Endoplasmic Reticulum Stress In Diabetes Research Articles

225-LB: Targeting ß-Cell Endoplasmic Reticulum (ER) TALK-1 Channels to Restore Ca2+ Homeostasis and Reduce ER Stress in Diabetes

Perturbations in Ca2+ homeostasis contribute to β-cell failure in diabetes, limiting insulin secretion and exacerbating hyperglycemia. TALK-1 is the most islet-restricted and most abundantly expressed K+ channel of the β-cell. ER TALK-1 channels provide K+ countercurrent for [Ca2+]ER release, thereby tuning [Ca2+]ER storage and release. During β-cell Ca2+ influx, TALK-1-enhanced [Ca2+]ER release leads to activation of the hyperpolarizing Ca2+ activated K+ channels, thus increasing the silent phase duration between Ca2+ oscillations. A gain-of-function mutation in TALK-1(L114P) causes MODY, by disrupting cytoplasmic and ER Ca2+ handling. However, the ability to probe ER TALK-1 channels as a therapeutic target to normalize β-cell Ca2+ handling and reduce ER stress in diabetes has not been determined. Here, using a high throughput fluorescence-based thallium (TI+) flux assay (z’ =0.64) in a TALK-1 overexpression cell line, we identified 80 TALK-1 selective inhibitors from a small molecule library of 50,000 compounds. 4 potent inhibitors that also inhibited the ER membrane TALK-1 channels were identified by monitoring cyclopiazonic acid-induced [Ca2+]ER release (e.g. K2P16-i7(IC50=5.3μM); 63.36% [Ca2+]ER recovery, p-value=0.02). We also tested these inhibitors for their efficacy in controlling mouse islet [Ca2+]ER storage as well as cytosolic Ca2+ oscillations. TALK-1 inhibitors significantly increased islet [Ca2+]ER storage and Ca2+ oscillation frequency (e.g.K2P16-i7; [Ca2+]ER AUC=60.9a.u., peaks/min=0.45 vs. DMSO; [Ca2+]ER AUC=49.7a.u., peaks/min= 0.17, p-value<0.05). Importantly, the inhibitors did not alter [Ca2+]ER and glucose-stimulated Ca2+ oscillation frequency in TALK-1 KO islets. In summary, we have identified the first ER TALK-1 selective inhibitors that can not only be utilized to assess the role of TALK-1 in human β-cell Ca2+ handling and GSIS, but also potentially reduce ER dysfunction during diabetes pathogenesis. Disclosure A. Y. Nakhe: None. P. Dadi: None. D. Westover: None. E. Days: None. J. Denton: None. D. Jacobson: None.

High-mobility group box-1 promotes vascular calcification in diabetic mice via endoplasmic reticulum stress.

Several studies reported the role of endoplasmic reticulum stress (ERS) in vascular calcification. High‐mobility group box‐1 (HMGB‐1) plays a substantial role in diabetes and its complications. However, relatively little information is available regarding the association between HMGB‐1 and calcification, and the underlying mechanism has still remained elusive. Therefore, in the present study, we attempted to indicate whether HMGB‐1 could promote vascular calcification via ERS in diabetes. After induction of diabetes by Streptozotocin (STZ), mice were treated with glycyrrhizin (Gly) or 4‐phenylbutyrate (4‐PBA). Mineral deposition was confirmed by reverse transcription‐polymerase chain reaction (RT‐PCR) and calcium assay. In cell experiments, calcification of vascular smooth muscle cells (VSMCs) was performed with Alizarin Red staining, alkaline phosphatase (ALP) activity and RT‐PCR. Expression and location of HMGB‐1 in aortic tissue were detected by Western blotting, immunocytochemistry (ICC) and immunohistochemistry (IHC). Diabetic mice demonstrated increased HMGB‐1 expression, ERS and vascular calcification. However, inhibition of HMGB‐1 with Gly or inhibition of ERS with 4‐PBA ameliorated the enhanced vascular calcification and ERS in diabetic mice. In vitro experiments unveiled that inhibition of HMGB‐1 attenuated advanced glycation end products (AGEs)‐induced ERS in VSMCs. In addition, AGEs promoted translocation and secretion of HMGB‐1 in VSMCs, which was reversed by 4‐PBA. Moreover, VSMCs exhibited increased mineralization and osteogenic gene expressions in response to HMGB‐1 and AGEs. However, inhibition of ERS with 4‐PBA partially, although noticeably, attenuated VSMC calcification induced by HMGB‐1. Thus, diabetes induced translocation and secretion of HMGB‐1 via ERS, which resulted in calcification in diabetic mice and in AGEs‐treated VSMCs.

Autophagy regulates insulin resistance following endoplasmic reticulum stress in diabetes.

