Involvement of oxidative stress-induced DNA damage, endoplasmic reticulum stress, and autophagy deficits in the decline of β-cell mass in Japanese type 2 diabetic patients

Pathological evaluation of the pathogenesis of diabetes mellitus and diabetic peripheral neuropathy

Currently, there are more than 10 million patients with diabetes mellitus in Japan. Therefore, the need to explore the pathogenesis of diabetes and the complications leading to its cure is becoming increasingly urgent. Pathological examination of pancreatic tissues from patients with type 2 diabetes reveals a decrease in the volume of beta cells because of a combination of various stresses. In human type 2 diabetes, islet amyloid deposition is a unique pathological change characterized by proinflammatory macrophage (M1) infiltration into the islets. The pathological changes in the pancreas with islet amyloid were different according to clinical factors, which suggests that type 2 diabetes can be further subclassified based on islet pathology. On the other hand, diabetic peripheral neuropathy is the most frequent diabetic complication. In early diabetic peripheral neuropathy, M1 infiltration in the sciatic nerve evokes oxidative stress or attenuates retrograde axonal transport, as clearly demonstrated by in vitro live imaging. Furthermore, islet parasympathetic nerve density and beta cell volume were inversely correlated in type 2 diabetic Goto-Kakizaki rats, suggesting that diabetic peripheral neuropathy itself may contribute to the decrease in beta cell volume. These findings suggest that the pathogenesis of diabetes mellitus and diabetic peripheral neuropathy may be interrelated.

Autophagy alterations in obesity, type 2 diabetes, and metabolic dysfunction-associated steatotic liver disease: the evidence from human studies

Autophagy is an evolutionarily conserved process that plays a pivotal role in the maintenance of cellular homeostasis and its impairment has been implicated in the pathogenesis of various metabolic diseases including obesity, type 2 diabetes (T2D), and metabolic dysfunction-associated steatotic liver disease (MASLD). This review synthesizes the current evidence from human studies on autophagy alterations under these metabolic conditions. In obesity, most data point to autophagy upregulation during the initiation phase of autophagosome formation, potentially in response to proinflammatory conditions in the adipose tissue. Autophagosome formation appears to be enhanced under hyperglycemic or insulin-resistant conditions in patients with T2D, possibly acting as a compensatory mechanism to eliminate damaged organelles and proteins. Other studies have proposed that prolonged hyperglycemia and disrupted insulin signaling hinder autophagic flux, resulting in the accumulation of dysfunctional cellular components that can contribute to β-cell dysfunction. Evidence from patients with MASLD supports autophagy inhibition in disease progression. Nevertheless, given the available data, it is difficult to ascertain whether autophagy is enhanced or suppressed in these conditions because the levels of autophagy markers depend on the overall metabolism of specific organs, tissues, experimental conditions, or disease duration. Owing to these constraints, determining whether the observed shifts in autophagic activity precede or result from metabolic diseases remains challenging. Additionally, autophagy-modulating strategies are shortly discussed. To conclude, more studies investigating autophagy impairment are required to gain a more comprehensive understanding of its role in the pathogenesis of obesity, T2D, and MASLD and to unveil novel therapeutic strategies for these conditions.

Developmental programming: An exploratory analysis of pancreatic islet compromise in female sheep resulting from gestational BPA exposure

Developmental exposure to endocrine disruptors like bisphenol A (BPA) are implicated in later-life metabolic dysfunction. Leveraging a unique sheep model of developmental programming, we conducted an exploratory analysis of the programming effects of BPA on the endocrine pancreas. Pregnant ewes were administered environmentally relevant doses of BPA during gestational days (GD) 30-90, and pancreata from female fetuses and adult offspring were analyzed. Prenatal BPA exposure induced a trend toward decreased islet insulin staining and β-cell count, increased glucagon staining and α-cell count, and increased α-cell/β-cell ratio. Findings were most consistent in fetal pancreata assessed at GD90 and in adult offspring exposed to the lowest BPA dose. While not assessed in fetuses, adult islet fibrosis was increased. Collectively, these data provide further evidence that early-life BPA exposure is a likely threat to human metabolic health. Future studies should corroborate these findings and decipher the molecular mechanisms of BPA's developmental endocrine toxicity.

Dysregulation of pancreatic β-cell autophagy and the risk of type 2 diabetes

Macroautophagy/autophagy is an essential degradation process that removes abnormal cellular components, maintains homeostasis within cells, and provides nutrition during starvation. Activated autophagy enhances cell survival during stressful conditions, although overactivation of autophagy triggers induction of autophagic cell death. Therefore, early-onset autophagy promotes cell survival whereas late-onset autophagy provokes programmed cell death, which can prevent disease progression. Moreover, autophagy regulates pancreatic β-cell functions by different mechanisms, although the precise role of autophagy in type 2 diabetes (T2D) is not completely understood. Consequently, this mini-review discusses the protective and harmful roles of autophagy in the pancreatic β cell and in the pathophysiology of T2D.

The Effect of Calcium Ions on hIAPP Channel Activity: Possible Implications in T2DM

The calcium ion (Ca2+) has been linked to type 2 diabetes mellitus (T2DM), although the role of Ca2+ in this disorder is the subject of intense investigation. Serum Ca2+ dyshomeostasis is associated with the development of insulin resistance, reduced insulin sensitivity, and impaired glucose tolerance. However, the molecular mechanisms involving Ca2+ ions in pancreatic β-cell loss and subsequently in T2DM remain poorly understood. Implicated in the decline in β-cell functions are aggregates of human islet amyloid polypeptide (hIAPP), a small peptide secreted by β-cells that shows a strong tendency to self-aggregate into β-sheet-rich aggregates that evolve toward the formation of amyloid deposits and mature fibrils. The soluble oligomers of hIAPP can permeabilize the cell membrane by interacting with bilayer lipids. Our study aimed to evaluate the effect of Ca2+ on the ability of the peptide to incorporate and form ion channels in zwitterionic planar lipid membranes (PLMs) composed of palmitoyl-oleoyl-phosphatidylcholine (POPC) and on the aggregation process of hIAPP molecules in solution. Our results may help to clarify the link between Ca2+ ions, hIAPP peptide, and consequently the pathophysiology of T2DM.

Heterogeneity of Islet Cells during Embryogenesis and Differentiation

Diabetes is caused by insufficient insulin secretion due to β-cell dysfunction and/or β-cell loss. Therefore, the restoration of functional β-cells by the induction of β-cell differentiation from embryonic stem (ES) and induced-pluripotent stem (iPS) cells, or from somatic non-β-cells, may be a promising curative therapy. To establish an efficient and feasible method for generating functional insulin-producing cells, comprehensive knowledge of pancreas development and β-cell differentiation, including the mechanisms driving cell fate decisions and endocrine cell maturation is crucial. Recent advances in single-cell RNA sequencing (scRNA-seq) technologies have opened a new era in pancreas development and diabetes research, leading to clarification of the detailed transcriptomes of individual insulin-producing cells. Such extensive high-resolution data enables the inference of developmental trajectories during cell transitions and gene regulatory networks. Additionally, advancements in stem cell research have not only enabled their immediate clinical application, but also has made it possible to observe the genetic dynamics of human cell development and maturation in a dish. In this review, we provide an overview of the heterogeneity of islet cells during embryogenesis and differentiation as demonstrated by scRNA-seq studies on the developing and adult pancreata, with implications for the future application of regenerative medicine for diabetes.

