Insulin secretion defects of human type 2 diabetic islets are corrected in vitro by a new reactive oxygen species scavenger

Tyrosine nitration of glucagon impairs its function: Extending the role of heme in T2D pathogenesis

New studies raise the possibility that the higher glucagon (GCG) level present in type 2 diabetes (T2D) is a compensatory mechanism to enhance β-cell function, rather than induce dysregulated glucose homeostasis, due to an important role for GCG that acts directly within the pancreas on insulin secretion by intra-islet GCG signaling. However, in states of poorly controlled T2D, pancreatic α cell mass increases (overproduced GCG) in response to insufficient insulin secretion, indicating decreased local GCG activity. The reason for this decrease is not clear. Recent evidence has uncovered a new role of heme in cellular signal transduction, and its mechanism involves reversible binding of heme to proteins. Considering that protein tyrosine nitration in diabetic islets increases and glucose-stimulated insulin secretion (GSIS) decreases, we speculated that heme modulates GSIS by transient interaction with GCG and catalyzing its tyrosine nitration, and the tyrosine nitration may impair GCG activity, leading to loss of intra-islet GCG signaling and markedly impaired insulin secretion. Data presented here elucidate a novel role for heme in disrupting local GCG signaling in diabetes. Heme bound to GCG and induced GCG tyrosine nitration. Two tyrosine residues in GCG were both sensitive to the nitrating species. Further, GCG was also demonstrated to be a preferred target peptide for tyrosine nitration by co-incubation with BSA. Tyrosine nitration impaired GCG stimulated cAMP-dependent signaling in islet β cells and decreased insulin release. Our results provided a new role of heme for impaired GSIS in the pathological process of diabetes.

Current updates on metabolites and its interlinked pathways as biomarkers for diabetic kidney disease: A systematic review

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes mellitus (DM) that poses a serious risk as it can lead to end-stage renal disease (ESRD). DKD is linked to changes in the diversity, composition, and functionality of the microbiota present in the gastrointestinal tract. The interplay between the gut microbiota and the host organism is primarily facilitated by metabolites generated by microbial metabolic processes from both dietary substrates and endogenous host compounds. The production of numerous metabolites by the gut microbiota is a crucial factor in the pathogenesis of DKD. However, a comprehensive understanding of the precise mechanisms by which gut microbiota and its metabolites contribute to the onset and progression of DKD remains incomplete. This review will provide a summary of the current scenario of metabolites in DKD and the impact of these metabolites on DKD progression. We will discuss in detail the primary and gut-derived metabolites in DKD, and the mechanisms of the metabolites involved in DKD progression. Further, we will address the importance of metabolomics in helping identify potential DKD markers. Furthermore, the possible therapeutic interventions and research gaps will be highlighted.

β-cell neogenesis: A rising star to rescue diabetes mellitus

BACKGROUND

Diabetes Mellitus (DM), a chronic metabolic disease characterized by elevated blood glucose, is caused by various degrees of insulin resistance and dysfunctional insulin secretion, resulting in hyperglycemia. The loss and failure of functional β-cells are key mechanisms resulting in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM).

AIM OF REVIEW

Elucidating the underlying mechanisms of β-cell failure, and exploring approaches for β-cell neogenesis to reverse β-cell dysfunction may provide novel strategies for DM therapy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Emerging studies reveal that genetic susceptibility, endoplasmic reticulum (ER) stress, oxidative stress, islet inflammation, and protein modification linked to multiple signaling pathways contribute to DM pathogenesis. Over the past few years, replenishing functional β-cell by β-cell neogenesis to restore the number and function of pancreatic β-cells has remarkably exhibited a promising therapeutic approach for DM therapy. In this review, we provide a comprehensive overview of the underlying mechanisms of β-cell failure in DM, highlight the effective approaches for β-cell neogenesis, as well as discuss the current clinical and preclinical agents research advances of β-cell neogenesis. Insights into the challenges of translating β-cell neogenesis into clinical application for DM treatment are also offered.

