Epigallocatechin-3-Gallate Inhibits Ethanol-Induced Apoptosis Through Neurod1 Regulating CHOP Expression in Pancreatic β-Cells

(-)-Epigallocatechin 3-gallate protects pancreatic β-cell against excessive autophagy-induced injury through promoting FTO degradation

Excessive macroautophagy/autophagy leads to pancreatic β-cell failure that contributes to the development of diabetes. Our previous study proved that the occurrence of deleterious hyperactive autophagy attributes to glucolipotoxicity-induced NR3C1 activation. Here, we explored the potential protective effects of (-)-epigallocatechin 3-gallate (EGCG) on β-cell-specific NR3C1 overexpression mice in vivo and NR3C1-enhanced β cells in vitro. We showed that EGCG protects pancreatic β cells against NR3C1 enhancement-induced failure through inhibiting excessive autophagy. RNA demethylase FTO (FTO alpha-ketoglutarate dependent dioxygenase) caused diminished m6A modifications on mRNAs of three pro-oxidant genes (Tlr4, Rela, Src) and, hence, oxidative stress occurs; by contrast, EGCG promotes FTO degradation by the ubiquitin-proteasome system in NR3C1-enhanced β cells, which alleviates oxidative stress, and thereby prevents excessive autophagy. Moreover, FTO overexpression abolishes the beneficial effects of EGCG on β cells against NR3C1 enhancement-induced damage. Collectively, our results demonstrate that EGCG protects pancreatic β cells against NR3C1 enhancement-induced excessive autophagy through suppressing FTO-stimulated oxidative stress, which provides novel insights into the mechanisms for the anti-diabetic effect of EGCG.Abbreviation 3-MA: 3-methyladenine; AAV: adeno-associated virus; Ad: adenovirus; ALD: aldosterone; AUC: area under curve; βNR3C1 mice: pancreatic β-cell-specific NR3C1 overexpression mice; Ctrl: control; CHX: cycloheximide; DEX: dexamethasone; DHE: dihydroethidium; EGCG: (-)-epigallocatechin 3-gallate; FTO: FTO alpha-ketoglutarate dependent dioxygenase; GSIS: glucose-stimulated insulin secretion; HFD: high-fat diet; HG: high glucose; i.p.: intraperitoneal; IOD: immunofluorescence optical density; KSIS: potassium-stimulated insulin secretion; m6A: N6-methyladenosine; MeRIP-seq: methylated RNA immunoprecipitation sequencing; NO: nitric oxide; NR3C1/GR: nuclear receptor subfamily 3, group C, member 1; NR3C1-Enhc.: NR3C1-enhancement; NAC: N-acetylcysteine; NC: negative control; PBS: phosphate-buffered saline; PI: propidium iodide; OCR: oxygen consumption rate; Palm.: palmitate; RELA: v-rel reticuloendotheliosis viral oncogene homolog A (avian); RNA-seq: RNA sequencing; O2.-: superoxide anion; SRC: Rous sarcoma oncogene; ROS: reactive oxygen species; T2D: type 2 diabetes; TEM: transmission electron microscopy; TLR4: toll-like receptor 4; TUNEL: terminal dUTP nick-end labeling; UTR: untranslated region; WT: wild-type.

Endoplasmic reticulum stress in pancreatic β-cell dysfunction: The potential therapeutic role of dietary flavonoids

Diabetes mellitus (DM) is a global health burden that is characterized by the loss or dysfunction of pancreatic β-cells. In pancreatic β-cells, endoplasmic reticulum (ER) stress is a fact of life that contributes to β-cell loss or dysfunction. Despite recent advances in research, the existing treatment approaches such as lifestyle modification and use of conventional therapeutics could not prevent the loss or dysfunction of pancreatic β-cells to abrogate the disease progression. Therefore, targeting ER stress and the consequent unfolded protein response (UPR) in pancreatic β-cells may be a potential therapeutic strategy for diabetes treatment. Dietary phytochemicals have therapeutic applications in human health owing to their broad spectrum of biochemical and pharmacological activities. Flavonoids, which are commonly obtained from fruits and vegetables worldwide, have shown promising prospects in alleviating ER stress. Dietary flavonoids including quercetin, kaempferol, myricetin, isorhamnetin, fisetin, icariin, apigenin, apigetrin, vitexin, baicalein, baicalin, nobiletin hesperidin, naringenin, epigallocatechin 3-O-gallate hesperidin (EGCG), tectorigenin, liquiritigenin, and acacetin have shown inhibitory effects on ER stress in pancreatic β-cells. Dietary flavonoids modulate ER stress signaling components, chaperone proteins, transcription factors, oxidative stress, autophagy, apoptosis, and inflammatory responses to exert their pharmacological effects on pancreatic β-cells ER stress. This review focuses on the role of dietary flavonoids as potential therapeutic adjuvants in preserving pancreatic β-cells from ER stress. Highlights of the underlying mechanisms of action are also presented as well as possible strategies for clinical translation in the management of DM.