Efficacy of IAPP suppression in mouse and human islets by GLP-1 analogue conjugated antisense oligonucleotide

Insulin resistance is the major risk factor for Type 2 diabetes (T2D). In vulnerable individuals, insulin resistance induces a progressive loss of insulin secretion with islet pathology revealing a partial deficit of beta cells and islet amyloid derived from islet amyloid polypeptide (IAPP). IAPP is co-expressed and secreted with insulin by beta cells, expression of both proteins being upregulated in response to insulin resistance. If IAPP expression exceeds the threshold for clearance of misfolded proteins, beta cell failure occurs exacerbated by the action of IAPP toxicity to compromise the autophagy lysosomal pathway. We postulated that suppression of IAPP expression by an IAPP antisense oligonucleotide delivered to beta cells by the GLP-1 agonist exenatide (eGLP1-IAPP-ASO) is a potential disease modifying therapy for T2D. While eGLP1-IAPP-ASO suppressed mouse IAPP and transgenic human IAPP expression in mouse islets, it had no discernable effects on IAPP expression in human islets under the conditions studied. Suppression of transgenic human IAPP expression in mouse islets attenuated disruption of the autophagy lysosomal pathway in beta cells, supporting the potential of this strategy.

Enhancement of glycolysis-dependent DNA repair regulated by FOXO1 knockdown via PFKFB3 attenuates hyperglycemia-induced endothelial oxidative stress injury

The accumulation of DNA damage induced by oxidative stress is a crucial pathogenic factor of endothelial loss in diabetic vascular complications, but it is still unknown whether aberrant glucose metabolism leads to defective DNA repair and accounts for hyperglycemia-induced endothelial oxidative stress injury. Here, we showed that Foxo1 knockdown alleviated diabetes-associated retinal DNA damage and vascular dysfunction. Mechanistically, FOXO1 knockdown avoided persistent DNA damage and cellular senescence under high glucose in endothelial cells by promoting DNA repair mediated by the MRN (MRE11-RAD50-NBS1 complex)-ATM pathway in response to oxidative stress injury. Moreover, FOXO1 knockdown mediated robust DNA repair by restoring glycolysis capacity under high glucose. During this process, the key glycolytic enzyme PFKFB3 was stimulated and, in addition to its promoting effect on glycolysis, directly participated in DNA repair. Under genotoxic stress, PFKFB3 relocated into oxidative stress-induced DNA damage sites and promoted DNA repair by interaction with the MRN-ATM pathway. Our study proposed that defective glycolysis-dependent DNA repair is present in diabetic endothelial cells and contributes to hyperglycemia-induced vascular dysfunction, which could provide novel therapeutic targets for diabetic vascular complications.

Regulation of glucose transporter-4 intervention with S. saman leaves extract in streptozotocin-induced diabetic rats

Diabetes mellitus is the leading disorder and affects more than millions of people worldwide. Nowadays, the usage of herbal drugs is said to control adiposity and hyperglycemia. The current research investigated the anti-adiposity and antidiabetic activity of S. saman leaf extract and bioactive compounds. Therefore, the results lower the sugar absorption into the blood and reveal the extract's antidiabetic properties. STZ-induced diabetic rats, Samanea saman methanolic extract show improvement in the parameters like fasting blood glucose levels, body weight, other biochemical parameters supported by the histopathological analysis, and an increase in serum levels in the experimental groups. The antioxidant plays a vital role by increasing SOD and catalase activity levels and decreasing lipid peroxidation levels. The methanolic extract protects the tissue from oxidation stress, which is responsible for the glycemic properties. According to the findings, diabetic-treated rats had overnight blood glucose levels lower and near standard biochemical markers. Histopathology of the liver, pancreas, kidneys, and adipose tissues supported the pharmacological observations. Further, we screened and documented S. saman extract used for in vitro and in vivo methods. In terms of effectiveness, the crude extracts exhibit 0.8-fold GLUT4 down-regulation. Consequently, this result contributes to clinical trials and develops antidiabetic therapy as a substitute for synthetic pharmaceuticals.

Gsk-3-Mediated Proteasomal Degradation of ATF4 Is a Proapoptotic Mechanism in Mouse Pancreatic β-Cells

Endoplasmic reticulum (ER) stress is a key pathogenic factor in type 1 and 2 diabetes. Glycogen synthase kinase 3 (Gsk-3) contributes to β-cell loss in mice. However, the mechanism by which Gsk-3 leads β-cell death remains unclear. ER stress was pharmacologically induced in mouse primary islets and insulinoma cells. We used insulinoma cells derived from Akita mice as a model of genetic ER stress. Gsk-3 activity was blocked by treating with Gsk-3 inhibitors or by introducing catalytically inactive Gsk-3β. Gsk-3 inhibition prevented proteasomal degradation of activating transcriptional factor 4 (ATF4) and alleviated apoptosis. We found that ATF4-S214 was phosphorylated by Gsk-3, and that this was required for a binding of ATF4 with βTrCP, which mediates polyubiquitination. The anti-apoptotic effect of Gsk-3 inhibition was attenuated by introducing DN-ATF4 or by knockdown of ATF4. Mechanistically, Gsk-3 inhibition modulated transcription targets of ATF4 and in turn facilitated dephosphorylation of eIF2α, altering the protein translational dynamism under ER stress. These observations were reproduced in the Akita mouse-derived cells. Thus, these results reveal the role of Gsk-3 in the regulation of the integrated stress response, and provide a rationale for inhibiting this enzyme to prevent β-cell death under ER stress conditions.

Effects of Sodium-Glucose Co-Transporter-2 Inhibitors on Pancreatic β-Cell Mass and Function

Sodium-glucose co-transporter-2 inhibitors (SGLT2is) not only have antihyperglycemic effects and are associated with a low risk of hypoglycemia but also have protective effects in organs, including the heart and kidneys. The pathophysiology of diabetes involves chronic hyperglycemia, which causes excessive demands on pancreatic β-cells, ultimately leading to decreases in β-cell mass and function. Because SGLT2is ameliorate hyperglycemia without acting directly on β-cells, they are thought to prevent β-cell failure by reducing glucose overload in this cell type. Several studies have shown that treatment with an SGLT2i increases β-cell proliferation and/or reduces β-cell apoptosis, resulting in the preservation of β-cell mass in animal models of diabetes. In addition, many clinical trials have shown that that SGLT2is improve β-cell function in individuals with type 2 diabetes. In this review, the preclinical and clinical data regarding the effects of SGLT2is on pancreatic β-cell mass and function are summarized and the protective effect of SGLT2is in β-cells is discussed.

Genetic ablation of p62/SQSTM1 demonstrates little effect on pancreatic β-cell function under autophagy deficiency

Autophagy is known to play an essential role in intracellular quality control through the degradation of damaged organelles and components. We previously demonstrated that β-cell-specific autophagy deficient mice, which lack Atg7, exhibited impaired glucose tolerance, accompanied by the accumulation of sequestosome 1/p62 (hereafter referred to as p62). Whereas p62 has been reported to play essential roles in regulating cellular homeostasis in the liver and adipose tissue, we previously showed that β-cell-specific p62 deficiency does not cause any apparent impairment in glucose metabolism. In the present study, we investigated the roles of p62 in β cells under autophagy-deficient conditions, by simultaneously inactivating both Atg7 and p62 in a β-cell specific manner. Whereas p62 accumulation was substantially reduced in the islets of Atg7 and p62 double-deficient mice, glucose tolerance and insulin secretion were comparable to Atg7 single-deficient mice. Taken together, these findings suggest that the p62 accumulation appears to have little effect on β-cell function under conditions of autophagy inhibition.

Cumulative autophagy insufficiency in mice leads to progression of β-cell failure

Autophagy is known to play a pivotal role in β-cell function. While the lifelong inhibition of autophagy through Atg7 deletion in β cells has been demonstrated to lead to impaired glucose tolerance together with β-cell dysfunction, the temporal association between autophagy inhibition and β-cell dysfunction remains unclear. To address such questions, inducible β-cell-specific Atg7-knockout (iβAtg7^KO) mice were generated, and autophagy inhibition was induced for two different time durations. Whereas 2 weeks of Atg7 ablation was sufficient to induce autophagy deficiency, confirmed by the accumulation of p62, iβAtg7^KO mice exhibited normal glucose tolerance. In contrast, prolonged autophagy deficiency for 6 weeks resulted in glucose intolerance together with impaired insulin secretion. Direct mRNA sequencing and pathway analysis revealed that the gene set associated with insulin secretion was downregulated only after the 6-week prolonged autophagy inhibition. Furthermore, we identified a novel gene, Sprr1a, which was expressed at more than 50-fold higher levels during both the 2-week and 6-week autophagy inhibition. These findings suggest that autophagy insufficiency cumulatively leads to β-cell failure after a certain interval, accompanied by stepwise alterations of gene expression patterns.

