Nutrient-dependent regulation of β-cell proinsulin content

Insulin is made from proinsulin, but the extent to which fasting/feeding controls the homeostatically regulated proinsulin pool in pancreatic β-cells remains largely unknown. Here, we first examined β-cell lines (INS1E and Min6, which proliferate slowly and are routinely fed fresh medium every 2-3 days) and found that the proinsulin pool size responds to each feeding within 1 to 2 h, affected both by the quantity of fresh nutrients and the frequency with which they are provided. We observed no effect of nutrient feeding on the overall rate of proinsulin turnover as quantified from cycloheximide-chase experiments. We show that nutrient feeding is primarily linked to rapid dephosphorylation of translation initiation factor eIF2α, presaging increased proinsulin levels (and thereafter, insulin levels), followed by its rephosphorylation during the ensuing hours that correspond to a fall in proinsulin levels. The decline of proinsulin levels is blunted by the integrated stress response inhibitor, ISRIB, or by inhibition of eIF2α rephosphorylation with a general control nonderepressible 2 (not PERK) kinase inhibitor. In addition, we demonstrate that amino acids contribute importantly to the proinsulin pool; mass spectrometry shows that β-cells avidly consume extracellular glutamine, serine, and cysteine. Finally, we show that in both rodent and human pancreatic islets, fresh nutrient availability dynamically increases preproinsulin, which can be quantified without pulse-labeling. Thus, the proinsulin available for insulin biosynthesis is rhythmically controlled by fasting/feeding cycles.

Beta Cell Dysfunction in Youth- and Adult-Onset Type 2 Diabetes: An Extensive Narrative Review with a Special Focus on the Role of Nutrients

Traditionally a disease of adults, type 2 diabetes (T2D) has been increasingly diagnosed in youth, particularly among adolescents and young adults of minority ethnic groups. Especially, during the recent COVID-19 pandemic, obesity and prediabetes have surged not only in minority ethnic groups but also in the general population, further raising T2D risk. Regarding its pathogenesis, a gradually increasing insulin resistance due to central adiposity combined with a progressively defective β-cell function are the main culprits. Especially in youth-onset T2D, a rapid β-cell activity decline has been observed, leading to higher treatment failure rates, and early complications. In addition, it is well established that both the quantity and quality of food ingested by individuals play a key role in T2D pathogenesis. A chronic imbalance between caloric intake and expenditure together with impaired micronutrient intake can lead to obesity and insulin resistance on one hand, and β-cell failure and defective insulin production on the other. This review summarizes our evolving understanding of the pathophysiological mechanisms involved in defective insulin secretion by the pancreatic islets in youth- and adult-onset T2D and, further, of the role various micronutrients play in these pathomechanisms. This knowledge is essential if we are to curtail the serious long-term complications of T2D both in pediatric and adult populations.

The CLOCK protein regulates insulin secretion related with L-type calcium channels in rat pancreatic beta cells

BACKGROUND

Circadian locomotor output cycles kaput protein (CLOCK) plays a crucial role in glucose homeostasis and controlling insulin secretion. However, the mechanism of the CLOCK regulating rhythmic insulin secretion has not been fully understood.

METHODS

Rhythmic expression of the CLOCK in rat pancreatic beta cell was detected. INS-1 cells were transfected with siRNAs to knockdown the CLOCK before the cells were incubated with different concentrations of glucose. Insulin secretion was analyzed by ELISA method. Expression of the L-type calcium channel protein (Cav1.2, Cacna1c) was determined both in the CLOCK-knockdown cells and the control cells. Calcium influx was probed by fluorescent. Chromatin immunoprecipitation (ChIP) test and dual-luciferase reporter gene experiments were applied to verify the relationship between the CLOCK and Cav1.2.

RESULTS

The CLOCK is abundantly expressed in rat pancreatic beta cells. Transcription level of the CLOCK showed rhythmicity in the beta cells. Compared to the control group, insulin release was significantly impaired with 25 mM glucose incubation in the CLOCK-knockdown group, but not showed with 2.5 mM glucose incubation. The expression of Cav1.2 and the influx of calcium were significantly decreased in the CLOCK-knockdown group with 25 mM glucose incubation. ChIP test indicted that the CLOCK bound to -444∼-454 region of the Cacna1c promoter of the INS-1 cells, but the binding was significantly reduced following the CLOCK-knockdown. Luciferase experiment was in accordance with the finding of ChIP.

CONCLUSIONS

The CLOCK mediating Cav1.2 expression may point out a potential pathway of circadian rhythm affecting insulin secretion.

Serum Orosomucoid Is Associated with Serum Adiponectin, Adipose Tissue Insulin Resistance Index, and a Family History of Type 2 Diabetes in Young Normal Weight Japanese Women

INTRODUCTION

Adipose tissue (AT) expandability may be facilitated by adiponectin and suppressed by orosomucoid, and reduced AT expandability may be associated with first-degree relatives of type 2 diabetes. We tested the hypothesis that orosomucoid may be associated not only with adiponectin and adipose tissue insulin resistance but also with a family history of type 2 diabetes (FHD). Research Design and Methods. Anthropometric and metabolic variables, adipokines, and measures of inflammatory and insulin resistance were cross-sectionally investigated in 153 young normal weight Japanese women. Stepwise multivariate linear regression analyses were used to identify the most important determinants of orosomucoid.

RESULTS

Orosomucoid was higher in women with positive (n = 57) compared to women with negative FHD and was associated positively with FHD (both p = 0.01). Orosomucoid also showed positive associations with fasting glucose (p < 0.001), free fatty acids (p = 0.001), and HbA1c (p = 0.007), whereas there was no association with fasting insulin and serum lipids. In addition, orosomucoid was associated inversely with adiponectin (p = 0.02) and positively with adipose tissue-insulin resistance index (AT-IR, the product of fasting insulin and free fatty acids; p = 0.001) but not with homeostasis model assessment-insulin resistance, leptin, and high-sensitivity C-reactive protein. In multivariate analyses, AT-IR (standardized β, 0.22; p = 0.003), serum adiponectin (standardized β, -0.163; p = 0.032), FHD+ (standardized β, 0.178; p = 0.029), and HbA1c (standardized β, 0.213; p = 0.005) emerged as independent determinants of orosomucoid and explained 15.2% of its variability.

CONCLUSIONS

These results are the first to demonstrate that orosomucoid is associated not only with adipose tissue-insulin resistance and adiponectin but also with FHD.

Carbon monoxide enhances calcium transients and glucose-stimulated insulin secretion from pancreatic β-cells by activating Phospholipase C signal pathway in diabetic mice

In early stage of diabetes, insulin secretion from pancreatic β-cells is increased to deal with the elevated blood glucose. Previous studies have reported that islet-produced carbon monoxide (CO) is associated with increased glucose-stimulated insulin secretion from β-cells. However, this compensatory mechanism by which CO may act to enhance β-cell function remain unclear. In this study, we revealed that CO promoted intracellular calcium ([Ca²⁺]_i) elevation and glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells in leptin receptor deficient db/db mice but not in C57 mice. The stimulatory effects of CO on β-cell function in db/db mice was blocked by inhibition of Phospholipase C (PLC) signaling pathway. We further demonstrated that CO triggered [Ca²⁺]_i transients and enhanced GSIS in C57 islets when β-cells overexpressed with PLCγ1 and PLCδ1, but not PLCβ1. On the other hand, reducing PLCγ1 and PLCδ1 expressions in db/db islets dramatically attenuated the stimulatory effects of CO on β-cell function, whereas interfering PLCβ1 expression had no effects on CO-induced β-cell function enhancement. Our findings showing that CO elevated [Ca²⁺]_i and enhanced GSIS by activating PLC signaling through PLCγ1 and PLCδ1 isoforms in db/db pancreatic β-cells may suggest an important mechanism by which CO promotes β-cell function to prevent hyperglycemia. Our study may also provide new insights into the therapy for type II diabetes and offer a potential target for therapeutic applications of CO.

Haplotypes of the genes (GCK and G6PC2) underlying the glucose/glucose-6-phosphate cycle are associated with pancreatic beta cell glucose sensitivity in patients with newly diagnosed type 2 diabetes from the VNDS study (VNDS 11)

BACKGROUND

Elevated fasting plasma glucose has been associated with increased risk for development of type 2 diabetes (T2D). The balance between glucokinase (GCK) and glucose-6-phosphate catalytic subunit 2 (G6PC2) activity are involved in glucose homeostasis through glycolytic flux, and subsequent insulin secretion.