The synergistic protection of EGCG and quercetin against streptozotocin (STZ)-induced NIT-1 pancreatic β cell damage via upregulation of BCL-2 expression by miR-16-5p

EGCG and quercetin are flavonoids which usually co-exist in edible plants and they exhibit anti-diabetes effects. This study aimed to explore the mechanisms by which quercetin and EGCG synergistically protected pancreatic β-cells from streptozotocin-induced apoptosis. EGCG, quercetin, and their combinations (both 15 μM) all reversed STZ-induced cells damage and enhanced glucose-stimulated insulin secretion, with the combination being more effective than a single compound. At the molecular level, the EGCG-quercetin combination upregulated BCL-2 expression and caused a greater reduction in miR-16-5p level than EGCG alone or quercetin alone. Overexpression of miR-16-5p could offset the down-regulated apoptotic genes caused by the synergistic action of the combination. These findings suggest that EGCG and quercetin exert synergistic anti-diabetes effect, possibly via decreasing the expression of miR-16-5p that targets directly BCL-2. This is the first report on a miRNA-based mechanism underlying the synergistic protective effect of EGCG and quercetin against pancreatic cell damage.

Effects of Polyphenols on Insulin Resistance

Insulin resistance (IR) is apparent when tissues responsible for clearing glucose from the blood, such as adipose and muscle, do not respond properly to appropriate signals. IR is estimated based on fasting blood glucose and insulin, but some measures also incorporate an oral glucose challenge. Certain (poly)phenols, as supplements or in foods, can improve insulin resistance by several mechanisms including lowering postprandial glucose, modulating glucose transport, affecting insulin signalling pathways, and by protecting against damage to insulin-secreting pancreatic β-cells. As shown by intervention studies on volunteers, the most promising candidates for improving insulin resistance are (-)-epicatechin, (-)-epicatechin-containing foods and anthocyanins. It is possible that quercetin and phenolic acids may also be active, but data from intervention studies are mixed. Longer term and especially dose-response studies on mildly insulin resistant participants are required to establish the extent to which (poly)phenols and (poly)phenol-rich foods may improve insulin resistance in compromised groups.

A CRISPR/Cas9 genome editing pipeline in the EndoC-βH1 cell line to study genes implicated in beta cell function

Type 2 diabetes (T2D) is a global pandemic with a strong genetic component, but most causal genes influencing the disease risk remain unknown. It is clear, however, that the pancreatic beta cell is central to T2D pathogenesis. In vitro gene-knockout (KO) models to study T2D risk genes have so far focused on rodent beta cells. However, there are important structural and functional differences between rodent and human beta cell lines. With that in mind, we have developed a robust pipeline to create a stable CRISPR/Cas9 KO in an authentic human beta cell line (EndoC-βH1). The KO pipeline consists of a dual lentiviral sgRNA strategy and we targeted three genes ( INS, IDE, PAM) as a proof of concept. We achieved a significant reduction in mRNA levels and complete protein depletion of all target genes. Using this dual sgRNA strategy, up to 94 kb DNA were cut out of the target genes and the editing efficiency of each sgRNA exceeded >87.5%. Sequencing of off-targets showed no unspecific editing. Most importantly, the pipeline did not affect the glucose-responsive insulin secretion of the cells. Interestingly, comparison of KO cell lines for NEUROD1 and SLC30A8 with siRNA-mediated knockdown (KD) approaches demonstrate phenotypic differences. NEUROD1-KO cells were not viable and displayed elevated markers for ER stress and apoptosis. NEUROD1-KD, however, only had a modest elevation, by 34%, in the pro-apoptotic transcription factor CHOP and a gene expression profile indicative of chronic ER stress without evidence of elevated cell death. On the other hand, SLC30A8-KO cells demonstrated no reduction in K ATP channel gene expression in contrast to siRNA silencing. Overall, this strategy to efficiently create stable KO in the human beta cell line EndoC-βH1 will allow for a better understanding of genes involved in beta cell dysfunction, their underlying functional mechanisms and T2D pathogenesis.