Autophagy is a kind of cell biological process that maintains the cell's energy level under nutrient-poor conditions, regulates the turnover of abnormal or aged proteins, and disposes of dysfunctional organelles. The autophagy system is activated as a novel signaling pathway in response to endoplasmic reticulum stress (ER stress)-induced insulin resistance (IR). Defective autophagy may be closely related to insulin resistance. There are at least three mechanistically distinct arms of ER stress that regulate the expression of key genes which not only function within the secretory pathway but also affect broad aspects of cell fate and the metabolism of proteins, amino acids, and lipids. ER stress-stimulated insulin resistance is mediated by the autophagy-dependent process. In settings of chronic ER stress, the associated autophagy may contribute to pathophysiological processes involved in a number of prevalent diseases, including diabetes. Whether autophagy plays a protective or harmful role in diabetes awaits further analysis. In this review, we will summarize the current knowledge about the emerging role of autophagy in ER stress-induced insulin resistance. Strategies to take advantage of the potential protective effect of autophagy remain important in the overall treatment of insulin resistance and type 2 diabetes.

Endoplasmic Reticulum Stress and Heart Complication in Diabetes

Diabetes mellitus (DM), including type 1 and type 2, is a significant and substantially enhancing burden in adolescents and adults. Diabetic patients are at high risk for several cardiovascular disorders: coronary heart disease, stroke, peripheral arterial disease, cardiomyopathy, and congestive heart failure. Cardiovascular diseases are the most predominant cause of morbidity and mortality among diabetic patients. The pathophysiology of the diabetes-associated cardiovascular disease has not been completely elucidated. Therefore, there is a critical need for new therapies and clear strategies for research, prevention and treatment to stop the progression of the disease. The purpose of this mini-review is to discuss the recent literature linking heart, endoplasmic reticulum stress, and to address the potential role of endoplasmic reticulum stress in diabetic heart and potential therapies targeting its modulation.

Effect of Liraglutide on endoplasmic reticulum stress in diabetes

Endoplasmic reticulum (ER) stress is associated with the development of diabetes. The present study sought to investigate the effect of Liraglutide, a glucagon like peptide 1 analogue, on ER stress in β-cells. We found that Liraglutide protected the pancreatic INS-1 cells from thapsigargin-induced ER stress and the ER stress associated cell apoptosis, mainly by suppressing the PERK and IRE1 pathways. We further tested the effects of Liraglutide in the Akita mouse, an ER-stress induced type 1 diabetes model. After administration of Liraglutide for 8weeks, p-eIF2α and p-JNK were significantly decreased in the pancreas of the Akita mouse, while the treatment showed no significant impact on the levels of insulin of INS-cells. Taken together, our findings suggest that Liraglutide may protect pancreatic cells from ER stress and its related cell death.

Musashi expression in β-cells coordinates insulin expression, apoptosis and proliferation in response to endoplasmic reticulum stress in diabetes

Diabetes is associated with the death and dysfunction of insulin-producing pancreatic β-cells. In other systems, Musashi genes regulate cell fate via Notch signaling, which we recently showed regulates β-cell survival. Here we show for the first time that human and mouse adult islet cells express mRNA and protein of both Musashi isoforms, as well Numb/Notch/Hes/neurogenin-3 pathway components. Musashi expression was observed in insulin/glucagon double-positive cells during human fetal development and increased during directed differentiation of human embryonic stem cells (hESCs) to the pancreatic lineage. De-differentiation of β-cells with activin A increased Msi1 expression. Endoplasmic reticulum (ER) stress increased Msi2 and Hes1, while it decreased Ins1 and Ins2 expression, revealing a molecular link between ER stress and β-cell dedifferentiation in type 2 diabetes. These effects were independent of changes in Numb protein levels and Notch activation. Overexpression of MSI1 was sufficient to increase Hes1, stimulate proliferation, inhibit apoptosis and reduce insulin expression, whereas Msi1 knockdown had the converse effects on proliferation and insulin expression. Overexpression of MSI2 resulted in a decrease in MSI1 expression. Taken together, these results demonstrate overlapping, but distinct roles for Musashi-1 and Musashi-2 in the control of insulin expression and β-cell proliferation. Our data also suggest that Musashi is a novel link between ER stress and the compensatory β-cell proliferation and the loss of β-cell gene expression seen in specific phases of the progression to type 2 diabetes.

Endoplasmic reticulum stress in diabetes: New insights of clinical relevance

The endoplasmic reticulum (ER) is a cellular compartment responsible for multiple important cellular functions including the biosynthesis and folding of newly synthesized proteins destined for secretion, such as insulin. A myriad of pathological and physiological factors perturb ER function and cause dysregulation of ER homeostasis, leading to ER stress. Accumulating evidence suggests that ER stress plays a role in the pathogenesis of diabetes, contributing to pancreatic β-cell loss and insulin resistance. ER stress may also link obesity, inflammation and insulin resistance 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 in diabetes and its complications. Special attention was given to elucidate how ER stress could explain some of the 'clinical paradoxes' such as secondary sulfonylurea failure, initial worsening of retinopathy during tight glycemic control, insulin resistance induced by protease inhibitors and other clinically relevant observations.