AIM

In this study, we evaluated the association between the genetic variability of G6PC2 and GCK genes and T2D-related quantitative traits.

METHODS

In 794 drug-naïve, GADA-negative, newly diagnosed T2D patients (VNDS; NTC01526720) we performed: genotyping of 6 independent tag-SNPs within GCK gene and 5 tag-SNPs within G6PC2 gene; euglycaemic insulin clamp to assess insulin sensitivity; OGTT to estimate beta-cell function (derivative and proportional control; DC, PC) by mathematical modeling. Genetic association analysis has been conducted using Plink software.

RESULTS

Two SNPs within GCK gene (rs882019 and rs1303722) were associated to DC in opposite way (both p < 0.004). Two G6PC2 variants (rs13387347 and rs560887) were associated to both parameters of insulin secretion (DC and PC) and to fasting C-peptide levels (all p < 0.038). Moreover, subjects carrying the A allele of rs560887 showed higher values of 2h-plasma glucose (2hPG) (p = 0.033). Haplotype analysis revealed that GCK (AACAAA) haplotype was associated to decreased fasting C-peptide levels, whereas, the most frequent haplotype of G6PC2 (GGAAG) was associated with higher fasting C-peptide levels (p = 0.001), higher PC (β = 6.87, p = 0.022) and the lower 2hPG (p = 0.012).

CONCLUSION

Our findings confirmed the role of GCK and G6PC2 in regulating the pulsatility in insulin secretion thereby influencing insulin-signaling and leading to a gradual modulation in glucose levels in Italian patients with newly diagnosed T2D.

Schisandrin C Affects Glucose-Stimulated Insulin Secretion in Pancreatic β-Cells and Glucose Uptake in Skeletal Muscle Cells

The aim of our study was to investigate the effect of three lignans (schisandrol A, schisandrol B, and schisandrin C) on insulin secretion in rat INS-1 pancreatic β-cells and glucose uptake in mouse C2C12 skeletal muscle cells. Schisandrol A and schisandrin C enhanced insulin secretion in response to high glucose levels with no toxic effects on INS-1 cells. The effect of schisandrin C was superior to that of gliclazide (positive control), a drug commonly used to treat type 2 diabetes (T2D). In addition, western blot analysis showed that the expression of associated proteins, including peroxisome proliferator-activated receptor γ (PPARγ), pancreatic and duodenal homeobox 1 (PDX-1), phosphatidylinositol 3-kinase (PI3K), Akt, and insulin receptor substrate-2 (IRS-2), was increased in INS-1 cells after treatment with schisandrin C. In addition, insulin secretion effect of schisandrin C were enhanced by the Bay K 8644 (L-type Ca²⁺ channel agonist) and glibenclamide (K⁺ channel blocker), were abolished by the nifedipine (L-type Ca²⁺ channel blocker) and diazoxide (K⁺ channel activator). Moreover, schisandrin C enhanced glucose uptake with no toxic effects on C2C12 cells. Western blot analysis showed that the expression of associated proteins, including insulin receptor substrate-1 (IRS-1), AMP-activated protein kinase (AMPK), PI3K, Akt, glucose transporter type 4 (GLUT-4), was increased in C2C12 cells after treatment with schisandrin C. Schisandrin C may improve hyperglycemia by enhancing insulin secretion in pancreatic β-cells and improving glucose uptake into skeletal muscle cells. Our findings may provide evidence that schisandrin C may be beneficial in devising novel anti-T2D strategies.

Exploring Insulin Production Following Alveolar Islet Transplantation (AIT)

Recent studies have demonstrated the feasibility of islet implantation into the alveoli. However, until today, there are no data on islet behavior and morphology at their transplant site. This study is the first to investigate islet distribution as well insulin production at the implant site. Using an ex vivo postmortem swine model, porcine pancreatic islets were isolated and aerosolized into the lung using an endoscopic spray-catheter. Lung tissue was explanted and bronchial airways were surgically isolated and connected to a perfusor. Correct implantation was confirmed via histology. The purpose of using this new lung perfusion model was to measure static as well as dynamic insulin excretions following glucose stimulation. Alveolar islet implantation was confirmed after aerosolization. Over 82% of islets were correctly implanted into the intra-alveolar space. The medium contact area to the alveolar surface was estimated at 60 +/− 3% of the total islet surface. The new constructed lung perfusion model was technically feasible. Following static glucose stimulation, insulin secretion was detected, and dynamic glucose stimulation revealed a biphasic insulin secretion capacity during perfusion. Our data indicate that islets secrete insulin following implantation into the alveoli and display an adapted response to dynamic changes in glucose. These preliminary results are encouraging and mark a first step toward endoscopically assisted islet implantation in the lung.

Collagen type I enhances cell growth and insulin biosynthesis in rat pancreatic cells

Type I collagen (collagen I) is the most abundant component of the extracellular matrix (ECM) in the pancreas. We previously reported that collagen I-coated culture dishes enhanced proliferation of rat pancreatic β cell line, INS-1 cells, via up-regulation of β-catenin nuclear translocation. In this study, we further investigated the effects of collagen I on insulin production of INS-1 cells. The results indicate that insulin synthesis as well as cell proliferation is increased in the INS-1 cells cultured on the dishes coated with collagen I. Up-regulation of insulin-like growth factor 1 receptor (IGF-1R) on the INS-1 cells cultured on the collagen-coated dishes is involved in up-regulation of cell proliferation and increase of insulin biosynthesis; however, up-regulation of insulin secretion in the INS-1 cells on collagen I-coated dishes was further enhanced by inhibition of IGF-1R. Autophagy of INS-1 cells on collagen I-coated dishes was repressed via IGF-1R upregulation, and inhibition of autophagy with 3MA further enhanced cell proliferation and insulin biosynthesis but did not affect insulin secretion. E-cadherin/β-catenin adherent junction complexes are stabilized by autophagy. That is, autophagy negatively regulates the nuclear translocation of β-catenin that leads to insulin biosynthesis and cell proliferation. In conclusion, IGF-1R/downregulation of autophagy/nuclear translocation of β-catenin is involved in collagen I-induced INS-1 cell proliferation and insulin synthesis.

Candidate plasticity gene 16 and jun dimerization protein 2 are involved in the suppression of insulin gene expression in rat pancreatic INS-1 β-cells

Chronic hyperglycemia causes pancreatic β-cell dysfunction, impaired insulin secretion and the suppression of insulin gene expression. This phenomenon is referred to as glucotoxicity, and is a critical component of the pathogenesis of type 2 diabetes. We previously reported that the expression of candidate plasticity gene 16 (CPG16) was higher in rat pancreatic INS-1 β-cells under glucotoxic conditions and CPG16 suppressed insulin promoter activity. However, the molecular mechanisms of the CPG16-mediated suppression of insulin gene expression are unclear. In this study, we found that CPG16 directly bound and phosphorylated jun dimerization protein 2 (JDP2), an AP-1 family transcription factor. CPG16 co-localized with JDP2 in the nucleus of INS-1 cells. JDP2 bound to the G1 element of the insulin promoter and up-regulated promoter activity. Finally, CPG16 suppressed the up-regulation of insulin promoter activity by JDP2 in a kinase activity-dependent manner. These results suggest that CPG16 suppresses insulin promoter activity by phosphorylating JDP2.

Evidence that extrapancreatic insulin production is involved in the mediation of sperm survival

Evidence is presented for expression of the insulin receptor on the surface of mammalian spermatozoa as well as transcripts for the receptor substrate adaptor proteins (IRS1-4) needed to mediate insulin action. Exposure to this hormone resulted in insulin receptor phosphorylation (pTyr972), activation of AKT (pSer473) and the stimulation of sperm motility. Intriguingly, the male germ line is also shown to be capable of generating insulin, possessing the relevant mRNA transcript and expressing strong immunocytochemical signals for both insulin and C-peptide. Insulin could be released from the spermatozoa by sonication in a concentration-dependent manner but was not secreted in response to glucose, fructose or stimulation with progesterone. However, insulin release could be induced by factors present in human uterine lavages. Furthermore, the endometrium was also shown to possess the machinery for insulin production and action (mRNA, insulin, C-peptide, proprotein convertase and insulin receptor), releasing insulin into the uterine lumen prior to ovulation. These studies emphasize the fundamental importance of extra-pancreatic insulin in regulating the reproductive process, particularly in the support of spermatozoa on their perilous voyage to the site of fertilization.