The Association and Mediating Biomarkers of Serum Retinol in Influencing the Development of Type 2 Diabetes: A Prospective Cohort Study in Middle-Aged and Elderly Population

The aims of this research are to elucidate whether serum retinol is associated with type 2 diabetes and to explore the underlying mechanisms of the association in a prospective cohort study. A total of 3,526 diabetes-free participants aged 40 years or older were enrolled at baseline in 2010–2012. Multivariable logistic regression was adopted to evaluate the associations of serum retinol and dietary vitamin A (VA) intake with type 2 diabetes. Mediation analyses were used to reveal potential mediators in their associations. After a mean follow-up of 5.3 years, 280 incident cases of type 2 diabetes occurred. Serum retinol was positively associated with the incidence of type 2 diabetes. The multivariable-adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for type 2 diabetes from the bottom to the top quintile of serum retinol were 1, 1.878 (1.202, 2.936), 2.110 (1.364, 3.263), 1.614 (1.027, 2.538), and 2.134 (1.377, 3.306) (p-trend = 0.009), respectively. Mediation analysis showed that increased homeostasis model assessment - insulin resistance HOMA-IR, triglycerides (TG), and serum xanthine oxidase (XO) activity could account for 8.5, 14.7, and 12.1% of the total effects of serum retinol on type 2 diabetes, respectively. Serum retinol concentration was not significantly associated with dietary VA intake (r = −0.010, p = 0.570). In addition, no significant relationship was observed between dietary VA intake and the risk of type 2 diabetes. Overall, elevated serum retinol might increase the risk of type 2 diabetes which is mainly mediated by increased insulin resistance, TG, or serum XO activity.

Islet microangiopathy and augmented β-cell loss in Japanese non-obese type 2 diabetes patients who died of acute myocardial infarction

AIMS/INTRODUCTION

Islets have microvessels that might develop pathological alterations similar to microangiopathy in type 2 diabetes patients. It remains unclear, however, whether the changes correlate with endocrine cell deficits or whether the presence of macroangiopathy influences the islet microvasculature in Japanese type 2 diabetes patients. In this study, we characterized changes of the islet microvessels and endocrine cells in Japanese non-obese patients with type 2 diabetes who died of acute myocardial infarction (AMI).

MATERIALS AND METHODS

Clinical profiles and islet pathology were examined for 35 diabetes patients who died of AMI (DM + AMI) and 13 diabetes patients who were free from AMI (DM). A total of 13 age-matched, individuals without diabetes who died of AMI and 16 individuals without diabetes who were free from AMI were also studied. Pancreata were subjected to morphometric evaluation of islets, including microvascular alterations of immunostained sections.

RESULTS

Body mass index in DM + AMI was comparable to those in DM. Compared with DM, DM + AMI showed greater glycated hemoglobin levels, higher prevalence of renal failure, hypertension, smaller β-cell volume density and greater amyloid area. DM + AMI showed an increased microvascular area and density compared with other groups. There was a significant increase in vascular basement membrane thickness and loss of pericytes in DM and DM + AMI compared with individuals without diabetes in each group, and the extent of thickening was correlated with the amyloid area and occurrence of β-cell loss in DM + AMI.

CONCLUSIONS

Islet microangiopathy was associated with augmented β-cell loss and amyloid deposition in non-obese Japanese type 2 diabetes patients who died of AMI.

Urolithin A suppresses glucolipotoxicity-induced ER stress and TXNIP/NLRP3/IL-1β inflammation signal in pancreatic β cells by regulating AMPK and autophagy

BACKGROUND

Pancreatic inflammation plays a key role in diabetes pathogenesis and progression. Urolithin A (UA), an intestinal flora metabolite of pomegranate, has anti-diabetic, anti-inflammatory and kidney protection effects among others. However, its effects on pancreatic inflammation and the potential mechanisms have not been clearly established.

PURPOSE

This study aimed at investigating the molecular mechanisms of UA anti-pancreatic inflammation under a diabetic environment.

METHODS

Diabetes induction in male C57BL/6 mice was achieved by a high fat diet and intraperitoneal streptozotocin injections. Then, diabetic mice were orally administered with UA for 8 weeks. In vitro, endoplasmic reticulum stress and MIN6 pancreatic β cell inflammation were induced using 25 mM glucose and 0.5 mM palmitic acid. The effects of UA were evaluated by immunohistochemistry, Western blot, and enzyme linked immunosorbent assays. Finally, the underlying mechanisms were elucidated using an autophagy inhibitor (chloroquine, CQ) and an AMPK inhibitor (dorsomorphin dihydrochloride).

RESULTS

UA significantly inhibited IL-1β secretion and TXNIP/NLRP3 expression in the pancreas of diabetic mice and in MIN6 pancreatic cells. UA downregulated the ER stress protein, p-PERK, and promoted AMPK phosphorylation. UA activated autophagy to inhibit TXNIP/NLRP3 IL-1β inflammatory signal, an effect that was reversed by CQ. Dorsomorphin 2HCL, reversed the autophagy-activation and anti-inflammatory effects of UA. Verapamil, clinically applied as an antiarrhythmic drug, is a TXNIP inhibitor for prevention of beta cell loss and diabetes development, but limited by its cardiac toxicity. In this study, verapamil (as positive control) inhibited NLRP3 /IL-1β signaling in MIN6 cells. Inhibitory effects of UA on TXNIP and IL-1β were weaker than those of verapamil (both at 50 μM, p < 0.05, p < 0.01). Conversely, inhibitory effects of UA on p62 were stronger, relative to those of verapamil (p < 0.05), and there were no differences in AMPK activation and LC3 enhancement effects between UA and verapamil.

CONCLUSION

UA is a potential anti-pancreatic inflammation agent that activates AMPK and autophagy to inhibit endoplasmic reticulum stress associated TXNIP/NLRP3/IL-1β signal pathway.

Diabetes in Humans Activates Pancreatic Stellate Cells via RAGE in Pancreatic Ductal Adenocarcinoma

Pancreatic stellate cells (PSCs) mainly consist of cancer-associating fibroblasts in pancreatic ductal adenocarcinoma (PDAC). The receptor for advanced glycation end products (RAGE) is implicated in the pathophysiology of diabetic complications. Here, we studied the implication of RAGE in PSC activation in PDAC. The activation of cultured mouse PSCs was evaluated by qPCR. The induction of epithelial mesenchymal transition (EMT) in PDAC cell lines was assessed under stimulation with culture supernatant from activated PSCs. A total of 155 surgically resected PDAC subjects (83 nondiabetic, 18 with ≦3-years and 54 with >3-years history of diabetes) were clinicopathologically evaluated. A high-fat diet increased the expression of activated markers in cultured PSCs, which was abrogated by RAGE deletion. Culture supernatant from activated PSCs facilitated EMT of PDAC cells with elevation of TGF-β and IL-6, but not from RAGE-deleted PSCs. Diabetic subjects complicated with metabolic syndrome, divided by cluster analysis, showed higher PSC activation and RAGE expression. In such groups, PDAC cells exhibited an EMT nature. The complication of metabolic syndrome with diabetes significantly worsened disease-free survival of PDAC subjects. Thus, RAGE in PSCs can be viewed as a new promoter and a future therapeutic target of PDAC in diabetic subjects with metabolic syndrome.

