellitus results from an autoimmune destruction of pancreatic beta-cells. Cytokines, such as interleukin-1 beta and interferon-gamma, are putative mediators of immune-induced beta-cell death and, under in vitro conditions, cause beta-cell apoptosis. We have recently shown that interleukin-1 beta + interferon-gamma modifies the expression of >200 genes in beta-cells. Several of these genes are putative targets for the transcription factor nuclear factor-kappa B (NF-kappa B), and in subsequent experiments we showed that NF-kappa B activation is mostly pro-apoptotic in beta-cells. To identify cytokine-induced and NF-kappa B-regulated genes in primary rat beta-cells, we presently combined two experimental approaches: 1) blocking of NF-kappa B activation in cytokine-exposed beta-cells by a recombinant adenovirus (AdI kappa B((SA)2)) containing an inhibitor of NF-kappa B alpha (I kappa Bac) super-repressor (S32A/S36A) and 2) study of gene expression by microarray analysis. We identified 66 cytokine-modified and NF-kappa B-regulated genes in beta-cells. Cytokine-induced NF-kappa B activation decreased Pdx-1 and increased c-Myc expression. This, together with NF-kappa B-dependent inhibition of Glut-2, pro-hormone convertase-1, and Isl-1 expression, probably contributes to the loss of differentiated beta-cell functions. NF-kappa B also regulates several genes encoding for chemokines and cytokines in beta-cells. The present data suggest that NF-kappa B is a key "switch regulator" of transcription factors and gene networks controlling cytokine-induced beta-cell dysfunction and death.

Free fatty acids cause pancreatic β-cell apoptosis and may contribute to β-cell loss in type 2 diabetes via the induction of endoplasmic reticulum stress. Reductions in eukaryotic translation initiation factor (eIF) 2α phosphorylation trigger β-cell failure and diabetes. Salubrinal selectively inhibits eIF2α dephosphorylation, protects other cells against endoplasmic reticulum stress-mediated apoptosis, and has been proposed as a β-cell protector. Unexpectedly, salubrinal induced apoptosis in primary β-cells, and it potentiated the deleterious effects of oleate and palmitate. Salubrinal induced a marked eIF2α phosphorylation and potentiated the inhibitory effects of free fatty acids on protein synthesis and insulin release. The synergistic activation of the PERK-eIF2α branch of the endoplasmic reticulum stress response, but not of the IRE1 and activating transcription factor-6 pathways, led to a marked induction of activating transcription factor-4 and the pro-apoptotic transcription factor CHOP. Our findings demonstrate that excessive eIF2α phosphorylation is poorly tolerated by β-cells and exacerbates free fatty acid-induced apoptosis. This modifies the present paradigm regarding the beneficial role of eIF2α phosphorylation in β-cells and must be taken into consideration when designing therapies to protect β-cells in type 2 diabetes.