Optogenetic stimulation of cholinergic fibers for the modulation of insulin and glycemia

Previous studies have demonstrated stimulation of endocrine pancreas function by vagal nerve electrical stimulation. While this increases insulin secretion, expected concomitant reductions in circulating glucose do not occur. A complicating factor is the non-specific nature of electrical nerve stimulation. Optogenetic tools, however, provide the potential for cell-type specific neural stimulation using genetic targeting and/or spatially shaped excitation light. Here, we demonstrate light-activated stimulation of the endocrine pancreas by targeting parasympathetic (cholinergic) axons. In a mouse model expressing ChannelRhodopsin2 (ChR2) in cholinergic cells, serum insulin and glucose were measured in response to (1) ultrasound image-guided optical stimulation of axon terminals in the pancreas or (2) optical stimulation of axons of the cervical vagus nerve. Measurements were made in basal-glucose and glucose-stimulated conditions. Significant increases in plasma insulin occurred relative to controls under both pancreas and cervical vagal stimulation, while a rapid reduction in glycemic levels were observed under pancreatic stimulation. Additionally, ultrasound-based measurements of blood flow in the pancreas were increased under pancreatic stimulation. Together, these results demonstrate the utility of in-vivo optogenetics for studying the neural regulation of endocrine pancreas function and suggest its therapeutic potential for the control of insulin secretion and glucose homeostasis.

The dual role of RFX6 in directing β cell development and insulin production

RFX6 transcription factor is believed to play a central role in directing cell development of insulin-producing pancreatic islet. RFX6 homozygous mutations cause syndromic neonatal diabetes with hypoplastic pancreas. However, RFX6 heterozygous mutations cause maturity-onset diabetes of the young (MODY) with normal pancreas development. Here, we show that RFX6 may control islet cell development and insulin production in different manners. The rfx6 knockout zebrafish generated by CRISPR/Cas9 exhibited an overt diabetes phenotype. Pancreatic islet failed to form compact structures in the knockout fish. While endocrine pancreatic islet non-β-cells were absent, insulin-producing β-cells were present in the knockout fish. Although insulin mRNA level was normal in the β-cells of the knockout fish, insulin protein level was decreased. High-throughput RNA sequencing (RNAseq) showed that differentially expressed genes were enriched in the translation term in islet β-cells from the knockout fish. Chromatin immunoprecipitation sequencing (ChIPseq) of normally developed islet β-cells from mice demonstrated that rfx6 interacted with translation initiation factors and controlled insulin translation. Our data indicate that Rfx6 may act as a transcription factor regulating the transcription of genes involved in mRNA translation, which may represent a new mechanism and treatment strategy for diseases.

Normal and defective pathways in biogenesis and maintenance of the insulin storage pool

Both basal and glucose-stimulated insulin release occur primarily by insulin secretory granule exocytosis from pancreatic β cells, and both are needed to maintain normoglycemia. Loss of insulin-secreting β cells, accompanied by abnormal glucose tolerance, may involve simple exhaustion of insulin reserves (which, by immunostaining, appears as a loss of β cell identity), or β cell dedifferentiation, or β cell death. While various sensing and signaling defects can result in diminished insulin secretion, somewhat less attention has been paid to diabetes risk caused by insufficiency in the biosynthetic generation and maintenance of the total insulin granule storage pool. This Review offers an overview of insulin biosynthesis, beginning with the preproinsulin mRNA (translation and translocation into the ER), proinsulin folding and export from the ER, and delivery via the Golgi complex to secretory granules for conversion to insulin and ultimate hormone storage. All of these steps are needed for generation and maintenance of the total insulin granule pool, and defects in any of these steps may, weakly or strongly, perturb glycemic control. The foregoing considerations have obvious potential relevance to the pathogenesis of type 2 diabetes and some forms of monogenic diabetes; conceivably, several of these concepts might also have implications for β cell failure in type 1 diabetes.

Pharmacological Inhibition of Inositol-Requiring Enzyme 1α RNase Activity Protects Pancreatic Beta Cell and Improves Diabetic Condition in Insulin Mutation-Induced Diabetes

β-cell ER stress plays an important role in β-cell dysfunction and death during the pathogenesis of diabetes. Proinsulin misfolding is regarded as one of the primary initiating factors of ER stress and unfolded protein response (UPR) activation in β-cells. Here, we found that the ER stress sensor inositol-requiring enzyme 1α (IRE1α) was activated in the Akita mice, a mouse model of mutant insulin gene-induced diabetes of youth (MIDY), a monogenic diabetes. Normalization of IRE1α RNase hyperactivity by pharmacological inhibitors significantly ameliorated the hyperglycemic conditions and increased serum insulin levels in Akita mice. These benefits were accompanied by a concomitant protection of functional β-cell mass, as shown by the suppression of β-cell apoptosis, increase in mature insulin production and reduction of proinsulin level. At the molecular level, we observed that the expression of genes associated with β-cell identity and function was significantly up-regulated and ER stress and its associated inflammation and oxidative stress were suppressed in islets from Akita mice treated with IRE1α RNase inhibitors. This study provides the evidence of the in vivo efficacy of IRE1α RNase inhibitors in Akita mice, pointing to the possibility of targeting IRE1α RNase as a therapeutic direction for the treatment of diabetes.

The mediation by GLP-1 receptors of glucagon-induced insulin secretion revisited in GLP-1 receptor knockout mice

To study whether activation of GLP-1 receptors importantly contributes to the insulinotropic action of exogenously administered glucagon, we have performed whole animal experiments in normal mice and in mice with GLP-1 receptor knockout. Glucagon (1, 3 or 10 μg/kg), the GLP-1 receptor antagonist exendin 9-39 (30 nmol/kg), glucose (0.35 g/kg) or the incretin hormone glucose-dependent insulinotropic polypeptide (GIP; 3 nmol/kg) was injected intravenously or glucose (75 mg) was given orally through gavage. Furthermore, islets were isolated and incubated in the presence of glucose with or without glucagon. It was found that the insulin response to intravenous glucagon was preserved in GLP-1 receptor knockout mice but that glucagon-induced insulin secretion was markedly suppressed in islets from GLP-1 receptor knockout mice. Similarly, the GLP-1 receptor antagonist markedly suppressed glucagon-induced insulin secretion in wildtype mice. These data suggest that GLP-1 receptors contribute to the insulinotropic action of glucagon and that there is a compensatory mechanism in GLP-1 receptor knockout mice that counteracts a reduced effect of glucagon. Two potential compensatory mechanisms (glucose and GIP) were explored. However, neither of these seemed to explain why the insulin response to glucagon is not suppressed in GLP-1 receptor knockout mice. Based on these data we confirm the hypothesis that glucagon-induced insulin secretion is partially mediated by GLP-1 receptors on the beta cells and we propose that a compensatory mechanism, the nature of which remains to be established, is induced in GLP-1 receptor knockout mice to counteract the expected impaired insulin response to glucagon in these mice.