Reactive Oxygen Species Mediate 6c-Induced Mitochondrial and Lysosomal Dysfunction, Autophagic Cell Death, and DNA Damage in Hepatocellular Carcinoma

Increasing the level of reactive oxygen species (ROS) in cancer cells has been suggested as a viable approach to cancer therapy. Our previous study has demonstrated that mitochondria-targeted flavone-naphthalimide-polyamine conjugate 6c elevates the level of ROS in cancer cells. However, the detailed role of ROS in 6c-treated cancer cells is not clearly stated. The biological effects and in-depth mechanisms of 6c in cancer cells need to be further investigated. In this study, we confirmed that mitochondria are the main source of 6c-induced ROS, as demonstrated by an increase in 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and MitoSox fluorescence. Compound 6c-induced mitochondrial ROS caused mitochondrial dysfunction and lysosomal destabilization confirmed by absolute quantitation (iTRAQ)-based comparative proteomics. Compound 6c-induced metabolic pathway dysfunction and lysosomal destabilization was attenuated by N-acetyl-L-cysteine (NAC). iTRAQ-based comparative proteomics showed that ROS regulated the expression of 6c-mediated proteins, and treatment with 6c promoted the formation of autophagosomes depending on ROS. Compound 6c-induced DNA damage was characterized by comet assay, p53 phosphorylation, and γH2A.X, which was diminished by pretreatment with NAC. Compound 6c-induced cell death was partially reversed by 3-methyladenine (3-MA), bafilomycin (BAF) A1, and NAC, respectively. Taken together, the data obtained in our study highlighted the involvement of mitochondrial ROS in 6c-induced autophagic cell death, mitochondrial and lysosomal dysfunction, and DNA damage.

Revisiting Regulators of Human β-cell Mass to Achieve β-cell-centric Approach Toward Type 2 Diabetes

Type 2 diabetes (T2DM) is characterized by insulin resistance and β-cell dysfunction. Because patients with T2DM have inadequate β-cell mass (BCM) and β-cell dysfunction worsens glycemic control and makes treatment difficult, therapeutic strategies to preserve and restore BCM are needed. In rodent models, obesity increases BCM about 3-fold, but the increase in BCM in humans is limited. Besides, obesity-induced changes in BCM may show racial differences between East Asians and Caucasians. Recently, the developmental origins of health and disease hypothesis, which states that the risk of developing noncommunicable diseases including T2DM is influenced by the fetal environment, has been proposed. It is known in rodents that animals with low birthweight have reduced BCM through epigenetic modifications, making them more susceptible to diabetes in the future. Similarly, in humans, we revealed that individuals born with low birthweight have lower BCM in adulthood. Because β-cell replication is more frequently observed in the 5 years after birth, and β cells are found to be more plastic in that period, a history of childhood obesity increases BCM. BCM in patients with T2DM is reduced by 20% to 65% compared with that in individuals without T2DM. However, since BCM starts to decrease from the stage of borderline diabetes, early intervention is essential for β-cell protection. In this review, we summarize the current knowledge on regulatory factors of human BCM in health and diabetes and propose the β-cell–centric concept of diabetes to enhance a more pathophysiology-based treatment approach for T2DM.

Diversity of pathophysiology in type 2 diabetes shown by islet pathology

The etiology of type 2 diabetes is multifactorial, in which environmental and genetic factors are involved to varying degrees. This suggests that its pathophysiology might vary depending on the individuals. Knowledge of the differences is critical, because these differences are directly linked to the care and treatment of the patients. Recent studies have attempted to carry out subclassifications of type 2 diabetes based on clinical and genetic differences. However, there is no pathological evidence to support these subclassifications. The pathophysiology of type 2 diabetes is generally divided into insulin resistance in peripheral tissues and pancreatic islet dysfunction. Among them, islet dysfunction causes a deficit in insulin secretion from β-cells. In particular, a deficit in insulin secretion is ascribed to a combination of disruption of the insulin secretory machinery and a decrease in β-cell volume in type 2 diabetes. Recent research has suggested that transdifferentiation and dedifferentiation are involved in the decrease in β-cell volume, and that it might change dynamically depending on the glucose metabolic state. However, it is possible that the numbers of islet cells are decreased in type 2 diabetes. In particular, the loss of endocrine cells due to islet amyloid deposits is an important pathological change in type 2 diabetes in humans. These results show that pathological changes of the islets can be different in each individuals with type 2 diabetes and reflect each pathophysiology, which is useful in establishing further subclassifications and developing tailor-made therapies for type 2 diabetes.

The Role of the α Cell in the Pathogenesis of Diabetes: A World beyond the Mirror

Type 2 Diabetes Mellitus (T2DM) is one of the most prevalent chronic metabolic disorders, and insulin has been placed at the epicentre of its pathophysiological basis. However, the involvement of impaired alpha (α) cell function has been recognized as playing an essential role in several diseases, since hyperglucagonemia has been evidenced in both Type 1 and T2DM. This phenomenon has been attributed to intra-islet defects, like modifications in pancreatic α cell mass or dysfunction in glucagon's secretion. Emerging evidence has shown that chronic hyperglycaemia provokes changes in the Langerhans' islets cytoarchitecture, including α cell hyperplasia, pancreatic beta (β) cell dedifferentiation into glucagon-positive producing cells, and loss of paracrine and endocrine regulation due to β cell mass loss. Other abnormalities like α cell insulin resistance, sensor machinery dysfunction, or paradoxical ATP-sensitive potassium channels (KATP) opening have also been linked to glucagon hypersecretion. Recent clinical trials in phases 1 or 2 have shown new molecules with glucagon-antagonist properties with considerable effectiveness and acceptable safety profiles. Glucagon-like peptide-1 (GLP-1) agonists and Dipeptidyl Peptidase-4 inhibitors (DPP-4 inhibitors) have been shown to decrease glucagon secretion in T2DM, and their possible therapeutic role in T1DM means they are attractive as an insulin-adjuvant therapy.

MEK/ERK Signaling in β-Cells Bifunctionally Regulates β-Cell Mass and Glucose-Stimulated Insulin Secretion Response to Maintain Glucose Homeostasis

In diabetic pathology, insufficiency in β-cell mass, unable to meet peripheral insulin demand, and functional defects of individual β-cells in production of insulin are often concurrently observed, collectively causing hyperglycemia. Here we show that the phosphorylation of ERK1/2 is significantly decreased in the islets of db/db mice as well as in those of a cohort of subjects with type 2 diabetes. In mice with abrogation of ERK signaling in pancreatic β-cells through deletion of Mek1 and Mek2, glucose intolerance aggravates under high-fat diet-feeding conditions due to insufficient insulin production with lower β-cell proliferation and reduced β-cell mass, while in individual β-cells dampening of the number of insulin exocytosis events is observed, with the molecules involved in insulin exocytosis being less phosphorylated. These data reveal bifunctional roles for MEK/ERK signaling in β-cells for glucose homeostasis, i.e., in regulating β-cell mass as well as in controlling insulin exocytosis in individual β-cells, thus providing not only a novel perspective for the understanding of diabetes pathophysiology but also a potential clue for new drug development for diabetes treatment.

Lactation improves pancreatic β cell mass and function through serotonin production

Pregnancy imposes a substantial metabolic burden on women through weight gain and insulin resistance. Lactation reduces the risk of maternal postpartum diabetes, but the mechanisms underlying this benefit are unknown. Here, we identified long-term beneficial effects of lactation on β cell function, which last for years after the cessation of lactation. We analyzed metabolic phenotypes including β cell characteristics in lactating and non-lactating humans and mice. Lactating and non-lactating women showed comparable glucose tolerance at 2 months after delivery, but after a mean of 3.6 years, glucose tolerance in lactated women had improved compared to non-lactated women. In humans, the disposition index, a measure of insulin secretory function of β cells considering the degree of insulin sensitivity, was higher in lactated women at 3.6 years after delivery. In mice, lactation improved glucose tolerance and increased β cell mass at 3 weeks after delivery. Amelioration of glucose tolerance and insulin secretion were maintained up to 4 months after delivery in lactated mice. During lactation, prolactin induced serotonin production in β cells. Secreted serotonin stimulated β cell proliferation through serotonin receptor 2B in an autocrine and paracrine manner. In addition, intracellular serotonin acted as an antioxidant to mitigate oxidative stress and improved β cell survival. Together, our results suggest that serotonin mediates the long-term beneficial effects of lactation on female metabolic health by increasing β cell proliferation and reducing oxidative stress in β cells.

