Pathogenic Mechanism of Diabetes Development Due to Dysfunction of Unfolded Protein Response

Retinal protective effect of curcumin metabolite hexahydrocurcumin against blue light-induced RPE damage

BACKGROUND

Age-related macular degeneration (AMD) is a disease of retinal pigment epithelium (RPE) cells. We have previously demonstrated that blue light can damage RPE cells and their underlying mechanisms. We found that hexahydrocurcumin (HHC), a metabolite of curcumin, had better retinal protection than curcumin. However, the involved mechanisms remain unclear.

METHODS

By exposing ARPE-19 human RPE cells and mouse primary RPE cells to blue light, the intracellular mechanisms of HHC in cells were investigated, including the proliferation of RPE cells and the effects of HHC on activating intracellular protective mechanisms and related factors. Next-generation sequencing (NGS) RNA sequencing revealed the underlying mechanisms involved in the induction and regulation of HHC treatment following blue light exposure.

RESULTS

HHC promoted autophagy by enhancing autophagic flux, reduced oxidative stress and endoplasmic reticulum (ER) stress, and effectively reversed blue light-induced cell death. RNA sequencing-based bioinformatics approaches comprehensively analyze HHC-mediated cellular processes.

CONCLUSION

Our findings elucidate the mechanisms of HHC against blue light damage in RPE cells and are beneficial for the development of natural metabolite-based preventive drugs or functional foods.

Monogenic Diabetes and Integrated Stress Response Genes Display Altered Gene Expression in Type 1 Diabetes

Type 1 diabetes (T1D) has a multifactorial autoimmune etiology, involving environmental prompts and polygenic predisposition. We hypothesized that pancreata from individuals with and at risk for T1D would exhibit dysregulated expression of genes associated with monogenic forms of diabetes caused by nonredundant single-gene mutations. Using a "monogenetic transcriptomic strategy," we measured the expression of these genes in human T1D, autoantibody-positive (autoantibody+), and control pancreas tissues with real-time quantitative PCR in accordance with the Minimum Information for Publication of Quantitative Real-Time PCR Experiments (MIQE) guidelines. Gene and protein expression was visualized in situ with use of immunofluorescence, RNAscope, and confocal microscopy. Two dozen monogenic diabetes genes showed altered expression in human pancreata from individuals with T1D versus unaffected control subjects. Six of these genes also saw dysregulation in pancreata from autoantibody+ individuals at increased risk for T1D. As a subset of these genes are related to cellular stress responses, we measured integrated stress response (ISR) genes and identified 20 with altered expression in T1D pancreata, including three of the four eIF2α-dependent kinases. Equally intriguing, we observed significant repression of the three arms of the ISR in autoantibody+ pancreata. Collectively, these efforts suggest monogenic diabetes and ISR genes are dysregulated early in the T1D disease process and likely contribute to the disorder's pathogenesis.

Role of Endoplasmic Reticulum Stress Sensor IRE1α in Cellular Physiology, Calcium, ROS Signaling, and Metaflammation

Inositol-requiring transmembrane kinase endoribonuclease-1α (IRE1α) is the most prominent and evolutionarily conserved unfolded protein response (UPR) signal transducer during endoplasmic reticulum functional upset (ER stress). A IRE1α signal pathway arbitrates yin and yang of cellular fate in objectionable conditions. It plays several roles in fundamental cellular physiology as well as in several pathological conditions such as diabetes, obesity, inflammation, cancer, neurodegeneration, and in many other diseases. Thus, further understanding of its molecular structure and mechanism of action during different cell insults helps in designing and developing better therapeutic strategies for the above-mentioned chronic diseases. In this review, recent insights into structure and mechanism of activation of IRE1α along with its complex regulating network were discussed in relation to their basic cellular physiological function. Addressing different binding partners that can modulate IRE1α function, UPRosome triggers different downstream pathways depending on the cellular backdrop. Furthermore, IRE1α are in normal cell activities outside the dominion of ER stress and activities under the weather of inflammation, diabetes, and obesity-related metaflammation. Thus, IRE1 as an ER stress sensor needs to be understood from a wider perspective for comprehensive functional meaning, which facilitates us with assembling future needs and therapeutic benefits.