Structural Lessons From the Mutant Proinsulin Syndrome

Insight into folding mechanisms of proinsulin has been provided by analysis of dominant diabetes-associated mutations in the human insulin gene (INS). Such mutations cause pancreatic β-cell dysfunction due to toxic misfolding of a mutant proinsulin and impairment in trans of wild-type insulin secretion. Anticipated by the "Akita" mouse (a classical model of monogenic diabetes mellitus; DM), this syndrome illustrates the paradigm endoreticulum (ER) stress leading to intracellular proteotoxicity. Diverse clinical mutations directly or indirectly perturb native disulfide pairing leading to protein misfolding and aberrant aggregation. Although most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships, but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of non-syndromic Type 2 DM. Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of non-foldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

Insulin-like and mimetic molecules from non-mammalian organisms: potential relevance for drug discovery

Insulin was first discovered in extracts of vertebrate pancreas during a focused search for a therapy for diabetes. Subsequent efforts to discover and isolate a similar active principle from yeast and plants driven by the hope to identify insulin-like/mimetic molecules with critical advantages in the pharmacokinetic profile and expenditure of production compared to authentic human insulin were not successful. As a consequence, it has generally been assumed that hormones evolved exclusively during course of the evolution of vertebrate endocrine organs, implying a rather recent origin. Concomitantly, the existence and physiological role of vertebrate hormones in lower multi- and unicellular eukaryotes have remained a rather controversial subject over decades, albeit there is some evidence that hormones and hormone-binding proteins resembling those of vertebrates are expressed in fungi and yeast. Past and recent findings on the existence of insulin-like and mimetic materials, such as the glucose tolerance factor, in lower eukaryotes, in particular Neurospora crassa and yeast, will be presented. These data provide further evidence for the provocative view that the evolutionary roots of the vertebrate endocrine system may be far more ancient than is generally believed and that the identification and characterisation of insulin-like/mimetic molecules from lower eukaryotes may be useful for future drug discovery efforts.

Management of refractory hypoglycaemia in a metastatic neuroendocrine tumour co-secreting serotonin and insulin

A 27-year-old otherwise healthy man of African descent presented to the hospital with initial symptoms of carcinoid syndrome that later evolved into symptoms of hyperinsulinemic hypoglycaemia. Investigations revealed a metastatic neuroendocrine tumour (NET), co-secreting both serotonin and insulin. Management involved a multimodal approach in an attempt to reduce tumour burden and achieve euglycaemia, which proved to be a significant challenge in the face of refractory hypoglycaemia despite the administration of multiple prohyperglycaemic agents in combination. Unfortunately, given the burden of metastatic disease and multiple medical complications that ensued, the patient passed away. This case highlights the clinical history of a NET co-secreting serotonin and insulin, the use of combination therapy in the treatment of refractory hypoglycaemia in a metastatic insulin-producing tumour and emerging therapeutic modalities in the treatment of these rare malignancies.

Presumption of innocence for beta cells: why are they vulnerable autoimmune targets in type 1 diabetes?

It is increasingly appreciated that the pathogenic mechanisms of type 1 diabetes involve both the autoimmune aggressors and their beta cell targets, which engage in a conflicting dialogue within and possibly outside the pancreas. Indeed, autoimmune CD8⁺ T cells, which are the final mediators of beta cell destruction, circulate at similar frequencies in type 1 diabetic and healthy individuals. Hence a universal state of 'benign' islet autoimmunity exists, and we hypothesise that its progression to type 1 diabetes may at least partially rely on a higher vulnerability of beta cells, which play a key, active role in disease development and/or amplification. We posit that this autoimmune vulnerability is rooted in some features of beta cell biology: the stress imposed by the high rate of production of insulin and other granule proteins, their dense vascularisation and the secretion of their products directly into the bloodstream. Gene variants that may predispose individuals to this vulnerability have been identified, e.g. MDA5, TYK2, PTPN2. They interact with environmental cues, such as viral infections, that may drive this genetic potential towards exacerbated local inflammation and progressive beta cell loss. On top of this, beta cells set up compensatory responses, such as the unfolded protein response, that become deleterious in the long term. The relative contribution of immune and beta cell drivers may vary and phenotypic subtypes (endotypes) are likely to exist. This dual view argues for the use of circulating biomarkers of both autoimmunity and beta cell stress for disease staging, and for the implementation of both immunomodulatory and beta cell-protective therapeutic strategies. Graphical abstract.

The making of insulin in health and disease

The discovery of insulin in 1921 has been one of greatest scientific achievements of the 20th century. Since then, the availability of insulin has shifted the focus of diabetes treatment from trying to keep patients alive to saving and improving the life of millions. Throughout this time, basic and clinical research has advanced our understanding of insulin synthesis and action, both in healthy and pathological conditions. Yet, multiple aspects of insulin production remain unknown. In this review, we focus on the most recent findings on insulin synthesis, highlighting their relevance in diabetes. Graphical abstract.

Glucose homeostasis is impaired in mice deficient in the neuropeptide 26RFa (QRFP)

INTRODUCTION

26RFa (pyroglutamyl RFamide peptide (QRFP)) is a biologically active peptide that has been found to control feeding behavior by stimulating food intake, and to regulate glucose homeostasis by acting as an incretin. The aim of the present study was thus to investigate the impact of 26RFa gene knockout on the regulation of energy and glucose metabolism.

RESEARCH DESIGN AND METHODS

26RFa mutant mice were generated by homologous recombination, in which the entire coding region of prepro26RFa was replaced by the iCre sequence. Energy and glucose metabolism was evaluated through measurement of complementary parameters. Morphological and physiological alterations of the pancreatic islets were also investigated.

RESULTS

Our data do not reveal significant alteration of energy metabolism in the 26RFa-deficient mice except the occurrence of an increased basal metabolic rate. By contrast, 26RFa mutant mice exhibited an altered glycemic phenotype with an increased hyperglycemia after a glucose challenge associated with an impaired insulin production, and an elevated hepatic glucose production. Two-dimensional and three-dimensional immunohistochemical experiments indicate that the insulin content of pancreatic β cells is much lower in the 26RFa^-/- mice as compared with the wild-type littermates.

CONCLUSION

Disruption of the 26RFa gene induces substantial alteration in the regulation of glucose homeostasis, with in particular a deficit in insulin production by the pancreatic islets. These findings further support the notion that 26RFa is an important regulator of glucose homeostasis.

Long-term effects of intracranial islet grafting on cognitive functioning in a rat metabolic model of sporadic Alzheimer's disease-like dementia

Accumulating evidence suggests that Alzheimer’s disease is associated with brain insulin resistance, as are some other types of dementia. Intranasal insulin administration has been suggested as a potential approach to overcoming brain insulin resistance and improving cognitive functions. Islet transplantation into the cranial subarachnoid cavity was used as an alternative route for insulin delivery into the brain. Recently, the authors showed the short-term beneficial cognitive effect of a small number of intracranially grafted islets in rats with cognitive dysfunction induced by intracerebroventricular administration of streptozotocin (icv-STZ). This was associated with continuous and safe insulin delivery to the rat brain. The current study investigated the long-term effect of intracranial grafting of islets on cognitive functioning in icv-STZ rats. Severe dementia, associated with obesity and cerebral amyloid-β angiopathy, was induced in Lewis inbred rats by icv-STZ. Two months after icv-STZ, one hundred syngeneic islets were transplanted into the cranial subarachnoid space. Two and six months later, cognitive alterations were assessed by Morris water-maze tests. Islet graft survival was evaluated by immunohistochemical and biochemical assays. Improvement was found in spatial learning and memory of grafted rats as opposed to the sham-operated icv-STZ rats. The grafted islets showed intact morphology, intensive expression of insulin, glucagon and glucose transporter 2. Normoglycemic obesity and cerebral amyloid-β angiopathy were found in both grafted and sham-operated icv-STZ rats. In conclusion, islet grafting into cranial subarachnoid space provides an efficient and safe approach for insulin delivery to the brain, leading to a long-term attenuation of icv-STZ-induced cognitive dysfunction.

Efficacy and Safety of Stem Cell Therapy for T1DM: An Updated Systematic Review and Meta-Analysis

BACKGROUND

The long-term insulin therapy for type 1 diabetes mellitus (T1DM) fails to achieve optimal glycemic control and avoid adverse events simultaneously. Stem cells have unique immunomodulatory capacities and have been considered as a promising interventional strategy for T1DM. Stem cell therapy in T1DM has been tried in many studies. However, the results were controversial. We thus performed a meta-analysis to update the efficacy and safety of stem cell therapy in patients with T1DM.

METHODS

We systematically searched the Medline, EMBASE, Cochrane Central Register of Controlled Trials, ClinicalTrials.gov, Web of Science, Wan Fang Data, China National Knowledge Infrastructure, VIP database, and the Chinese Biomedical Literature Database (SinoMed) for relevant studies published before March 19, 2019. The outcomes included parameters for glycemic control (i.e., glycosylated hemoglobin (HbA1c) levels and insulin dosages), β cell function (i.e., fasting C-peptide levels and area-under-curve of C-peptide concentration (AUCC)), and relative risk of adverse events. Statistical analysis was conducted by using RevMan 5.3 and Stata 12.0.