Urolithin A, a pomegranate metabolite, protects pancreatic β cells from apoptosis by activating autophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Urolithin A is an active metabolite of plant polyphenol ellagic acid generated by intestinal flora, which is derived from strawberry or traditional anti-diabetic Chinese medicine such as Punica granatum L. and Phyllanthus emblica. The present study aimed to whether urolithin A can protect against glycolipid-toxicity-induced apoptosis of pancreatic β-cells and the underlying mechanisms.

MATERIALS AND METHODS

Apoptosis was induced in the pancreas of mice with type 2 diabetes and MIN6 pancreatic β-cells. CC-8 assay was conducted to determine cell viability. Flow cytometry, JC-1 fluorescent probe, and western blot assays were performed to assess apoptosis. Immunofluorescence and western blot assays were used to detect changes in autophagy. The mechanism of apoptosis was elucidated using autophagy inhibitor chloroquine.

RESULTS

Urolithin A intervention significantly reduced pancreatic cell apoptosis in diabetic mice and MIN6 β cells. This was achieved by the downregulation of cleaved-caspase 3, cleaved-caspase 1, and restoration of cell viability, cell morphology and mitochondrial membrane potential, accompanied with the downregulation of autophagic protein SQSTM1/p62 and upregulation of LC3II. Chloroquine, an autophagy inhibitor, reversed the anti-glucolipotoxic and anti-apoptotic effects of urolithin A.

CONCLUSION

These findings suggest that urolithin A protects against glucolipotoxicity-induced apoptosis in pancreatic β-cells by inducing activation of autophagy.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

The potential roles of chitosan oligosaccharide in prevention of kidney injury in obese and diabetic conditions

Obesity is closely associated with insulin resistance (IR). The most likely links between the two are obesity-mediated systemic low-grade chronic inflammation, endoplasmic reticulum stress and mitochondrial dysfunction, which are all known to contribute to the development of type 2 diabetes (T2DM) and eventually diabetic nephropathy (DN). Chitosan oligosaccharide (COS) is an oligomer of chitosan prepared by the deacetylation of chitin commonly found in exoskeletons of crustaceans such as shrimp and crab as well as the cell walls of fungi. COS has various biological effects including lipid lowering, anti-inflammation, anti-diabetes, and anti-oxidant effects. Therefore, COS is a potential new therapeutic agent for treatment of the obesity-induced DN condition. It is an abundant natural polymer and therefore freely available. This review includes information regarding the relationship between obesity, IR, T2DM, and DN as well as the potential usefulness of COS in controlling lipid and cholesterol metabolism, T2DM and kidney injury models in both in vivo and in vitro studies. However, evidence is limited regarding the effect of COS on the DN model. Further studies, especially in obesity-induced DN, are needed to support the mechanisms proposed in this review.

Increased insulin sensitivity and diminished pancreatic beta-cell function in DNA repair deficient Ercc1d/- mice

BACKGROUND

Type 2 diabetes (T2DM) is an age-associated disease characterized by hyperglycemia due to insulin resistance and decreased beta-cell function. DNA damage accumulation has been associated with T2DM, but whether DNA damage plays a role in the pathogenesis of the disease is unclear. Here, we used mice deficient for the DNA excision-repair gene Ercc1 to study the impact of persistent endogenous DNA damage accumulation on energy metabolism, glucose homeostasis and beta-cell function.

METHODS

ERCC1-XPF is an endonuclease required for multiple DNA repair pathways and reduced expression of ERCC1-XPF causes accelerated accumulation of unrepaired endogenous DNA damage and accelerated aging in humans and mice. In this study, energy metabolism, glucose metabolism, beta-cell function and insulin sensitivity were studied in Ercc1^d/- mice, which model a human progeroid syndrome.

RESULTS

Ercc1^d/- mice displayed suppression of the somatotropic axis and altered energy metabolism. Insulin sensitivity was increased, whereas, plasma insulin levels were decreased in Ercc1^d/- mice. Fasting induced hypoglycemia in Ercc1^d/- mice, which was the result of increased glucose disposal. Ercc1^d/- mice exhibit a significantly reduced beta-cell area, even compared to control mice of similar weight. Glucose-stimulated insulin secretion in vivo was decreased in Ercc1^d/- mice. Islets isolated from Ercc1^d/- mice showed increased DNA damage markers, decreased glucose-stimulated insulin secretion and increased susceptibility to apoptosis.

CONCLUSION

Spontaneous DNA damage accumulation triggers an adaptive response resulting in improved insulin sensitivity. Loss of DNA repair, however, does negatively impacts beta-cell survival and function in Ercc1^d/- mice.

Islet cell dedifferentiation is a pathologic mechanism of long-standing progression of type 2 diabetes

Dedifferentiation has been implicated in β cell dysfunction and loss in rodent diabetes. However, the pathophysiological significance in humans remains unclear. To elucidate this, we analyzed surgically resected pancreatic tissues of 26 Japanese subjects with diabetes and 11 nondiabetic subjects, who had been overweight during adulthood but had no family history of diabetes. The diabetic subjects were subclassified into 3 disease stage categories, early, advanced, and intermediate. Despite no numerical changes in endocrine cells immunoreactive for chromogranin A (ChgA), diabetic islets showed profound β cell loss, with an increase in α cells without an increase in insulin and glucagon double-positive cells. The proportion of dedifferentiated cells that retain ChgA immunoreactivity without 4 major islet hormones was strikingly increased in diabetic islets and rose substantially during disease progression. The increased dedifferentiated cell ratio was inversely correlated with declining C-peptide index. Moreover, a subset of islet cells converted into exocrine-like cells during disease progression. These results indicate that islet remodeling with dedifferentiation is the underlying cause of β cell failure during the course of diabetes progression in humans.

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)1

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field.

Beta-Cell Mass in Obesity and Type 2 Diabetes, and Its Relation to Pancreas Fat: A Mini-Review

Type 2 diabetes (T2DM) is characterized by insulin resistance and beta-cell dysfunction. Although insulin resistance is assumed to be a main pathophysiological feature of the development of T2DM, recent studies have revealed that a deficit of functional beta-cell mass is an essential factor for the pathophysiology of T2DM. Pancreatic fat contents increase with obesity and are suggested to cause beta-cell dysfunction. Since the beta-cell dysfunction induced by obesity or progressive decline with disease duration results in a worsening glycemic control, and treatment failure, preserving beta-cell mass is an important treatment strategy for T2DM. In this mini-review, we summarize the current knowledge on beta-cell mass, beta-cell function, and pancreas fat in obesity and T2DM, and we discuss treatment strategies for T2DM in relation to beta-cell preservation.

Alleviation by Mahuang Fuzi and Shenzhuo Decoction in High Glucose-Induced Podocyte Injury by Inhibiting the Activation of Wnt/β-Catenin Signaling Pathway, Resulting in Activation of Podocyte Autophagy

Background

Organ fibrosis is a common endpoint of a variety of diseases. Many studies have shown that the pathogenesis of diabetic kidney disease (DKD) is related to the excessive activation of the Wnt/β-catenin signaling pathway on podocytes, so the treatment of DKD starts from this signaling pathway. At the same time, DKD, as a metabolic disease, has many connections related to podocyte autophagy.

Objectives

We experimented the effects of Mahuang Fuzi and Shenzhuo decoction (MFSD) which is the combination of Mahuang Fuzi decoction and Shenzhuo decoction in traditional Chinese medicine compounds used "The Golden Chamber" in high glucose-induced podocytes, determined whether this effect was related to Wnt/β-catenin signaling pathway, and further investigated the relationship between this effect and autophagy.