Metabolites of endophytic fungi isolated from leaves of Bauhinia variegata exhibit antioxidant activity and agonist activity on peroxisome proliferator-activated receptors α, β/δ and γ

Diabetes mellitus is a metabolic disorder that affects millions of people worldwide and is linked to oxidative stress and inflammation. Thiazolidinediones (TZD) improve insulin sensitization and glucose homeostasis mediated by the activation of peroxisome proliferator-activated receptors γ (PPARγ) in patients with type 2 diabetes. However, their use is associated with severe adverse effects such as loss of bone mass, retention of body fluids, liver and heart problems, and increased risk of bladder cancer. Partial PPARγ agonists can promote the beneficial effects of thiazolidinediones with fewer adverse effects. Endophytic fungi colonize plant tissues and have a particularly active metabolism caused by the interaction with them, which leads to the production of natural products with significant biological effects that may be like that of the colonized plant. Here, we identify seven endophytic fungi isolated from Bauhinia variegata leaves that have antioxidant activities. Also, one of the extracts presented pan-agonist activity on PPAR, and another showed activity in PPARα and PPARβ/δ. A better understanding of this relationship could help to comprehend the mechanism of action of antioxidants in treating diabetes and its complications. Moreover, compounds with these capabilities to reduce oxidative stress and activate the receptor that promotes glucose homeostasis are promising candidates in treatment of diabetes.

Oxidative stress and impaired insulin secretion in cystic fibrosis pig pancreas

Cystic fibrosis-related diabetes (CFRD) is one the most common comorbidities in cystic fibrosis (CF). Pancreatic oxidative stress has been postulated in the pathogenesis of CFRD, but no studies have been done to show an association. The main obstacle is the lack of suitable animal models and no immediate availability of pancreas tissue in humans. In the CF porcine model, we found increased pancreatic total glutathione (GSH), glutathione disulfide (GSSG), 3-nitrotyrosine- and 4-hydroxynonenal-modified proteins, and decreased copper zinc superoxide dismutase (CuZnSOD) activity, all indicative of oxidative stress. CF pig pancreas demonstrated increased DHE oxidation (as a surrogate marker of superoxide) in situ compared to non-CF and this was inhibited by a SOD-mimetic (GC4401). Catalase and glutathione peroxidase activities were not different between CF and non-CF pancreas. Isolated CF pig islets had significantly increased DHE oxidation, peroxide production, reduced insulin secretion in response to high glucose and diminished secretory index compared to non-CF islets. Acute treatment with apocynin or an SOD mimetic failed to restore insulin secretion. These results are consistent with the hypothesis that CF pig pancreas is under significant oxidative stress as a result of increased O₂^●− and peroxides combined with reduced antioxidant defenses against reactive oxygen species (ROS). We speculate that insulin secretory defects in CF may be due to oxidative stress.

Tinospora cordifolia (Willd.) Miers: Protection mechanisms and strategies against oxidative stress-related diseases

ETHNOPHARMACOLOGICAL RELEVANCE

Tinospora cordifolia (Willd.) Miers (Menispermaceae) is a Mediterranean herb, used in Ayurvedic, Siddha, Unani, and folk medicines. The herb is also used in conventional medicine to treat oxidative stress-related diseases and conditions, including inflammation, pain, diarrhea, asthma, respiratory infections, cancer, diabetes, and gastrointestinal disorders.

AIM OF THE REVIEW

The taxonomy, botanical classification, geographical distribution, and ethnobotanical uses of T. cordifolia, as well as the phytochemical compounds found in the herb, the toxicology of and pharmacological and clinical studies on the effects of T. cordifolia are all covered in this study.

MATERIALS AND METHODS

To gather information on T. cordifolia, we used a variety of scientific databases, including Scopus, Google Scholar, PubMed, and Science Direct. The information discussed focuses on biologically active compounds found in T. cordifolia, and common applications and pharmacological activity of the herb, as well as toxicological and clinical studies on its properties.