Resveratrol protects against ethanol-induced impairment of insulin secretion in INS-1 cells through SIRT1-UCP2 axis

SIRT1 has been proposed to enhance insulin secretion in β-cell through repressing the expression of uncoupling protein2 (UCP2), but whether ethanol-induced β-cell dysfunction is mediated by the disrupted SIRT1-UCP2 axis remains unknown. This study was conducted to explore the underlying mechanisms by which ethanol resulted in β-cell dysfunction and the potential protective effects of resveratrol in this process. INS-1 cells (rat pancreatic β-cell line) were cultured with ethanol in the presence or absence of resveratrol (2.5, 12.5 μmol/L). The results showed that ethanol exposure reduced glucose-stimulated insulin secretion, ATP production and SIRT1 expression but increased UCP2 expression, while supplementation with resveratrol restored the function of INS-1 cell by upregulating SIRT1 and inhibiting UCP2. Moreover, the critical role of SIRT1-UCP2 axis was further supported by the results that SIRT1 activator SRT1720 reversed ethanol-induced impairment of glucose-stimulated insulin secretion by decreasing UCP2, while SIRT1 inhibitor Ex527 abolished the beneficial effects of resveratrol. Meanwhile, NAD⁺ booster nicotinamide mononucleotide also counteracted the deleterious effects of ethanol by increasing SIRT1, suggesting the regulation of SIRT1-UCP2 axis may be associated with cellular NAD⁺/NADH ratio. In conclusion, our observations imply that ethanol induces impaired insulin secretion from INS-1 cell through disrupting SIRT1-UCP2 axis, while resveratrol may reverse this process by augmenting SIRT1 and inhibiting UCP2.

A CRISPR/Cas9 genome editing pipeline in the EndoC-βH1 cell line to study genes implicated in beta cell function

Type 2 diabetes (T2D) is a global pandemic with a strong genetic component, but most causal genes influencing the disease risk remain unknown. It is clear, however, that the pancreatic beta cell is central to T2D pathogenesis. In vitro gene-knockout (KO) models to study T2D risk genes have so far focused on rodent beta cells. However, there are important structural and functional differences between rodent and human beta cell lines. With that in mind, we have developed a robust pipeline to create a stable CRISPR/Cas9 KO in an authentic human beta cell line (EndoC-βH1). The KO pipeline consists of a dual lentiviral sgRNA strategy and we targeted three genes ( INS, IDE, PAM) as a proof of concept. We achieved a significant reduction in mRNA levels and complete protein depletion of all target genes. Using this dual sgRNA strategy, up to 94 kb DNA were cut out of the target genes and the editing efficiency of each sgRNA exceeded >87.5%. Sequencing of off-targets showed no unspecific editing. Most importantly, the pipeline did not affect the glucose-responsive insulin secretion of the cells. Interestingly, comparison of KO cell lines for NEUROD1 and SLC30A8 with siRNA-mediated knockdown (KD) approaches demonstrate phenotypic differences. NEUROD1-KO cells were not viable and displayed elevated markers for ER stress and apoptosis. NEUROD1-KD, however, only had a modest elevation, by 34%, in the pro-apoptotic transcription factor CHOP and a gene expression profile indicative of chronic ER stress without evidence of elevated cell death. On the other hand, SLC30A8-KO cells demonstrated no reduction in K ATP channel gene expression in contrast to siRNA silencing. Overall, this strategy to efficiently create stable KO in the human beta cell line EndoC-βH1 will allow for a better understanding of genes involved in beta cell dysfunction, their underlying functional mechanisms and T2D pathogenesis.