Methods

The mice podocytes were stimulated by using 30 mmol/L of high glucose and serum containing MFSD or Wnt/β-catenin signaling pathway inhibitor DKK1 (100 ng/ml) was used to intervene podocytes before high glucose stimulation. Podocyte injury-related proteins, Wnt/β-catenin signaling pathway-related proteins, and autophagy-related proteins were detected by using western blotting and immunofluorescence analysis.

Results

Our results showed that DKK1 and MFSD treatment significantly upregulated the protein expressions of nephrin, podocin, podocalyxin, and podoplanin in high glucose-induced podocytes and downregulated the β-catenin protein expression. Furthermore, the protein expressions of beclin1, LC3B, and P62 were also significantly increased in high glucose-induced podocytes.

Conclusion

Our experiments confirmed that the destruction of podocytes in DKD is related to the excessive activation of Wnt/β-catenin signaling pathway and the inhibition of autophagy after activation. MFSD treatment can inhibit the activation of Wnt/β-catenin signaling pathway in podocytes stimulated by high glucose and helpful in reducing the podocyte injury. This protective mechanism can be related to the enhancement of podocyte autophagy by MFSD treatment.

Association of glucose tolerance status with pancreatic β- and α-cell mass in community-based autopsy samples of Japanese individuals: The Hisayama Study

AIMS/INTRODUCTION

Changes in histologically quantified β- and α-cell mass during the development of glucose intolerance have not been fully elucidated. The aim of the present study was to explore differences in β- and α-cell mass according to the glucose tolerance status.

MATERIALS AND METHODS

Autopsy samples from a total of 103 individuals (40 with normal glucose tolerance, 31 with prediabetes and 32 with type 2 diabetes mellitus) who underwent a 75-g oral glucose tolerance test within 5 years before death were selected from 643 community-based autopsy samples collected from 2002 to 2016. Fractional β-cell area (BCA) and α-cell area were quantified with Image Pro Plus software. Associations of BCA and α-cell area with glucose tolerance status were assessed using a linear regression analysis, and Spearman's correlation coefficients between glycemic markers and β-cell function were estimated.

RESULTS

The mean values of BCA decreased significantly with worsening glucose tolerance status (mean ± standard error 1.85 ± 0.10% in normal glucose tolerance, 1.59 ± 0.11% in prediabetes and 1.17 ± 0.11% in type 2 diabetes mellitus, P for trend < 0.001), whereas there was no significant association between α-cell area and glucose tolerance status. BCA was inversely correlated with fasting and 2-h plasma glucose levels during oral glucose tolerance test and glycated hemoglobin measurement, and positively correlated with disposition index (all P < 0.01).

CONCLUSIONS

β-Cell mass decreased significantly with worsening glucose tolerance, from the stage of prediabetes, in the Japanese population. Prevention of declining β-cell mass before the onset of glucose intolerance is important to reduce the burden of type 2 diabetes mellitus.

An emerging new concept for the management of type 2 diabetes with a paradigm shift from the glucose-centric to beta cell-centric concept of diabetes - an Asian perspective

INTRODUCTION

Recent advances in anti-diabetic medications and glucose monitoring have led to a paradigm shift in diabetes care. Newer anti-diabetic medications such as DPP-4 inhibitors, GLP-1 receptor agonists (GLP-1RAs), and SGLT2 inhibitors have enabled optimal glycemic control to be achieved without increasing the risk of hypoglycemia and weight gain. Treatment with GLP-1RAs and SGLT2 inhibitors has been demonstrated to improve cardiorenal outcomes, positioning these agents as the mainstay of treatment for patients with type 2 diabetes (T2DM). The development of these newer agents has also prompted a paradigm shift in the concept of T2DM, highlighting the importance of beta cell dysfunction in the pathophysiology of T2DM.

AREAS COVERED

Recent advances in pharmacotherapy for diabetes are summarized with a focus on the role of incretin-based drugs and SGLT2 inhibitors. The importance of a paradigm shift from a glucose-centric to a beta cell-centric concept of T2DM is also discussed, given from an Asian perspective.

EXPERT OPINION

Management of T2DM including lifestyle modification as well as pharmacotherapy should be focused on reducing beta cell workload, to preserve functional beta cell mass. A paradigm shift from a glucose-centric to a beta cell-centric concept of T2DM enhances the implementation of person-centered diabetes care.

Beneficial effects of combination therapy of canagliflozin and teneligliptin on diabetic polyneuropathy and β-cell volume density in spontaneously type 2 diabetic Goto-Kakizaki rats

AIMS

Parasympathetic nerve (PN) signaling plays a crucial role in the maintenance of pancreatic β-cell volume density (V_β). PN may be pathologically affected in diabetic polyneuropathy (DPN). However, the association between the reduction of PNs in islets and V_β and the therapeutic effects of a DPP4 inhibitor (DPP4i) and an SGLT2 inhibitor (SGLT2i) in nonobese type 2 diabetes mellitus (T2DM) Goto-Kakizaki rats (GK) have not been investigated.

MATERIALS AND METHODS

We divided 5-week old male GK and Wistar rats (W) into a DPP4i-treated group (GKTe), SGLT2i-treated group (GKCa), and combination-treated group (GKCaTe). After 25 weeks, the pancreata was pathologically evaluated.

RESULTS

V_β in GK was significantly decreased (p < 0.01 vs. W), whereas V_β was the most well preserved in GKCaTe (p < 0.05 vs. GKTe), followed by GKTe (p < 0.05 vs. GK). The decreased amount of PNs in the islets and intraepidermal nerve fiber density (IENFD) in GK was significantly improved in the treated groups compared with GK (p < 0.05 vs. GKCa and GKTe and p < 0.01 vs. GKCaTe). PN density and IENFD were significantly correlated with V_β (r = 0.55, p < 0.01 and r = 0.54, p < 0.01, respectively). IENFD was identified as a surrogate marker for the prediction of V_β (cutoff value, 16.39).

CONCLUSIONS

The combination therapy of DPP4i and SGLT2i improved V_β accompanied by PNs density and IENFD. IENFD was proportionally correlated with V_β. Therefore, the prevention of DPN development may be concurrently beneficial for the preservation of V_β in nonobese T2DM.

Autophagic dysfunction of β cell dysfunction in type 2 diabetes, a double-edged sword

Diabetes is an age-related disease, most of which is type 2 diabetes, and islet β cell dysfunction and insulin resistance are the main mechanisms of type 2 diabetes. Recent studies have revealed that autophagy plays an important role in maintaining the structure and function of islet beta cells and inhibiting insulin resistance and apoptosis induced by oxidative stress. In this review, we discussed the positive and negative effects of autophagy and its dysfunction on type 2 diabetes mellitus, which is the so-called double-edged sword, analysed its possible mechanism, and identified possible research hot spots.

Nutrient Sensor mTOR and OGT: Orchestrators of Organelle Homeostasis in Pancreatic β-Cells

The purpose of this review is to integrate the role of nutrient-sensing pathways into β-cell organelle dysfunction prompted by nutrient excess during type 2 diabetes (T2D). T2D encompasses chronic hyperglycemia, hyperlipidemia, and inflammation, which each contribute to β-cell failure. These factors can disrupt the function of critical β-cell organelles, namely, the ER, mitochondria, lysosomes, and autophagosomes. Dysfunctional organelles cause defects in insulin synthesis and secretion and activate apoptotic pathways if homeostasis is not restored. In this review, we will focus on mTORC1 and OGT, two major anabolic nutrient sensors with important roles in β-cell physiology. Though acute stimulation of these sensors frequently improves β-cell function and promotes adaptation to cell stress, chronic and sustained activity disturbs organelle homeostasis. mTORC1 and OGT regulate organelle function by influencing the expression and activities of key proteins, enzymes, and transcription factors, as well as by modulating autophagy to influence clearance of defective organelles. In addition, mTORC1 and OGT activity influence islet inflammation during T2D, which can further disrupt organelle and β-cell function. Therapies for T2D that fine-tune the activity of these nutrient sensors have yet to be developed, but the important role of mTORC1 and OGT in organelle homeostasis makes them promising targets to improve β-cell function and survival.