RESULTS

The findings of this study reveal a connection between the use of T. cordifolia in conventional medicine and its antioxidant, anti-inflammatory, antihypertensive, antidiabetic, anticancer, immunomodulatory, and other biological effects. The entire plant, stem, leaves, root, and extracts of T. cordifolia have been shown to have a variety of biological activities, including antioxidant, antimicrobial, antiviral, antiparasitic, antidiabetic, anticancer, anti-inflammatory, analgesic and antipyretic, hepatoprotective, and cardioprotective impact. Toxicological testing demonstrated that this plant may have medicinal applications. T. cordifolia contains a variety of biologically active compounds from various chemical classes, including alkaloids, terpenoids, sitosterols, flavonoids, and phenolic acids. Based on the reports researched for this review, we believe that chemicals in T. cordifolia may activate Nrf2, which leads to the overexpression of antioxidant enzymes such as CAT, GPx, GST, and GR, and thereby induces the adaptive response to oxidative stress. T. cordifolia is also able to reduce NF-κB signalling by inhibiting PI3K/Akt, activating AMPK and sirtuins, and downregulating PI3K/Akt.

CONCLUSIONS

Our findings indicate that the pharmacological properties displayed by T. cordifolia back up its conventional uses. Antimicrobial, antiviral, antioxidant, anticancer, anti-inflammatory, antimutagenic, antidiabetic, nephroprotective, gastroprotective, hepatoprotective, and cardioprotective activities were all demonstrated in T. cordifolia stem extracts. To validate pharmacodynamic targets, further research is needed to evaluate the molecular mechanisms of the known compounds against gastrointestinal diseases, inflammatory processes, and microbial infections, as immunostimulants, and in chemotherapy. The T. cordifolia safety profile was confirmed in a toxicological analysis, which prompted pharmacokinetic assessment testing to confirm its bioavailability.

Improvement of Lipotoxicity-Induced Islet β Cellular Insulin Secretion Disorder by Osteocalcin

BACKGROUND

Osteocalcin (OCN) has been proved to be closely related with the development of type 2 diabetes mellitus (T2DM). We aimed to study if OCN could improve the disorder of islet cell caused by lipotoxicity.

METHODS

Alizarin red staining was used to investigate the mineralization. Western blotting and ELISA methods were used to measure protein expression. Immunofluorescence staining was used to investigate the protein nuclear transfer.

RESULTS

High glucose and high fat inhibited the differentiation of osteoblast precursors. Overexpression of insulin receptor (InsR^OE) significantly promoted the Runx2 and OCN expression. The increase of insulin, Gprc6a, and Glut2 by osteoblast culture medium overexpressing insulin receptor was reversed by osteocalcin neutralizing antibody. Undercarboxylated osteocalcin (ucOC) suppressed the lipotoxic islet β-cell damage caused by palmitic acid. The FOXO1 from intranuclear to extranuclear was also significantly increased after ucOC treatment compared with the group PA. Knockdown of Gprc6a or suppression of PI3K/AKT signal pathway could reverse the upregulation of GPRC6A/PI3K/AKT/FoxO1/Pdx1 caused by ucOC.

CONCLUSION

OCN could activate the FOXO1 signaling pathway to regulate GLUT2 expression and improve the insulin secretion disorder caused by lipotoxicity.

Metabolism as a central regulator of β-cell chromatin state

Pancreatic β-cells are critical mediators of glucose homeostasis in the body, and proper cellular nutrient metabolism is critical to β-cell function. Several interacting signaling networks that uniquely control β-cell metabolism produce essential substrates and co-factors for catalytic reactions, including reactions that modify chromatin. Chromatin modifications, in turn, regulate gene expression. The reactions that modify chromatin are therefore well-positioned to adjust gene expression programs according to nutrient availability. It follows that dysregulation of nutrient metabolism in β-cells may impact chromatin state and gene expression through altering the availability of these substrates and co-factors. Metabolic disorders such as type 2 diabetes (T2D) can significantly alter metabolite levels in cells. This suggests that a driver of β-cell dysfunction during T2D may be the altered availability of substrates or co-factors necessary to maintain β-cell chromatin state. Induced changes in the β-cell chromatin modifications may then lead to dysregulation of gene expression, in turn contributing to the downward cascade of events that leads to the loss of functional β-cell mass, and loss of glucose homeostasis, that occurs in T2D.