Mechanisms of impaired pancreatic β‑cell function in high‑fat diet‑induced obese mice: The role of endoplasmic reticulum stress

The aim of the study was to examine whether there was excessive endoplasmic reticulum stress (ERs) in the islets of high-fat diet (HFD)-induced obese mice, as well as the effects of ERs on β-cell function. Male C57BL/6J mice were fed a HFD for 16 weeks. Pancreatic β-cell function was evaluated using intraperitoneal glucose tolerance and insulin release tests, and via electron microscopy. The expression of activating transcription factor 6 (ATF6) and phosphorylated (p)-eukaryotic initiation factor 2α (eIF2α) were detected via immunofluorescence staining to determine whether exposure to a HFD induced ERs in pancreatic islets. In vitro, ERs was induced by palmitate (PA) in INS-1 cells, and the protein expression of ATF6, and mRNA expression of ATF6 and insulin were examined via western blot and quantitative PCR (qPCR) analyses, respectively. The nuclear localization of ATF6 was examined by immunofluorescence. Finally, small interfering (si)RNA was used to downregulate ATF6 expression in INS-1 cells to further determine whether ATF6 mediated the ERs-induced impairment of insulin gene transcription. After 16 weeks of induction, the obese mice showed impaired glucose tolerance, insulin resistance and hyperinsulinemia. Immunohistochemistry staining showed increased p-eIF2α and ATF6 expression in pancreatic islets in the obesity group compared with the normal group. Electron microscopy indicated that the microstructures and secretory functions of β-cells were impaired. After 24 h of incubation, ATF mRNA and protein expression in the PA group was significantly higher compared with the control group. However, the insulin mRNA expression in the PA group was significantly decreased. Furthermore, qPCR showed that the insulin mRNA expression was significantly increased 24 h after PA treatment in cells transfected with ATF6-siRNA compared with the negative control group. The present suggested shows that ERs-induced activation of ATF6 may play an important role in the development of β-cell dysfunction in obese mice.

Ginsenosides reduce body weight and ameliorate hepatic steatosis in high fat diet‑induced obese mice via endoplasmic reticulum stress and p‑STAT3/STAT3 signaling

Obesity has been increasing globally for over three decades. According to previous studies, dietary obesity is usually associated with endoplasmic reticulum stress (ERS) and STAT3 signaling, which result in interference with the homeostatic control of energy and lipid metabolism. Ginsenosides (GS) administered to mice will modulate adiposity and food intake; however, the mechanism of food inhibition is unknown. The aim of the present study was to investigate whether GS may inhibit ERS and regulate STAT3 phosphorylation in GT1‑7 cells (a mouse hypothalamus gonadotropin‑releasing hormone neuron cell line) and the hypothalamus in order to reduce the body weight and ameliorate hepatic steatosis in high fat diet (HFD)‑induced obese mice. In the present study, GS inhibited the appetite, reduced the body weight, visceral fat, body fat content and blood glucose, and ameliorated the glucose tolerance of the obese mice compared with HFD mice. In addition, the levels of aspartate aminotransferase and alanine aminotransferase, triglyceride (TG), leptin and insulin in the serum were reduced compared with HFD mice. There was less TG in the liver, but more in the feces compared with HFD mice. Using hematoxylin and eosin staining of HepG2 cells and liver tissues, GS were demonstrated to improve the non‑alcoholic fatty liver of the HFD‑induced obese mice and reduce the diameter of the fat cells compared with HFD mice. GS also increased oxygen consumption and carbon dioxide emissions in the metabolic cage data compared with HFD mice. In the GT1‑7 cells, GS alleviated the ERS induced by tunicamycin and enhanced the activation of the STAT3 phosphorylation pathway. Furthermore the ERS of the liver was relieved to achieve the aforementioned pharmacological effects. GS were used in the homeostatic control of the energy and lipid metabolism of a diet‑induced obesity model. In conclusion, present studies suggest that GS exert these effects by increasing STAT3 phosphorylation expression and reducing the ERS. Thus, GS reduce body weight and ameliorate hepatic steatosis in HFD‑induced obese mice.