Nutrient Metabolism, Subcellular Redox State, and Oxidative Stress in Pancreatic Islets and β-Cells

Insulin-secreting pancreatic β-cells play a critical role in blood glucose homeostasis and the development of type 2 diabetes (T2D) in the context of insulin resistance. Based on data obtained at the whole cell level using poorly specific chemical probes, reactive oxygen species (ROS) such as superoxide and hydrogen peroxide have been proposed to contribute to the stimulation of insulin secretion by nutrients (positive role) and to the alterations of cell survival and secretory function in T2D (negative role). This raised the controversial hypothesis that any attempt to decrease β-cell oxidative stress and apoptosis in T2D would further impair insulin secretion. Over the last decade, the development of genetically-encoded redox probes that can be targeted to cellular compartments of interest and are specific of redox couples allowed the evaluation of short- and long-term effects of nutrients on β-cell redox changes at the subcellular level. The data indicated that the nutrient regulation of β-cell redox signaling and ROS toxicity is far more complex than previously thought and that the subcellular compartmentation of these processes cannot be neglected when evaluating the mechanisms of ROS production or the efficacy of antioxidant enzymes and antioxidant drugs under glucolipotoxic conditions and in T2D. In this review, we present what is currently known about the compartmentation of redox homeostatic systems and tools to investigate it. We then review data about the effects of nutrients on β-cell subcellular redox state under normal conditions and in the context of T2D and discuss challenges and opportunities in the field.

The challenge of correctly reporting hormones content and secretion in isolated human islets

Background

An increased access of research laboratories to isolated human islets has improved our understanding of the biology of the endocrine pancreas and hence the mechanisms causing diabetes. However, in vitro studies of human islets remain technically challenging, and optimal use of such precious material requires a minimum of rigor and coordination to optimize the reliability and share of the information. A detailed report of the demographics of pancreas donors and of the procedures of islet handling after isolation is important but insufficient. Correct characterization of islet basic functions (a token of quality) at the time of experimentation is also crucial.

Scope of review

I have analyzed the literature reporting measurements of insulin and glucagon in the human pancreas or isolated human islets. The published information is often fragmentary. Elementary features such as islet size, insulin content, or rate of hormone secretion are either unreported or incorrectly reported in many papers. Although internal comparisons between control and test groups may remain valid, comparisons with data from other laboratories are problematic. The drawbacks, pitfalls and errors of common ways of expressing hormone content or secretion rates are discussed and alternatives to harmonize data presentation are proposed.

Major Conclusions

Greater coherence and rigor in the report of in vitro studies using human islets are necessary to ensure optimal progress in our understanding of the pathogenesis of diabetes.

The Mitochondrial Antioxidant SS-31 Modulates Oxidative Stress, Endoplasmic Reticulum Stress, and Autophagy in Type 2 Diabetes

Mitochondrial dysfunction has been shown to play a central role in the pathophysiology of type 2 diabetes (T2D), and mitochondria-targeted agents such as SS-31 are emerging as a promising strategy for its treatment. We aimed to study the effects of SS-31 on leukocytes from T2D patients by evaluating oxidative stress, endoplasmic reticulum (ER) stress and autophagy. Sixty-one T2D patients and 53 controls were included. Anthropometric and analytical measurements were performed. We also assessed reactive oxygen species (ROS) production, calcium content, the expression of ER stress markers GRP78, CHOP, P-eIF2α, and autophagy-related proteins Beclin1, LC3 II/I, and p62 in leukocytes from T2D and control subjects treated or not with SS-31. Furthermore, we have evaluated the action of SS-31 on leukocyte-endothelium interactions. T2D patients exhibited elevated ROS concentration, calcium levels and presence of ER markers (GRP78 and CHOP gene expression, and GRP78 and P-eIF2α protein expression), all of which were reduced by SS-31 treatment. SS-31 also led to a drop in BECN1 gene expression, and Beclin1 and LC3 II/I protein expression in T2D patients. In contrast, the T2D group displayed reduced p62 protein levels that were restored by SS-31. SS-20 (with non-antioxidant activity) did not change any analyzed parameter. In addition, SS-31 decreased rolling flux and leukocyte adhesion, and increased rolling velocity in T2D patients. Our findings suggest that SS-31 exerts potentially beneficial effects on leukocytes of T2D patients modulating oxidative stress and autophagy, and ameliorating ER stress.

Breaking Bad and Breaking Good: β-Cell Autophagy Pathways in Diabetes

For many decades the lysosome has been recognized as the terminal center of cellular waste disposal. Products of lysosomal degradation are either recycled in biosynthetic pathways or are further metabolized to produce energy. As such the lysosome was attributed a rather passive role in cellular metabolism merely transforming bulk material into small metabolites. More recently, however, the emerging evidence has brought the lysosome to the center of nutrient sensing as the organelle that harbors a complex signaling machinery which dynamically and actively regulates cell metabolism. The pancreatic β cell is unique in as much as nutrient sensing is directly coupled to insulin secretion. Importantly, defects in insulin secretion constitute a hallmark in the progression of patients from a state of impaired glucose tolerance to full blown type 2 diabetes (T2D). However, mechanisms linking nutrient-dependent lysosomal function to insulin secretion and more generally to β cell health have evolved only very recently. This review discusses emerging concepts in macroautophagy and macroautophagy-independent processes of cargo delivery to lysosomes as well as nutrient-dependent lysosomal signaling specifically in the context of β cell function in health and disease. Such mechanisms may provide a novel source of therapeutic targets to be exploited in the context of β cell failure in diabetes in the near future.

Pregnancy in human IAPP transgenic mice recapitulates beta cell stress in type 2 diabetes

AIMS/HYPOTHESIS

Islet amyloid polypeptide (IAPP) misfolding and toxic oligomers contribute to beta cell loss and stress in type 2 diabetes. Pregnancy-related diabetes predicts subsequent risk for type 2 diabetes but little is known about the impact of pregnancy on beta cell mass, turnover and stress. Availability of human pancreas tissue in pregnancy is limited and most widely used mouse models of type 2 diabetes do not develop pregnancy-related diabetes, possibly because rodent IAPP is not prone to form toxic oligomers. We hypothesised that mice transgenic for human IAPP (hIAPP) are prone to pregnancy-related diabetes with beta cell responses reflective of those in type 2 diabetes.

METHODS

We evaluated the impact of a first and second pregnancy on glucose homeostasis, beta cell mass and turnover and markers of beta cell stress in hIAPP transgenic (hTG) mice.

RESULTS

Pregnancy induced both endoplasmic reticulum stress and oxidative stress and compromised autophagy in beta cells in hTG mice, which are characteristic of beta cells in type 2 diabetes. Beta cell stress persisted after pregnancy, resulting in subsequent diabetes before or during a second pregnancy.

CONCLUSIONS/INTERPRETATION

High expression of hIAPP in response to pregnancy recapitulates mechanisms contributing to beta cell stress in type 2 diabetes. We hypothesise that, in individuals prone to type 2 diabetes, pregnancy-induced increased expression of IAPP inflicts beta cell damage that persists and is compounded by subsequent additive stress such as further pregnancy. The hTG mouse model is a novel model for pregnancy-related diabetes.

C/EBPβ mediates palmitate-induced musclin expression via the regulation of PERK/ATF4 pathways in myotubes

Musclin is a muscle-secreted cytokine that disrupts glucose uptake and glycogen synthesis in type 2 diabetes. The purpose of this study was to investigate the mechanisms responsible for the regulation of musclin gene expression in response to treatment with palmitate. RNA sequencing results showed that biological processes activated by palmitate are mainly enriched in endoplasmic reticulum (ER) stress. The protein kinase RNA-like ER kinase (PERK) signaling pathway is involved in the regulation of musclin expression induced by palmitate. Chromatin immunoprecipitation data showed that activating transcription factor 4 (ATF4)-downstream of PERK-bound to the promoter of the C/EBPβ gene. Notably, C/EBPβ also contains a binding site in the region -94~-52 of the musclin gene promoter. Knockdown or knockout of PERK and ATF4 using short hairpin RNA or CRISPR-Cas9 decreased the expression of C/EBPβ and musclin induced by palmitate. Furthermore, knockdown and knockout of C/EBPβ alleviated the high expression of musclin in response to treatment with palmitate. Moreover, CRISPR-Cas9 knockout of the region -94~-52 in which C/EBPβ binds to the promoter of musclin abrogated the induction of high musclin expression caused by palmitate. Collectively, these findings suggest that treatment with palmitate activates the PERK/ATF4 signaling pathway, which in turn increases the expression of C/EBPβ. C/EBPβ binds directly to the promoter of the musclin gene and upregulates its expression.