Unbiased Profiling of the Human Proinsulin Biosynthetic Interaction Network Reveals a Role for Peroxiredoxin 4 in Proinsulin Folding

The β-cell protein synthetic machinery is dedicated to the production of mature insulin, which requires the proper folding and trafficking of its precursor, proinsulin. The complete network of proteins that mediate proinsulin folding and advancement through the secretory pathway, however, remains poorly defined. Here we used affinity purification and mass spectrometry to identify, for the first time, the proinsulin biosynthetic interaction network in human islets. Stringent analysis established a central node of proinsulin interactions with endoplasmic reticulum (ER) folding factors, including chaperones and oxidoreductases, that is remarkably conserved in both sexes and across three ethnicities. The ER-localized peroxiredoxin PRDX4 was identified as a prominent proinsulin-interacting protein. In β-cells, gene silencing of PRDX4 rendered proinsulin susceptible to misfolding, particularly in response to oxidative stress, while exogenous PRDX4 improved proinsulin folding. Moreover, proinsulin misfolding induced by oxidative stress or high glucose was accompanied by sulfonylation of PRDX4, a modification known to inactivate peroxiredoxins. Notably, islets from patients with type 2 diabetes (T2D) exhibited significantly higher levels of sulfonylated PRDX4 than islets from healthy individuals. In conclusion, we have generated the first reference map of the human proinsulin interactome to identify critical factors controlling insulin biosynthesis, β-cell function, and T2D.

Tyrosine nitration as a key event of signal transduction that regulates functional state of the cell

This review is dedicated to the role of nitration of proteins by tyrosine residues in physiological and pathological conditions. First of all, we analyze the biochemical evidence of peroxynitrite formation and reactions that lead to its formation, types of posttranslational modifications (PTMs) induced by reactive nitrogen species, as well as three biological pathways of tyrosine nitration. Then, we describe two possible mechanisms of protein nitration that are involved in intracellular signal transduction, as well as its interconnection with phosphorylation/dephosphorylation of tyrosine. Next part of the review is dedicated to the role of proteins nitration in different pathological conditions. In this section, special attention is devoted to the role of nitration in changes of functional properties of actin-protein that undergoes PTMs both in normal and pathological conditions. Overall, this review is devoted to the main features of protein nitration by tyrosine residue and the role of this process in intracellular signal transduction in basal and pathological conditions.

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.

Mitigating effects of antioxidant properties of Artemisia herba alba aqueous extract on hyperlipidemia and oxidative damage in alloxan-induced diabetic rats

Chronic hyperglycemia and excess reactive oxygen species overproduced in diabetes were associated with oxidative stress, led to continuous injury and functions damage to different organs: eyes, kidneys, neural and cardiovascular system. The present study was undertaken to evaluate the protective effect of Artemisia herba alba (AHA) leaf powder against alloxane-induced oxidative damage in diabetic rats. Rats were randomly divided into four groups: Group I controls received saline solution 9%; Group II was treated with 150 mg alloxane/(kg body weight) administered by intraperitoneal. Rats of Group III have received saline solution and treated with 400 mg AHA/(kg body weight). Animals of Group IV were treated with alloxane and AHA. Alloxane exposure led to increased blood glucose, total cholesterol, triglycerides, malondialdehyde, and a decrease in the antioxidants enzymes activities (catalase, glutathione peroxidase and glutathione-S-transferase). Administration of AHA aqueous extract ameliorated these parameters. These results demonstrate that AHA ameliorates hyperglycemia, hyperlipidemia and oxidative damage in alloxan-induced diabetes in rats.

Could the high consumption of high glycaemic index carbohydrates and sugars, associated with the nutritional transition to the Western type of diet, be the common cause of the obesity epidemic and the worldwide increasing incidences of Type 1 and Type 2 diabetes?

The globally increasing incidences of Type 1 diabetes (T1DM) and Type 2 diabetes (T2DM) can have a common background. If challenged by the contemporary high level of nutritional glucose stimulation, the β-cells in genetically predisposed individuals are at risk for damage which can lead to the diseases. The fat to carbohydrate dietary shift can also contribute to the associated obesity epidemic.

Single-cell gene expression analysis reveals β-cell dysfunction and deficit mechanisms in type 2 diabetes

BACKGROUND

Type 2 diabetes (T2D) is one of the most common chronic diseases. Studies on T2D are mainly built upon bulk-cell data analysis, which measures the average gene expression levels for a population of cells and cannot capture the inter-cell heterogeneity. The single-cell RNA-sequencing technology can provide additional information about the molecular mechanisms of T2D at single-cell level.