RESULTS

Five randomized controlled trials (RCTs) and eight nonrandomized concurrent control trials (NRCCTs) with a total of 396 individuals were finally included into the meta-analysis. Among RCTs, stem cell therapy could significantly reduce HbA1c levels (MD = -1.20, 95% CI -1.91 to -0.49, P = 0.0009) and increase fasting C-peptide levels (MD = 0.25, 95% CI 0.04 to 0.45, P = 0.02) and AUCC (SMD = 0.66, 95% CI 0.13 to 1.18, P = 0.01). Stem cell therapy could also reduce insulin dosages (SMD = -2.65, 95% CI -4.86 to -0.45, P = 0.02) at 6 months after treatment. NRCCTs also had consistent results. Furthermore, RCTs showed stem cell therapy did not increase relative risk of gastrointestinal symptom (RR = 0.69, 95% CI 0.14 to 3.28, P = 0.64) and infection (RR = 0.97, 95% CI 0.40 to 2.34, P = 0.95). However, NRCCTs showed stem cell therapy increased relative risk of gastrointestinal symptom (RR = 44.49, 95% CI 9.20 to 215.18, P < 0.00001).

CONCLUSION

Stem cell therapy for T1DM may improve glycemic control and β cell function without increasing the risk of serious adverse events. Stem cell therapy may also have a short-term (3-6 months) effect on reducing insulin dosages.

Study of Possible Relation between Fasting Plasma Glucagon, Gestational Diabetes and Development of Type 2 DM

BACKGROUND AND AIMS

Women who develop GDM (gestational diabetes mellitus) have a relative insulin secretion deficiency, the severity of which may be predictive for later development of diabetes. This study aimed to investigate the role of fasting plasma glucagon in the prediction of later development of diabetes in pregnant women with GDM.

MATERIALS AND METHODS

The study was conducted on 150 pregnant women with GDM after giving informed oral and written consents and being approved by the research ethical committee according to the declaration of Helsinki. The study was conducted in two phases, first phase during pregnancy and the second one was 6 months post-partum, as we measured fasting plasma glucagon before and after delivery together with fasting and 2 hour post-prandial plasma sugar.

RESULTS

Our findings suggested that glucagon levels significantly increased after delivery in the majority 14/25 (56%) of GDM women who developed type 2 DM within 6 months after delivery compared to 6/20 (30%) patients with impaired fasting plasma glucose (IFG) and only 22/105 (20%) non DM women, as the median glucagon levels were 80,76, 55, respectively. Also, there was a high statistical difference between fasting plasma glucagon post-delivery among diabetic and non-diabetic women (p ≤ 0.001). These results indicated the useful role of assessing fasting plasma glucagon before and after delivery in patients with GDM to predict the possibility of type 2 DM.

CONCLUSION

There is a relatively high glucagon level in GDM patients, which is a significant pathogenic factor in the incidence of subsequent diabetes in women with a history of GDM. This could be important in the design of follow-up programs for women with previous GDM.

Anti-diabetic effect of -D-mannuronic acid (M2000) as a novel NSAID with immunosuppressive property on insulin production, blood glucose, and inflammatory markers in the experimental diabetes model

This research aimed to evaluate the anti-diabetic effects of β-d-mannuronic acid (M2000) on blood glucose, insulin production, and inflammatory markers in streptozotocin-induced diabetic rats. Our data showed that the final fasting serum glucose level was significantly lower in the M2000-treated group compared to the diabetic control group (p < .05). In addition, the final fasting serum insulin level significantly increased in the M2000-treated group compared to the diabetic control group (p < .05). Our finding revealed that the serum level of hs-CRP and IL-6 decreased significantly in the M2000-treated group compared to the diabetic control group (p < .05). This study showed that M2000, as a new NSAID, was able to decrease serum glucose levels and increase serum insulin levels and this drug could significantly decrease the inflammatory markers in the M2000-treated group. Collectively, treatment with M2000 might be recommended reducing the severity of diabetes-induced inflammatory symptoms.

Requirement for translocon-associated protein (TRAP) α in insulin biogenesis

The mechanistic basis for the biogenesis of peptide hormones and growth factors is poorly understood. Here, we show that the conserved endoplasmic reticulum membrane translocon-associated protein α (TRAPα), also known as signal sequence receptor 1, plays a critical role in the biosynthesis of insulin. Genetic analysis in the nematode Caenorhabditis elegans and biochemical studies in pancreatic β cells reveal that TRAPα deletion impairs preproinsulin translocation while unexpectedly disrupting distal steps in insulin biogenesis including proinsulin processing and secretion. The association of common intronic single-nucleotide variants in the human TRAPα gene with susceptibility to type 2 diabetes and pancreatic β cell dysfunction suggests that impairment of preproinsulin translocation and proinsulin trafficking may contribute to the pathogenesis of type 2 diabetes.

Lrrc55 is a novel prosurvival factor in pancreatic islets

Pancreatic islets adapt to the increase in insulin demand during pregnancy by upregulating β-cell number, insulin synthesis, and secretion. These changes require prolactin receptor (PrlR) signaling, as mice with PrlR deletion are glucose intolerant with a lower β-cell mass. Prolactin also prevents β-cell apoptosis. Many genes participate in these adaptive changes in the islet, and Lrrc55 is one of the most upregulated genes with unknown function in islets. Because Lrrc55 expression increases in parallel to the increase in β-cell number and insulin production during pregnancy, we hypothesize that Lrrc55 might regulate β-cell proliferation/apoptosis (thus β-cell number) and insulin synthesis. Here, we found that Lrrc55 expression was upregulated by >60-fold during pregnancy in a PrlR-dependent manner, and this increase was restricted only to the islets. Overexpression of Lrrc55 in β-cells had minimal effect on β-cell proliferation and glucose-stimulated insulin secretion but protected β-cells from glucolipotoxicity-induced reduction in insulin gene expression. Moreover, Lrrc55 protects β-cells from glucolipotoxicity-induced apoptosis, with upregulation of prosurvival signals and downregulation of proapoptotic signals of the endoplasmic reticulum (ER) stress pathway. Furthermore, Lrrc55 attenuated calcium depletion induced by glucolipotoxicity, which may contribute to its antiapoptotic effect. Hence our findings suggest that Lrrc55 is a novel prosurvival factor that is upregulated specifically in islets during pregnancy, and it prevents conversion of adaptive unfolded protein response to unresolved ER stress and apoptosis in β-cells. Lrrc55 could be a potential therapeutic target in diabetes by reducing ER stress and promoting β-cell survival.

Stem Cell Therapies for Treating Diabetes: Progress and Remaining Challenges

Restoration of insulin independence and normoglycemia has been the overarching goal in diabetes research and therapy. While whole-organ and islet transplantation have become gold-standard procedures in achieving glucose control in diabetic patients, the profound lack of suitable donor tissues severely hampers the broad application of these therapies. Here, we describe current efforts aimed at generating a sustainable source of functional human stem cell-derived insulin-producing islet cells for cell transplantation and present state-of-the-art efforts to protect such cells via immune modulation and encapsulation strategies.

Recent advances in organoid culture for insulin production and diabetes therapy: methods and challenges

Breakthroughs in stem cell technology have contributed to disease modeling and drug screening via organoid technology. Organoid are defined as three-dimensional cellular aggregations derived from adult tissues or stem cells. They recapitulate the intricate pattern and functionality of the original tissue. Insulin is secreted mainly by the pancreatic β cells. Large-scale production of insulin-secreting β cells is crucial for diabetes therapy. Here, we provide a brief overview of organoids and focus on recent advances in protocols for the generation of pancreatic islet organoids from pancreatic tissue or pluripotent stem cells for insulin secretion. The feasibility and limitations of organoid cultures derived from stem cells for insulin production will be described. As the pancreas and gut share the same embryological origin and produce insulin, we will also discuss the possible application of gut organoids for diabetes therapy. Better understanding of the challenges associated with the current protocols for organoid culture facilitates development of scalable organoid cultures for applications in biomedicine. [BMB Reports 2019; 52(5): 295-303].