What Is the Sweetest UPR Flavor for the β-cell? That Is the Question

Unfolded protein response (UPR) is a process conserved from yeasts to mammals and, based on the generally accepted dogma, helps the secretory performance of a cell, by improving its capacity to cope with a burden in the endoplasmic reticulum (ER). The ER of β-cells, "professional secretory cells", has to manage tremendous amounts of insulin, which elicits a strong pressure on the ER intrinsic folding capacity. Thus, the constant demand for insulin production results in misfolded proinsulin, triggering a physiological upregulation of UPR to restore homeostasis. Most diabetic disorders are characterized by the loss of functional β-cells, and the pathological side of UPR plays an instrumental role. The transition from a homeostatic to a pathological UPR that ultimately leads to insulin-producing β-cell decay entails complex cellular processes and molecular mechanisms which remain poorly described so far. Here, we summarize important processes that are coupled with or driven by UPR in β-cells, such as proliferation, inflammation and dedifferentiation. We conclude that the UPR comes in different "flavors" and each of them is correlated with a specific outcome for the cell, for survival, differentiation, proliferation as well as cell death. All these greatly depend on the way UPR is triggered, however what exactly is the switch that favors the activation of one UPR as opposed to others is largely unknown. Substantial work needs to be done to progress the knowledge in this important emerging field as this will help in the development of novel and more efficient therapies for diabetes.

Misfolded proinsulin in the endoplasmic reticulum during development of beta cell failure in diabetes

The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta cells can synthesize 6000 proinsulin molecules per second. Ordinarily, nascent proinsulin entering the ER rapidly folds via the formation of three evolutionarily conserved disulfide bonds (B7-A7, B19-A20, and A6-A11). A modest amount of proinsulin misfolding, including both intramolecular disulfide mispairing and intermolecular disulfide-linked protein complexes, is a natural by-product of proinsulin biosynthesis, as is the case for many proteins. The steady-state level of misfolded proinsulin-a potential ER stressor-is linked to (1) production rate, (2) ER environment, (3) presence or absence of naturally occurring (mutational) defects in proinsulin, and (4) clearance of misfolded proinsulin molecules. Accumulation of misfolded proinsulin beyond a certain threshold begins to interfere with the normal intracellular transport of bystander proinsulin, leading to diminished insulin production and hyperglycemia, as well as exacerbating ER stress. This is most obvious in mutant INS gene-induced Diabetes of Youth (MIDY; an autosomal dominant disease) but also likely to occur in type 2 diabetes owing to dysregulation in proinsulin synthesis, ER folding environment, or clearance.

Endoplasmic Reticulum (ER) Stress and Endocrine Disorders

The endoplasmic reticulum (ER) is the organelle where secretory and membrane proteins are synthesized and folded. Unfolded proteins that are retained within the ER can cause ER stress. Eukaryotic cells have a defense system called the “unfolded protein response” (UPR), which protects cells from ER stress. Cells undergo apoptosis when ER stress exceeds the capacity of the UPR, which has been revealed to cause human diseases. Although neurodegenerative diseases are well-known ER stress-related diseases, it has been discovered that endocrine diseases are also related to ER stress. In this review, we focus on ER stress-related human endocrine disorders. In addition to diabetes mellitus, which is well characterized, several relatively rare genetic disorders such as familial neurohypophyseal diabetes insipidus (FNDI), Wolfram syndrome, and isolated growth hormone deficiency type II (IGHD2) are discussed in this article.

Endoplasmic reticulum stress in beta cells and autoimmune diabetes

Type 1 diabetes results from the autoimmune destruction of pancreatic β cells, leading to insulin deficiency and hyperglycemia. Although multiple attempts have been made to slow the autoimmune process using immunosuppressive or immunomodulatory agents, there are still no effective treatments that can delay or reverse the progression of type 1 diabetes in humans. Recent studies support endoplasmic reticulum (ER) as a novel target for preventing the initiation of the autoimmune reaction, propagation of inflammation, and β cell death in type 1 diabetes. This review highlights recent findings on ER stress in β cells and development of type 1 diabetes and introduces potential new treatments targeting the ER to combat this disorder.