RESULTS

In this work, we analyze three datasets of single-cell transcriptomes to reveal β-cell dysfunction and deficit mechanisms in T2D. Focused on the expression levels of key genes, we conduct discrimination of healthy and T2D β-cells using five machine learning classifiers, and extracted major influential factors by calculating correlation coefficients and mutual information. Our analysis shows that T2D β-cells are normal in insulin gene expression in the scenario of low cellular stress (especially oxidative stress), but appear dysfunctional under the circumstances of high cellular stress. Remarkably, oxidative stress plays an important role in affecting the expression of insulin gene. In addition, by analyzing the genes related to apoptosis, we found that the TNFR1-, BAX-, CAPN1- and CAPN2-dependent pathways may be crucial for β-cell apoptosis in T2D. Finally, personalized analysis indicates cell heterogeneity and individual-specific insulin gene expression.

CONCLUSIONS

Oxidative stress is an important influential factor on insulin gene expression in T2D. Based on the uncovered mechanism of β-cell dysfunction and deficit, targeting key genes in the apoptosis pathway along with alleviating oxidative stress could be a potential treatment strategy for T2D.

A Syntenic Cross Species Aneuploidy Genetic Screen Links RCAN1 Expression to β-Cell Mitochondrial Dysfunction in Type 2 Diabetes

Type 2 diabetes (T2D) is a complex metabolic disease associated with obesity, insulin resistance and hypoinsulinemia due to pancreatic β-cell dysfunction. Reduced mitochondrial function is thought to be central to β-cell dysfunction. Mitochondrial dysfunction and reduced insulin secretion are also observed in β-cells of humans with the most common human genetic disorder, Down syndrome (DS, Trisomy 21). To identify regions of chromosome 21 that may be associated with perturbed glucose homeostasis we profiled the glycaemic status of different DS mouse models. The Ts65Dn and Dp16 DS mouse lines were hyperglycemic, while Tc1 and Ts1Rhr mice were not, providing us with a region of chromosome 21 containing genes that cause hyperglycemia. We then examined whether any of these genes were upregulated in a set of ~5,000 gene expression changes we had identified in a large gene expression analysis of human T2D β-cells. This approach produced a single gene, RCAN1, as a candidate gene linking hyperglycemia and functional changes in T2D β-cells. Further investigations demonstrated that RCAN1 methylation is reduced in human T2D islets at multiple sites, correlating with increased expression. RCAN1 protein expression was also increased in db/db mouse islets and in human and mouse islets exposed to high glucose. Mice overexpressing RCAN1 had reduced in vivo glucose-stimulated insulin secretion and their β-cells displayed mitochondrial dysfunction including hyperpolarised membrane potential, reduced oxidative phosphorylation and low ATP production. This lack of β-cell ATP had functional consequences by negatively affecting both glucose-stimulated membrane depolarisation and ATP-dependent insulin granule exocytosis. Thus, from amongst the myriad of gene expression changes occurring in T2D β-cells where we had little knowledge of which changes cause β-cell dysfunction, we applied a trisomy 21 screening approach which linked RCAN1 to β-cell mitochondrial dysfunction in T2D.

Research-Focused Isolation of Human Islets From Donors With and Without Diabetes at the Alberta Diabetes Institute IsletCore

Recent years have seen an increased focus on human islet biology, and exciting findings in the stem cell and genomic arenas highlight the need to define the key features of mature human islets and β-cells. Donor and organ procurement parameters impact human islet yield, although for research purposes islet yield may be secondary in importance to islet function. We examined the feasibility of a research-only human islet isolation, distribution, and biobanking program and whether key criteria such as cold ischemia time (CIT) and metabolic status may be relaxed and still allow successful research-focused isolations, including from donors with type 1 diabetes and type 2 diabetes. Through 142 isolations over approximately 5 years, we confirm that CIT and glycated hemoglobin each have a weak negative impacts on isolation purity and yield, and extending CIT beyond the typical clinical isolation cutoff of 12 hours (to ≥ 18 h) had only a modest impact on islet function. Age and glycated hemoglobin/type 2 diabetes status negatively impacted secretory function; however, these and other biological (sex, body mass index) and procurement/isolation variables (CIT, time in culture) appear to make only a small contribution to the heterogeneity of human islet function. This work demonstrates the feasibility of extending acceptable CIT for research-focused human islet isolation and highlights the biological variation in function of human islets from donors with and without diabetes.