14-3-3 Protein Protects Against Cardiac Endoplasmic Reticulum Stress (ERS) and ERS-Initiated Apoptosis in Experimental Diabetes

Diabetic cardiomyopathy and nephropathy induce endoplasmic reticulum stress (ERS) and ERS-initiated apoptosis. The primary function of 14-3-3 protein is to inhibit apoptosis, but the roles of this protein in protecting against cardiac ERS and apoptosis in the diabetic heart are largely unknown. In this study, we investigated the in vivo role of 14-3-3 protein in diabetic ERS and apoptosis using streptozotocin (STZ)-induced transgenic mice that showed cardiac-specific expression of a dominant negative (DN) 14-3-3η protein mutant. The expression levels of cardiac glucose-regulated protein (GRP) 78, inositol-requiring enzyme (Ire) 1α, and tumor necrosis factor receptor (TNFR)-associated factor (TRAF) 2 protein were significantly increased in the diabetic DN 14-3-3η mice compared with the diabetic wild-type. Moreover, cardiac apoptosis and the expression of CCAAT / enhancer binding protein homology protein (CHOP), caspase-12, and cleaved caspase-12 protein were significantly increased in the diabetic DN 14-3-3η mice. In conclusion, partial depletion of 14-3-3 protein in the diabetic heart exacerbates cardiac ERS and activates ERS-induced apoptosis pathways, at least in part, through the regulation of CHOP and caspase-12 via the Ire1α/TRAF2 pathway. The enhancement of 14-3-3 protein expression can be used as a novel protective therapy against ERS and ERS-initiated apoptosis in the diabetic heart.

Glucagon-Like Peptide-1 Agonists Protect Pancreatic β-Cells From Lipotoxic Endoplasmic Reticulum Stress Through Upregulation of BiP and JunB

OBJECTIVEChronic exposure of pancreatic β-cells to saturated free fatty acids (FFAs) causes endoplasmic reticulum (ER) stress and apoptosis and may contribute to β-cell loss in type 2 diabetes. Here, we evaluated the molecular mechanisms involved in the protection of β-cells from lipotoxic ER stress by glucagon-like peptide (GLP)-1 agonists utilized in the treatment of type 2 diabetes.RESEARCH DESIGN AND METHODSINS-1E or fluorescence-activated cell sorter–purified primary rat β-cells were exposed to oleate or palmitate with or without the GLP-1 agonist exendin-4 or forskolin. Cyclopiazonic acid was used as a synthetic ER stressor, while the activating transcription factor 4–C/EBP homologous protein branch was selectively activated with salubrinal. The ER stress signaling pathways modulated by GLP-1 agonists were studied by real-time PCR and Western blot. Knockdown by RNA interference was used to identify mediators of the antiapoptotic GLP-1 effects in the ER stress response and downstream mitochondrial cell death mechanisms.RESULTSExendin-4 and forskolin protected β-cells against FFAs via the induction of the ER chaperone BiP and the antiapoptotic protein JunB that mediate β-cell survival under lipotoxic conditions. On the other hand, exendin-4 and forskolin protected against synthetic ER stressors by inactivating caspase 12 and upregulating Bcl-2 and X-chromosome–linked inhibitor of apoptosis protein that inhibit mitochondrial apoptosis.CONCLUSIONSThese observations suggest that GLP-1 agonists increase in a context-dependent way the β-cell defense mechanisms against different pathways involved in ER stress–induced apoptosis. The identification of the pathways modulated by GLP-1 agonists allows for targeted approaches to alleviate β-cell ER stress in diabetes.

Increased stress protein ORP150 autoantibody production in Type 1 diabetic patients

Various genetic and environmental stresses interfere with protein folding in the endoplasmic reticulum (ER), which leads to the induction of ER stress. It has recently been reported that ER stress is involved in the development of diabetes in diabetic animal models. The aim of this study is to estimate ER stress levels in Type 1 diabetic patients. We recruited Type 1 diabetic patients undergoing periodic follow-up examinations (n = 91) and healthy non-diabetic individuals (n = 37), and measured their serum anti-oxygen-related protein (ORP)150 autoantibody levels. Anti-ORP150 autoantibody levels in Type 1 diabetic patients were significantly higher compared with those in healthy non-diabetic subjects. Furthermore, the serum autoantibody levels in Type 1 diabetic patients correlated with HbA(1c) (F > 3.0, P = 0.079), indicating that hyperglycaemia itself induces ER stress in diabetes. Anti-ORP150 autoantibody levels in Type 1 diabetic patients are higher compared with non-diabetic subjects, suggesting that ER stress is increased in Type 1 diabetes.