Circulating Adrenomedullin Is Elevated in Gestational Diabetes and Its Role in Impaired Insulin Production by β-Cells

CONTEXT

Defective pancreatic β-cell adaptation in pregnancy plays an important role in the pathophysiology of gestational diabetes mellitus (GDM), but the molecular basis remains unclear. Objectives of this study were to determine if circulating levels of adrenomedullin (ADM) in women with GDM are elevated and to assess the effects of ADM on insulin synthesis and secretion by human pancreatic β-cells.

DESIGN

A stable gene product of ADM precursor, midregional pro-adrenomedullin (MR-proADM), was measured in plasma of pregnant women with normal glucose tolerance (NGT, n = 10) or GDM (n = 11). The β-Lox5 cell line, derived from human pancreatic β-cells, was transduced with homeodomain transcription factor pancreatic-duodenal homeobox (PDX) factor 1 (PDX1) encoding lentiviral vector and treated with different doses of ADM. mRNA for insulin, ADM, and its receptor components in β-Lox5 cells and insulin in media were measured.

RESULTS

Plasma MR-proADM levels were significantly higher in GDM compared with patients with NGT. Pancreatic β-Lox5 cells express mRNA for insulin, ADM, and its receptor components. PDX1 transduction and cell-cell contact synergistically promote β-Lox5 cells insulin mRNA and secretion. Furthermore, ADM dose-dependently inhibited mRNA and secretion of insulin in β-Lox5 cell aggregates. These inhibitory effects were blocked by ADM antagonist ADM22-52, cAMP-dependent protein kinase A inhibitor KT5720, and Erk inhibitor PD98059, but not by PI-3K the inhibitor wortmannin.

CONCLUSIONS

Circulating ADM concentrations were elevated in pregnant women with GDM. ADM suppresses insulin synthesis and secretion by pancreatic β-cells in vitro. Thus, increased circulating ADM may contribute to the defective adaptation of β-cells in diabetic pregnancies, and blockade of ADM actions with its antagonists may improve β-cell functions.

Differentiation Potential of Mesenchymal Stem Cells into Pancreatic β-Cells

Stem cells having the capability to differentiate into other type of cells and renewing themselves, gained so much importance in recent years. Investigations in stem cells revealed that mesenchymal stem cells can successfully differentiate into other type of cells like adipocytes, hepatocytes, osteocytes, neurocytes and chondrocytes. In addition, these cells can also differentiate into insulin-producing beta cells. Insulin is a crucial hormone for glucose balance of the body. Insufficiency or unavailability of insulin is called diabetes. External insulin intake, as well as pancreas or islet transplantation, is the most basic treatment of diabetes. In vivo and in vitro studies demonstrate that stem cell therapy is also used in the cure of diabetes. Differentiation process of stem cells into beta cells releasing insulin is quite complicated. There are many different reports for the differentiation of stem cells in the literature. The success of differentiation of stem cells into beta cells depends on several factors like the source of stem cells, chemicals added into the differentiation medium and the duration of differentiation protocol. Distinct studies for the differentiation of stem cells into insulin-secreting cells are available in the literature. Moreover, thanks to the superior differentiation capacity of stem cells, they are being preferred in clinical studies. Stem cells were clinically used to heal diabetic ulcer, to increase c-peptide level and insulin secretion in both type 1 and type 2 diabetes. Mesenchymal stem cells having high differentiation potential to insulin-secreting cells are encouraging vehicles for both in vivo and in vitro studies together with clinical trials for diabetes mellitus.

Gene expression patterns in synchronized islet populations

In vivo levels of insulin are oscillatory with a period of ~5-10 minutes, indicating that the islets of Langerhans within the pancreas are synchronized. While the synchronizing factors are still under investigation, one result of this behavior is expected to be coordinated and oscillatory intracellular factors, such as intracellular Ca²⁺ levels, throughout the islet population. In other cell types, oscillatory intracellular signals, like intracellular Ca²⁺, have been shown to affect specific gene expression. To test how the gene expression landscape may differ between a synchronized islet population with its reproducible intracellular oscillations and an unsynchronized islet population with heterogeneous oscillations, gene set enrichment analysis (GSEA) was used to compare an islet population that had been synchronized using a glucose wave with a 5-min period, and an unsynchronized islet population. In the population exposed to the glucose wave, 58/62 islets showed synchronization as evidenced by coordinated intracellular Ca²⁺ oscillations with an average oscillation period of 5.1 min, while in the unsynchronized population 29/62 islets showed slow oscillations with an average period of 5.2 min. The synchronized islets also had a significantly smaller drift of their oscillation period during the experiment as compared to the unsynchronized population. GSEA indicated that the synchronized population had reduced expression of gene sets related to protein translation, protein turnover, energy expenditure, and insulin synthesis, while those that were related to maintenance of cell morphology were increased.

Reprogramming of Human Pancreatic Organoid Cells into Insulin-Producing β-Like Cells by Small Molecules and in Vitro Transcribed Modified mRNA Encoding Neurogenin 3 Transcription Factor

Reprogramming of non-endocrine pancreatic cells into insulin-producing cells represents a promising therapeutic approach for the restoration of endogenous insulin production in diabetic patients. In this paper, we report that human organoid cells derived from the pancreatic tissue can be reprogrammed into the insulin-producing cells (IPCs) by the combination of in vitro transcribed modified mRNA encoding transcription factor neurogenin 3 and small molecules modulating the epigenetic state and signalling pathways. Upon the reprogramming, IPCs formed 4.6 ± 1.2 % of the total cells and expressed typical markers (insulin, glucokinase, ABCC8, KCNJ11, SLC2A2, SLC30A8) and transcription factors (PDX1, NEUROD1, MAFA, NKX2.2, NKX6.1, PAX4, PAX6) needed for the proper function of pancreatic β-cells. Additionally, we have revealed a positive effect of ALK5 inhibitor RepSox on the overall reprogramming efficiency. However, the reprogrammed IPCs possessed only a partial insulin-secretory capacity, as they were not able to respond to the changes in the extracellular glucose concentration by increasing insulin secretion. Based on the achieved results we conclude that due to the incomplete reprogramming, the IPCs have immature character and only partial properties of native human β-cells.

microRNA-96 protects pancreatic β-cell function by targeting PAK1 in gestational diabetes mellitus

Gestational diabetes mellitus (GDM) is a disease condition in which a woman develops high blood sugar levels during pregnancy, which might be induced by multiple factors. Among those relative factors, microRNA (miRNA) is well-known to be involved in GDM development. In this study, we investigated the role of miRNA in GDM by analyzing miRNA expression profiling in placenta tissues from healthy or GDM pregnancies. We found that miR-96 was the most down-regulated miRNA in GDM samples. Furthermore, miRNA target gene prediction revealed that p21-activated kinase 1 (PAK1) is a potential target of miR-96. Functional assays showed that miR-96 enhanced β-cell function, whereas PAK1 inhibited β-cell function and cell viability. Our findings demonstrate that miR-96 plays a critical role in GDM development by regulating PAK1 expression, insulin secretion, and β-cell function. © 2018 BioFactors, 44(6):539-547, 2018.

Establishment of a rapid and footprint-free protocol for differentiation of human embryonic stem cells into pancreatic endocrine cells with synthetic mRNAs encoding transcription factors

BACKGROUND

Transplantation of pancreatic β cells generated in vitro from pluripotent stem cells (hPSCs) such as embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) has been proposed as an alternative therapy for diabetes. Though many differentiation protocols have been developed for this purpose, lentivirus-mediated forced expression of transcription factors (TF)-PDX1 and NKX6.1-has been at the forefront for its relatively fast and straightforward approach. However, considering that such cells will be used for therapeutic purposes in the future, it is desirable to develop a procedure that does not leave any footprint on the genome, as any changes of DNAs could potentially be a source of unintended, concerning effects such as tumorigenicity. In this study, we attempted to establish a novel protocol for rapid and footprint-free hESC differentiation into a pancreatic endocrine lineage by using synthetic mRNAs (synRNAs) encoding PDX1 and NKX6.1. We also tested whether siPOU5F1, which reduces the expression of pluripotency gene POU5F1 (also known as OCT4), can enhance differentiation as reported previously for mesoderm and endoderm lineages.