Antioxidant activity of crude ethanolic extract and fractions of Ziziphus mauritiana Lam. (Rhamnaceae) leaves from Burkina Faso

Background Ziziphus mauritiana Lam. is a plant used in traditional medicine in Burkina Faso in the treatment of several diseases, of which diabetes is characterized by oxidative stress. The aim of this study was to evaluate the in vitro antioxidant potential of the extracts of leaves of this plant. Methods The crude hydroethanolic extract (HEE) of the leaves of Z. mauritiana and their partitionates in n-hexane, dichloromethane, and ethyl acetate, and in the residual aqueous solution (the F1, F2, F3, and F4 fractions, respectively) were first prepared. The content of polyphenols was determined and the antioxidant effects of the extracts were evaluated by their 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity, inhibition of lipid peroxidation (TBARS), and the ferric reducing antioxidant power (FRAP). Results The HEE as well as the F3 and F4 fractions were rich in polyphenols with contents between 58 and 84 mg equivalent gallic acid per 100 mg. The flavonoid content was 4 mg quercetin equivalents in the HEE and the F4 fraction. Except for the F1 fraction, the HEE and the other fractions showed significant DPPH scavenging activity (IC50 between 8 and 12 μg/mL). The IC50 of TBARS by different extracts was in the range 1-5 μg/mL, and the FRAP activity was 7-85 mg ascorbic acid equivalent per 100 mg. Total polyphenol content was highly correlated with the antioxidant activities. Conclusions The HEE, F3, and F4 fractions were found to be the richest in polyphenols and had the best antioxidant activity. The antioxidant activity of the extracts of the leaves of Z. mauritiana is due to these polyphenolic compounds.

Expression of LRP1 and CHOP genes associated with peripheral neuropathy in type 2 diabetes mellitus: Correlations with nerve conduction studies

AIM

Diabetic peripheral neuropathy (DPN) is a frequent and debilitating complication of diabetes mellitus. The low-density lipoprotein receptor-related protein-1 (LRP-1) is a multifunctional cell surface receptor playing critical roles in lipoprotein metabolism and several cell signaling processes. C/EBP homologous protein (CHOP) is a main conduit to endoplasmic reticulum stress-induced apoptosis. We aimed to investigate LRP1 and CHOP gene expression in peripheral blood cells of type 2 diabetes mellitus (T2DM) subjects to clarify its possible relation to DPN pathogenesis.

METHOD

The study included 20 non-complicated T2DM subjects, 20 subjects with DPN and 20 healthy controls. Quantitative real time PCR was used to study gene expression.

RESULTS

There was a significant reduction in LRP1 mRNA expression and a significant increase in CHOP mRNA expression in subjects with DPN compared to non-complicated group and healthy controls. Both LRP1 and CHOP expression levels were inversely correlated, and both showed significant correlation with HbA1c, hyperlipidemia, hs-CRP, and different electrophysiological parameters. Receiver operating characteristics (ROC) analysis suggested that both LRP1 and CHOP mRNA expression and hs-CRP levels had great potential advantages to predict the progression of DPN.

CONCLUSION

LRP1 and CHOP might be involved in DPN pathogenesis and progression, thus providing opportunities for early detection and treatment.

Associations between Fatty Acid-Binding Protein 4⁻A Proinflammatory Adipokine and Insulin Resistance, Gestational and Type 2 Diabetes Mellitus

There is ample scientific evidence to suggest a link between the fatty acid-binding protein 4 (FABP4) and insulin resistance, gestational (GDM), and type 2 (T2DM) diabetes mellitus. This novel proinflammatory adipokine is engaged in the regulation of lipid metabolism at the cellular level. The molecule takes part in lipid oxidation, the regulation of transcription as well as the synthesis of membranes. An involvement of FABP4 in the pathogenesis of obesity and insulin resistance seems to be mediated via FABP4-dependent peroxisome proliferator-activated receptor γ (PPARγ) inhibition. A considerable number of studies have shown that plasma concentrations of FABP4 is increased in obesity and T2DM, and that circulating FABP4 levels are correlated with certain clinical parameters, such as body mass index, insulin resistance, and dyslipidemia. Since plasma-circulating FABP4 has the potential to modulate the function of several types of cells, it appears to be of extreme interest to try to develop potential therapeutic strategies targeting the pathogenesis of metabolic diseases in this respect. In this manuscript, representing a detailed review of the literature on FABP4 and the abovementioned metabolic disorders, various mechanisms of the interaction of FABP4 with insulin signaling pathways are thoroughly discussed. Clinical aspects of insulin resistance in diabetic patients, including women diagnosed with GDM, are analyzed as well.

Inhibition of COX2/PGD2-Related Autophagy Is Involved in the Mechanism of Brain Injury in T2DM Rat

The present study was designed to observe the effect of COX2/PGD2-related autophagy on brain injury in type 2 diabetes rats. The histopathology was detected by haematoxylin–eosin staining. The learning and memory functions were evaluated by Morris water maze. The levels of insulin and PGD2 were measured by enzyme-linked immunosorbent assay. The expressions of COX2, p-AKT(S473), p-AMPK(T172), Aβ, Beclin1, LC3BII, and p62 were measured by immunohistochemistry and Western blotting. In model rats, we found that the body weight was significantly decreased, the blood glucose levels were significantly increased, the plasma insulin content was significantly decreased, the learning and memory functions were impaired and the cortex and hippocampus neurons showed significant nuclear pyknosis. The levels of COX2, p-AKT(S473), PGD2, Aβ, Beclin1 and p62 were significantly increased, whereas the expression of p-AMPK(T172) and LC3BII was significantly decreased in the cortex and hippocampus of model rats. In meloxicam-treated rats, the body weight, blood glucose and the content of plasma insulin did not significantly change, the learning and memory functions were improved and nuclear pyknosis was improved in the cortex and hippocampus neurons. The expression of p-AMPK(T172), Beclin1 and LC3BII was significantly increased, and the levels of COX2, p-AKT(S473), PGD2, Aβ, and p62 were significantly decreased in the cortex and hippocampus of meloxicam-treated rats. Our results suggested that the inhibition of COX2/PGD2-related autophagy was involved in the mechanism of brain injury caused by type 2 diabetes in rats.

Complement Component C3 Is Highly Expressed in Human Pancreatic Islets and Prevents β Cell Death via ATG16L1 Interaction and Autophagy Regulation

We show here that human pancreatic islets highly express C3, which is both secreted and present in the cytosol. Within isolated human islets, C3 expression correlates with type 2 diabetes (T2D) donor status, HbA1c, and inflammation. Islet C3 expression is also upregulated in several rodent diabetes models. C3 interacts with ATG16L1, which is essential for autophagy. Autophagy relieves cellular stresses faced by β cells during T2D and maintains cellular homeostasis. C3 knockout in clonal β cells impaired autophagy and led to increased apoptosis after exposure of cells to palmitic acid and IAPP. In the absence of C3, autophagosomes do not undergo fusion with lysosomes. Thus, C3 may be upregulated in islets during T2D as a cytoprotective factor against β cell dysfunction caused by impaired autophagy. Therefore, we revealed a previously undescribed intracellular function for C3, connecting the complement system directly to autophagy, with a broad potential importance in other diseases and cell types.