Preserving Mafa expression in diabetic islet β-cells improves glycemic control in vivo

The murine Mafa transcription factor is a key regulator of postnatal islet β-cell activity, affecting insulin transcription, insulin secretion, and β-cell mass. Human MAFA expression is also markedly decreased in islet β-cells of type 2 diabetes mellitus (T2DM) patients. Moreover, levels are profoundly reduced in db/db islet β-cells, a mouse model of T2DM. To examine the significance of this key islet β-cell-enriched protein to glycemic control under diabetic conditions, we generated transgenic mice that conditionally and specifically produced Mafa in db/db islet β-cells. Sustained expression of Mafa resulted in significantly lower plasma glucose levels, higher plasma insulin, and augmented islet β-cell mass. In addition, there was increased expression of insulin, Slc2a2, and newly identified Mafa-regulated genes involved in reducing β-cell stress, like Gsta1 and Gckr. Importantly, the levels of human GSTA1 were also compromised in T2DM islets. Collectively, these results illustrate how consequential the reduction in Mafa activity is to islet β-cell function under pathophysiological conditions.

Natural history of β-cell adaptation and failure in type 2 diabetes

Type 2 diabetes mellitus (T2D) is a complex disease characterized by β-cell failure in the setting of insulin resistance. The current evidence suggests that genetic predisposition, and environmental factors can impair the capacity of the β-cells to respond to insulin resistance and ultimately lead to their failure. However, genetic studies have demonstrated that known variants account for less than 10% of the overall estimated T2D risk, suggesting that additional unidentified factors contribute to susceptibility of this disease. In this review, we will discuss the different stages that contribute to the development of β-cell failure in T2D. We divide the natural history of this process in three major stages: susceptibility, β-cell adaptation and β-cell failure, and provide an overview of the molecular mechanisms involved. Further research into mechanisms will reveal key modulators of β-cell failure and thus identify possible novel therapeutic targets and potential interventions to protect against β-cell failure.

Novel approaches to drug discovery for the treatment of type 2 diabetes

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is a chronic, complex and multifactorial metabolic disorder, which has become a serious global health problem. The side effects of known drugs and the deficiency of long-term safety data, in addition to the already determined adverse effects for the current preclinical drugs against T2DM, have largely called upon the urgent exploration of novel therapeutic and preventative strategies against this disease.

AREAS COVERED

The authors highlight the potential approaches for anti-T2DM drug discovery by focusing on: the restoration of pancreatic β-cell mass, the promotion of insulin secretion, the regulation of oxidative stress and endoplasmic reticulum (ER) stress and the modulation of autophagy.

EXPERT OPINION

T2DM is based on the gradual development of insulin resistance and β-cell dysfunction. Thus, the restoration of β-cell function is considered as one of the promising therapeutic strategies against T2DM. The stress factors, such as oxidative stress, ER stress and autophagy, play potent roles in the regulation of β-cell apoptosis, insulin secretion and sensitivity in the development of T2DM involving complicated cross-talks. Based on multiplex stress-involved regulatory networks, more and more novel potential targets have been discovered and the multi-targeted drug leads are expected to help develop more effective clinical agents for the treatment of T2DM.

Inactivation of specific β cell transcription factors in type 2 diabetes

Type 2 diabetes (T2DM) commonly arises from islet β cell failure and insulin resistance. Here, we examined the sensitivity of key islet-enriched transcription factors to oxidative stress, a condition associated with β cell dysfunction in both type 1 diabetes (T1DM) and T2DM. Hydrogen peroxide treatment of β cell lines induced cytoplasmic translocation of MAFA and NKX6.1. In parallel, the ability of nuclear PDX1 to bind endogenous target gene promoters was also dramatically reduced, whereas the activity of other key β cell transcriptional regulators was unaffected. MAFA levels were reduced, followed by a reduction in NKX6.1 upon development of hyperglycemia in db/db mice, a T2DM model. Transgenic expression of the glutathione peroxidase-1 antioxidant enzyme (GPX1) in db/db islet β cells restored nuclear MAFA, nuclear NKX6.1, and β cell function in vivo. Notably, the selective decrease in MAFA, NKX6.1, and PDX1 expression was found in human T2DM islets. MAFB, a MAFA-related transcription factor expressed in human β cells, was also severely compromised. We propose that MAFA, MAFB, NKX6.1, and PDX1 activity provides a gauge of islet β cell function, with loss of MAFA (and/or MAFB) representing an early indicator of β cell inactivity and the subsequent deficit of more impactful NKX6.1 (and/or PDX1) resulting in overt dysfunction associated with T2DM.