METHODS

synRNA-PDX1 and synRNA-NKX6.1 were synthesized in vitro and were transfected five times to hESCs with a lipofection reagent in a modified differentiation culture condition. siPOU5F1 was included only in the first transfection. Subsequently, cells were seeded onto a low attachment plate and aggregated by an orbital shaker. At day 13, the degree of differentiation was assessed by quantitative RT-PCR (qRT-PCR) and immunohistochemistry for endocrine hormones such as insulin, glucagon, and somatostatin.

RESULTS

Both PDX1 and NKX6.1 expression were detected in cells co-transfected with synRNA-PDX1 and synRNA-NKX6.1 at day 3. Expression levels of insulin in the transfected cells at day 13 were 450 times and 14 times higher by qRT-PCR compared to the levels at day 0 and in cells cultured without synRNA transfection, respectively. Immunohistochemically, pancreatic endocrine hormones were not detected in cells cultured without synRNA transfection but were highly expressed in cells transfected with synRNA-PDX1, synRNA-NKX6.1, and siPOU5F1 at as early as day 13.

CONCLUSIONS

In this study, we report a novel protocol for rapid and footprint-free differentiation of hESCs to endocrine cells.

Subversion of Systemic Glucose Metabolism as a Mechanism to Support the Growth of Leukemia Cells

From an organismal perspective, cancer cell populations can be considered analogous to parasites that compete with the host for essential systemic resources such as glucose. Here, we employed leukemia models and human leukemia samples to document a form of adaptive homeostasis, where malignant cells alter systemic physiology through impairment of both host insulin sensitivity and insulin secretion to provide tumors with increased glucose. Mechanistically, tumor cells induce high-level production of IGFBP1 from adipose tissue to mediate insulin sensitivity. Further, leukemia-induced gut dysbiosis, serotonin loss, and incretin inactivation combine to suppress insulin secretion. Importantly, attenuated disease progression and prolonged survival are achieved through disruption of the leukemia-induced adaptive homeostasis. Our studies provide a paradigm for systemic management of leukemic disease.

Taurine Increases Insulin Expression in STZ-Treated Rat Islet Cells In Vitro

This research aims at figure out the effects and the pathway of taurine on insulin in islet cells cultured in vitro treated by STZ. In the experiment, islet cells were isolated from pancreatic tissue by in situ perfusion with collagenase V. The pancreatic islet cells, maintained in RPMI 1640 culture medium were divided into six groups: C: control, E: supplemented with 10 mmol/L of taurine, group M, T1, T2 and T3 was treated with STZ (0.5 mmol/L), at the same time, taurine were added in group T1,T2 and T3 for 30 min, and then culture medium were collected by centrifugation and then insulin levels were detected by radioimmunoassay, the cells were then rinsed with Hanks, and 0,10, 0, 5, 10, 20 mmol/L of taurine in group C, E, M, T1, T2 and T3 were added for 24 h respectively. Total RNA was extracted, then insulin gene and its transcription regulator such as PDX-1, NeuroD1 were amplified by semi-quantitative RT-PCR. The results showed that, the release of insulin from islet cells treated by STZ could be inhibited by taurine, gene expression of insulin, PDX-1 and NeuroD1 in STZ group decreased significantly, which were up-regulated by taurine administration. In conclusion, taurine exerts a certain degree of protective and reconstructive effects on islet cells treated by STZ.

Mesenchymal stem cell (MSC)-mediated survival of insulin producing pancreatic β-cells during cellular stress involves signalling via Akt and ERK1/2

Mesenchymal stem cells (MSC) are of interest for cell therapy since their secreted factors mediate immunomodulation and support tissue regeneration. This study investigated the direct humoral interactions between MSC and pancreatic β-cells using human telomerase-immortalized MSC (hMSC-TERT) and rat insulinoma-derived INS-1E β-cells. hMSC-TERT supported survival of cocultured INS-1E β-cells during cellular stress by alloxan (ALX) and streptozotocin (STZ), but not in response to IL-1β. Accordingly, hMSC-TERT had no effect on inflammatory cytokine-related signalling via NF-kB and p-JNK but maintained p-Akt and upregulated p-ERK1/2. Inhibition of either p-Akt or p-ERK1/2 did not abolish protection by hMSC-TERT but activated the respective non-inhibited pathway. This suggests that one pathway compensates for the other. Main results were confirmed in mouse islets except hMSC-TERT-mediated upregulation of p-ERK1/2. Therefore, MSC promote β-cell survival by preservation of p-Akt signalling and further involve p-ERK1/2 activation in certain conditions such as loss of p-Akt or insulinoma background.

tRNA modifications and islet function

Efficient and accurate protein translation is essential to producing insulin in pancreatic β-cells. Transfer RNA (tRNA) is known as the key component of the protein translational machinery. Interestingly, tRNA contains a wide variety of chemical modifications, which are posttranscriptionally catalysed by tRNA modifying enzymes. Recent advances in genome-sequencing technology have unveiled a number of genetic variations that are associated with the development of type 2 diabetes (T2D). Some of these mutations are located in the genes of tRNA modifying enzymes. Using cellular and animal models, it has been showed that dysregulation of tRNA modification impairs protein translation in pancreatic β-cells and leads to aberrant insulin production. In this review, we discuss the recent findings in the molecular functions of tRNA modifications and their involvement in the development of T2D.

Insulin Is Transcribed and Translated in Mammalian Taste Bud Cells

We and others have reported that taste cells in taste buds express many peptides in common with cells in the gut and islets of Langerhans in the pancreas. Islets and taste bud cells express the hormones glucagon and ghrelin, the same ATP-sensitive potassium channel responsible for depolarizing the insulin-secreting β cell during glucose-induced insulin secretion, as well as the propeptide-processing enzymes PC1/3 and PC2. Given the common expression of functionally specific proteins in taste buds and islets, it is surprising that no one has investigated whether insulin is synthesized in taste bud cells. Using immunofluorescence, we demonstrated the presence of insulin in mouse, rat, and human taste bud cells. By detecting the postprocessing insulin molecule C-peptide and green fluorescence protein (GFP) in taste cells of both insulin 1-GFP and insulin 2-GFP mice and the presence of the mouse insulin transcript by in situ hybridization, we further proved that insulin is synthesized in individual taste buds and not taken up from the parenchyma. In addition to our cytology data, we measured the level of insulin transcript by quantitative RT-PCR in the anterior and posterior lingual epithelia. These analyses showed that insulin is translated in the circumvallate and foliate papillae in the posterior, but only insulin transcript was detected in the anterior fungiform papillae of the rodent tongue. Thus, some taste cells are insulin-synthesizing cells generated from a continually replenished source of precursor cells in the adult mammalian lingual epithelium.

In vitro preconditioning of insulin-producing cells with growth factors improves their survival and ability to release insulin

Glucose-induced oxidative stress in the diabetic pancreas directly affects viability and the consequent therapeutic outcome of transplanted stem cells. Pretreatment of stem cells with growth factors induces tolerance in them against various stresses (hypoxia, thermal or hyperglycaemic). This study investigated the effect of pretreatment on insulin-producing cells (IPCs) differentiated from adipose-derived mesenchymal stem cells (ADMSCs), with a combination of stromal cell-derived factor 1 alpha (SDF1 α) and basic fibroblast growth factor (bFGF) against hyperglycaemic stress (17 or 33 mM glucose). The results showed that IPCs pretreated with a combination of SDF1α and bFGF exhibited maximally alleviated apoptosis, senescence and cell damage with a concomitantly increased release of insulin, enhanced cell proliferation and greater upregulation of Insulin 1, Insulin 2, Ngn3, Pdx1 and Nkx6.2 when stressed with 33 mM glucose. These findings may offer an improved therapeutic outcome for the treatment of diabetes.

MicroRNAs in islet hormone secretion

Pancreatic islet hormone secretion is central in the maintenance of blood glucose homeostasis. During development of hyperglycaemia, the β-cell is under pressure to release more insulin to compensate for increased insulin resistance. Failure of the β-cells to secrete enough insulin results in type 2 diabetes (T2D). MicroRNAs (miRNAs) are short non-coding RNA molecules suitable for rapid regulation of the changes in target gene expression needed in β-cell adaptations. Moreover, miRNAs are involved in the maintenance of α-cell and β-cell phenotypic identities via cell-specific, or cell-enriched expression. Although many of the abundant miRNAs are highly expressed in both cell types, recent research has focused on the role of miRNAs in β-cells. It has been shown that highly abundant miRNAs, such as miR-375, are involved in several cellular functions indispensable in maintaining β-cell phenotypic identity, almost acting as "housekeeping genes" in the context of hormone secretion. Despite the abundance and importance of miR-375, it has not been shown to be differentially expressed in T2D islets. On the contrary, the less abundant miRNAs such as miR-212/miR-132, miR-335, miR-130a/b and miR-152 are deregulated in T2D islets, wherein the latter three miRNAs were shown to play key roles in regulating β-cell metabolism. In this review, we focus on β-cell function and describe miRNAs involved in insulin biosynthesis and processing, glucose uptake and metabolism, electrical activity and Ca²⁺ -influx and exocytosis of the insulin granules. We present current status on miRNA regulation in α-cells, and finally we discuss the involvement of miRNAs in β-cell dysfunction underlying T2D pathogenesis.

Islet prohormone processing in health and disease

Biosynthesis of peptide hormones by pancreatic islet endocrine cells is a tightly orchestrated process that is critical for metabolic homeostasis. Like neuroendocrine peptides, insulin and other islet hormones are first synthesized as larger precursor molecules that are processed to their mature secreted products through a series of proteolytic cleavages, mediated by the prohormone convertases Pc1/3 and Pc2, and carboxypeptidase E. Additional posttranslational modifications including C-terminal amidation of the β-cell peptide islet amyloid polypeptide (IAPP) by peptidyl-glycine α-amidating monooxygenase (Pam) may also occur. Genome-wide association studies (GWAS) have showed genetic linkage of these processing enzymes to obesity, β-cell dysfunction, and type 2 diabetes (T2D), pointing to their important roles in metabolism and blood glucose regulation. In both type 1 diabetes (T1D) and T2D, and in the face of metabolic or inflammatory stresses, islet prohormone processing may become impaired; indeed elevated proinsulin:insulin (PI:I) ratios are a hallmark of the β-cell dysfunction in T2D. Recent studies suggest that genetic or acquired defects in proIAPP processing may lead to the production and secretion of incompletely processed forms of proIAPP that could contribute to T2D pathogenesis, and additionally that impaired processing of both PI and proIAPP may be characteristic of β-cell dysfunction in T1D. In islet α-cells, the prohormone proglucagon is normally processed to bioactive glucagon by Pc2 but may express Pc1/3 under certain conditions leading to production of GLP-1(7-36_NH2 ). A better understanding of how β-cell processing of PI and proIAPP, as well as α-cell processing of proglucagon, are impacted by genetic susceptibility and in the face of diabetogenic stresses, may lead to new therapeutic approaches for improving islet function in diabetes.

Islet stress, degradation and autoimmunity

β-cell destruction in type 1 diabetes (T1D) results from the effect of inflammation and autoimmunity. In response to inflammatory signals, islet cells engage adaptive mechanisms to restore and maintain cellular homeostasis. Among these mechanisms, the unfolded protein response (UPR) leads to a reduction of the general protein translation rate, increased production of endoplasmic reticulum chaperones and the initiation of degradation by activation of the ER associated degradation pathway (ERAD) in which newly synthetized proteins are ubiquitinylated and processed through the proteasome. This adaptive phase is also believed to play a critical role in the development of autoimmunity by the generation of neoantigens. While we have previously investigated the effect of stress on transcription, translation and post-translational events as possible source for neoantigens, the participation of the degradation machinery, yet crucial in the generation of antigenic peptides, remains to be investigated in the context of T1D pathology. In this review, we will describe the relation between the unfolded protein response and the Ubiquitin Proteasome System (UPS) and address the role of the cellular degradation machinery in the generation of antigens. Learning from tumour immunology, we propose how these processes may unmask β-cells by triggering the generation of aberrant peptides recognized by the immune cells.

Biosynthesis, structure, and folding of the insulin precursor protein

Insulin synthesis in pancreatic β-cells is initiated as preproinsulin. Prevailing glucose concentrations, which oscillate pre- and postprandially, exert major dynamic variation in preproinsulin biosynthesis. Accompanying upregulated translation of the insulin precursor includes elements of the endoplasmic reticulum (ER) translocation apparatus linked to successful orientation of the signal peptide, translocation and signal peptide cleavage of preproinsulin-all of which are necessary to initiate the pathway of proper proinsulin folding. Evolutionary pressures on the primary structure of proinsulin itself have preserved the efficiency of folding ("foldability"), and remarkably, these evolutionary pressures are distinct from those protecting the ultimate biological activity of insulin. Proinsulin foldability is manifest in the ER, in which the local environment is designed to assist in the overall load of proinsulin folding and to favour its disulphide bond formation (while limiting misfolding), all of which is closely tuned to ER stress response pathways that have complex (beneficial, as well as potentially damaging) effects on pancreatic β-cells. Proinsulin misfolding may occur as a consequence of exuberant proinsulin biosynthetic load in the ER, proinsulin coding sequence mutations, or genetic predispositions that lead to an altered ER folding environment. Proinsulin misfolding is a phenotype that is very much linked to deficient insulin production and diabetes, as is seen in a variety of contexts: rodent models bearing proinsulin-misfolding mutants, human patients with Mutant INS-gene-induced Diabetes of Youth (MIDY), animal models and human patients bearing mutations in critical ER resident proteins, and, quite possibly, in more common variety type 2 diabetes.

A Genetic Interaction Map of Insulin Production Identifies Mfi as an Inhibitor of Mitochondrial Fission

Insulin production by the pancreatic β cell is critical for the glucose homeostasis of the whole organism. Although the transcription factors required for insulin production are known, the upstream pathways that control insulin production are less clear. To further elucidate this regulatory network, we created a genetic interaction map of insulin production by performing ∼20,000 pairwise RNA interference knockdowns of insulin promoter regulators. Our map correctly predicted known physical complexes in the electron transport chain and a role for Spry2 in the unfolded protein response. To further validate our map, we used it to predict the function of an unannotated gene encoding a 37-kDa protein with no identifiable domains we have termed mitochondrial fission factor interactor (Mfi). We have shown that Mfi is a binding partner of the mitochondrial fission factor and that Mfi inhibits dynamin-like protein 1 recruitment to mitochondria. Our data provide a resource to understand the regulatory network of insulin promoter activity.

Pro-inflammatory cytokines attenuate glucose-stimulated insulin secretion from INS-1E insulinoma cells by restricting mitochondrial pyruvate oxidation capacity - Novel mechanistic insight from real-time analysis of oxidative phosphorylation

Pro-inflammatory cytokines cause pancreatic beta cell failure during the development of type 2 diabetes. This beta cell failure associates with mitochondrial dysfunction, but the precise effects of cytokines on mitochondrial respiration remain unclear. To test the hypothesis that pro-inflammatory cytokines impair glucose-stimulated insulin secretion (GSIS) by inhibiting oxidative ATP synthesis, we probed insulin release and real-time mitochondrial respiration in rat INS-1E insulinoma cells that were exposed to a combination of 2 ng/mL interleukin-1-beta and 50 ng/mL interferon-gamma. We show that 24-h exposure to these cytokines dampens both glucose- and pyruvate-stimulated insulin secretion (P < 0.0001 and P < 0.05, respectively), but does not affect KCl-induced insulin release. Mirroring secretory defects, glucose- and pyruvate-stimulated mitochondrial respiration are lowered after cytokine exposure (P < 0.01). Further analysis confirms that cytokine-induced mitochondrial respiratory defects occur irrespective of whether fuel oxidation is coupled to, or uncoupled from, ATP synthesis. These observations demonstrate that pro-inflammatory cytokines attenuate GSIS by restricting mitochondrial pyruvate oxidation capacity. Interleukin-1-beta and interferon-gamma also increase mitochondrial superoxide levels (P < 0.05), which may reinforce the inhibition of pyruvate oxidation, and cause a modest (20%) but significant (P < 0.01) loss of INS-1E cells. Cytokine-induced INS-1E cell failure is insensitive to palmitoleate and linoleate, which is at odds with the cytoprotection offered by unsaturated fatty acids against harm caused by nutrient excess. Our data disclose a mitochondrial mechanism for cytokine-impaired GSIS in INS-1E cells, and suggest that inflammatory and nutrient-related beta cell failure emerge, at least partly, through distinct paths.