The pancreatic beta cells in human type 2 diabetes

Bell-cell (beta-cell) impairment is central to the development and progression of human diabetes, as a result of the combined effects of genetic and acquired factors. Reduced islet number and/or reduced beta cells amount in the pancreas of individuals with Type 2 diabetes have been consistently reported. This is mainly due to increased beta cell death, not adequately compensated for by regeneration. In addition, several quantitative and/or qualitative defects of insulin secretion have been observed in Type 2 diabetes, both in vivo and ex vivo with isolated islets. All this is associated with modifications of islet cell gene and protein expression. With the identification of several susceptible Type 2 diabetes loci, the role of genotype in affecting beta-cell function and survival has been addressed in a few studies and the relationships between genotype and beta-cell phenotype investigated. Among acquired factors, the importance of metabolic insults (in particular glucotoxicity and lipotoxicity) in the natural history of beta-cell damage has been widely underlined. Continuous improvements in our knowledge of the beta cells in human Type 2 diabetes will lead to more targeted and effective strategies for the prevention and treatment of the disease.

Management of type 2 diabetes: new and future developments in treatment

The increasing prevalence, variable pathogenesis, progressive natural history, and complications of type 2 diabetes emphasise the urgent need for new treatment strategies. Longacting (eg, once weekly) agonists of the glucagon-like-peptide-1 receptor are advanced in development, and they improve prandial insulin secretion, reduce excess glucagon production, and promote satiety. Trials of inhibitors of dipeptidyl peptidase 4, which enhance the effect of endogenous incretin hormones, are also nearing completion. Novel approaches to glycaemic regulation include use of inhibitors of the sodium-glucose cotransporter 2, which increase renal glucose elimination, and inhibitors of 11β-hydroxysteroid dehydrogenase 1, which reduce the glucocorticoid effects in liver and fat. Insulin-releasing glucokinase activators and pancreatic-G-protein-coupled fatty-acid-receptor agonists, glucagon-receptor antagonists, and metabolic inhibitors of hepatic glucose output are being assessed. Early proof of principle has been shown for compounds that enhance and partly mimic insulin action and replicate some effects of bariatric surgery.

Zinc and zinc transporter regulation in pancreatic islets and the potential role of zinc in islet transplantation

The critical trace element zinc is essential for normal insulin production, and plays a central role in cellular protection against apoptosis and oxidative stress. The regulation of zinc within the pancreas and β-cells is controlled by the zinc transporter families ZnT and ZIP. Pancreatic islets display wide variability in the occurrence of these molecules. The zinc transporter, ZnT8 is an important target for autoimmunity in type 1 diabetes. Gene polymorphisms of this transporter confer sensitivity for immunosuppressive drugs used in islet transplantation. Understanding the biology of zinc transport within pancreatic islets will provide insight into the mechanisms of β-cell death, and may well reveal new pathways for improvement of diabetes therapy, including islet transplantation. This review discusses the possible roles of zinc in β-cell physiology with a special focus on islet transplantation.

Renal glucose reabsorption inhibitors to treat diabetes

Current therapies to reduce hyperglycaemia in type 2 diabetes mellitus (T2DM) mostly involve insulin-dependent mechanisms and lose their effectiveness as pancreatic β-cell function declines. In the kidney, filtered glucose is reabsorbed mainly via the high-capacity, low-affinity sodium glucose cotransporter-2 (SGLT2) at the luminal surface of cells lining the first segment of the proximal tubules. Selective inhibitors of SGLT2 reduce glucose reabsorption, causing excess glucose to be eliminated in the urine; this decreases plasma glucose. In T2DM, the glucosuria produced by SGLT2 inhibitors is associated with weight loss, and mild osmotic diuresis might assist a reduction in blood pressure. The mechanism is independent of insulin and carries a low risk of hypoglycaemia. This review examines the potential of SGLT2 inhibitors as a novel approach to the treatment of hyperglycaemia in T2DM.

G-protein-coupled receptor 40 (GPR40) expression and its regulation in human pancreatic islets: the role of type 2 diabetes and fatty acids

BACKGROUND AND AIMS

GPR40 is a membrane-bound receptor paired with medium and long-chain fatty acids (FFA) as endogenous ligands. Its acute activation potentiates insulin secretion from beta cells, whereas prolonged binding might contribute to the deleterious effects of chronic exposure to FFA. Little information is available on the expression of GPR40 and its regulation in human islets (HI).

MATERIAL AND METHODS

HI were prepared by enzymatic digestion and gradient separation from the pancreas of 20 non-diabetic (Ctrl) and 13 type 2 diabetic (T2DM) multiorgan donors, and functional and molecular studies were then performed.

RESULTS

By qualitative and quantitative PCR experiments, mRNA expression was shown in HI. Both in T2DM islets and in Ctrl islets pre-exposed for 24 h to 1.0 mmol/l FFA (palmitate:oleate, 2:1), GPR40 mRNA expression was significantly reduced (p<0.01) in the T2DM cells as compared to Ctrl cells. A significant positive correlation was found between glucose-stimulated insulin secretion and GPR40 expression.

CONCLUSIONS

These results show the expression of GPR40 in human pancreatic islets which are regulated by FFA. The finding that T2DM islets have a lower GPR40 expression, and the correlation of these genes with insulin secretion, raises the possibility of an involvement of GPR40 in human diabetes beta-cell dysfunction.

Role of glucotoxicity and lipotoxicity in the pathophysiology of Type 2 diabetes mellitus and emerging treatment strategies

Type 2 diabetes mellitus is a disease characterized by persistent and progressive deterioration of glucose tolerance. Both insulin resistance and impaired insulin secretion contribute to development of Type 2 diabetes. However, whilst insulin resistance is fully apparent in the pre-diabetic condition, impairment of insulin secretion worsens over the time, being paralleled by a progressive decline in both pancreatic B-cell function and B-cell mass. Intense research has identified a number of genetic variants that may predispose to impaired B-cell function, but such predisposition can be precipitated and worsened by toxic effects of hyperglycaemia (glucotoxicity) and elevated levels of free fatty acids (lipotoxicity). All these aspects of the pathogenesis of Type 2 diabetes are discussed in this review. Moreover, treatments that target reduction in glucotoxicity or lipotoxicity are outlined, including emerging strategies that target the role of glucagon-like peptide 1 and sodium glucose co-transporter 2.

Glucose-modulated tyrosine nitration in beta cells: targets and consequences

Hyperglycemia, key factor of the pre-diabetic and diabetic pathology, is associated with cellular oxidative stress that promotes oxidative protein modifications. We report that protein nitration is responsive to changes in glucose concentrations in islets of Langerhans and insulinoma beta cells. Alterations in the extent of tyrosine nitration as well as the cellular nitroproteome profile correlated tightly with changing glucose concentrations. The target proteins we identified function in protein folding, energy metabolism, antioxidant capacity, and membrane permeability. Nitration of heat shock protein 60 in vitro was found to decrease its ATP hydrolysis and interaction with proinsulin, suggesting a mechanism by which protein nitration could diminish insulin secretion. This was supported by our finding of a decrease in stimulated insulin secretion following glycolytic stress in cultured cells. Our results reveal that protein tyrosine nitration may be a previously unrecognized factor in beta-cell dysfunction and the pathogenesis of diabetes.

Beneficial Effect of the Nonpeptidyl Low Molecular Weight Radical Scavenger IAC on Cultured Human Islet Function

We examined a possible protective effect of the nonpeptidyl low molecular weight radical scavenger IAC [bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)decanedioate di-hydrochloride] on isolated human islet cells against isolation and culture oxidative stress. Islets isolated from pancreases of nondiabetic multiorgan donors by collagenase digestion were purified by density gradient centrifugation. After the isolation, islets were either exposed or not exposed for 7 days to 10 μmol/L IAC. We found that IAC markedly reduced oxidative stress and ameliorated islets function. These results suggest that the use of IAC could be an interesting pharmacological approach for the treatment of the islets before transplantation.