Molecular insights into insulin action and secretion

Molecular exploration of the diurnal alteration of glycogen structural fragility and stability in time-restricted-feeding mouse liver

The structure of glycogen α particles in healthy mouse liver has two states: stability and fragility. In contrast, glycogen α particles in diabetic liver present consistent fragility, which may exacerbate hyperglycemia. Currently, the molecular mechanism behind glycogen structural alteration is still unclear. In this study, we characterized the fine molecular structure of liver glycogen α particles in healthy mice under time-restricted feeding (TRF) mode during a 24-h cycle. Then, differentially expressed genes (DEGs) in the liver during daytime and nighttime were revealed via transcriptomics, which identified that the key downregulated DEGs were mainly related to insulin secretion in daytime. Furthermore, GO annotation and KEGG pathway enrichment found that negative regulation of the glycogen catabolic process and insulin secretion process were significantly downregulated in the daytime. Therefore, transcriptomic analyses indicated that the structural stability of glycogen α particles might be correlated with the glycogen degradation process via insulin secretion downregulation. Further molecular experiments confirmed the significant upregulation of glycogen phosphorylase (PYGL), phosphorylated PYGL (p-PYGL), and glycogen debranching enzyme (AGL) at the protein level during the daytime. Overall, we concluded that the downregulation of insulin secretion in the daytime under TRF mode facilitated glycogenolysis, contributing to the structural stability of glycogen α-particles.

Preptin Deficiency Does Not Protect against High-Fat Diet-Induced Metabolic Dysfunction or Bone Loss in Mice

Preptin is derived from the cleavage of the E-peptide of pro-insulin-like growth factor (IGF)-II and is an insulin secretagogue. Observational studies have linked elevated circulating preptin to metabolic dysfunction in humans; however, a causal role for preptin in metabolic dysfunction has not been established. Additionally, preptin can promote osteoblast proliferation and differentiation, suggesting a link with skeletal health. We previously described a global preptin knockout (KO) model. In this study, we sought to uncover the impact of preptin KO in mice on the response to a moderately high-fat diet (HFD) and low-fat diet (LFD). HFD groups had higher weight and fat mass gain, lower trabecular and cortical bone volume and fracture load, and higher liver triglycerides. In males, preptin deficiency led to lower blood glucose than wild-type (WT) mice under LFD conditions. This was accompanied by differences in bone microarchitecture, including lower trabecular bone volume fraction, trabecular number, and lower cortical thickness. These differences were absent in female mice, although KO females had a HFD-driven increase in fat mass and liver triglycerides that was absent in WT mice. Female WT mice had increased glucose-stimulated insulin secretion under HFD conditions that was absent in female KO mice. Overall, preptin may have a detrimental impact on metabolism and a positive impact on bone health in male mice and may protect against liver fat storage in females while enabling islet compensation under HFD conditions. When we consider that serum preptin levels are elevated in humans of both sexes in pathological states in which insulin levels are elevated, the impact of preptin on comorbidity risk needs to be better understood. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Genetic ablation of the preptin-coding portion of Igf2 impairs pancreatic function in female mice

Preptin is a 34-amino acid peptide derived from the E-peptide of pro-insulin-like growth factor 2 and is co-secreted with insulin from β-cells. Little is understood about the effects of endogenous preptin on whole body glucose metabolism. We developed a novel mouse model in which the preptin portion of Igf2 was genetically ablated in all tissues, hereafter referred to as preptin knockout (KO), and tested the hypothesis that the removal of preptin will lead to a decreased insulin response to a metabolic challenge. Preptin KO and wild-type (WT) mice underwent weekly fasting blood glucose measurements, intraperitoneal insulin tolerance tests (ITT) at 9, 29, and 44 wk of age, and an oral glucose tolerance test (GTT) at 45 wk of age. Preptin KO mice of both sexes had similar Igf2 exon 2-3 mRNA expression in the liver and kidney compared with WT mice, but Igf2 exon 3-4 (preptin) expression was not detectable. Western blot analysis of neonatal serum indicated that processing of pro-IGF2 translated from the KO allele may be altered. Preptin KO mice had similar body weight, body composition, β-cell area, and fasted glucose concentrations compared with WT mice in both sexes up to 47 wk of age. Female KO mice had a diminished ability to mount an insulin response following glucose stimulation in vivo. This effect was absent in male KO mice. Although preptin is not essential for glucose homeostasis, when combined with previous in vitro and ex vivo findings, these data show that preptin positively impacts β-cell function.NEW & NOTEWORTHY This is the first study to describe a model in which the preptin-coding portion of the Igf2 gene has been genetically ablated in mice. The mice do not show reduced size at birth associated with Igf2 knockout suggesting that IGF2 functionality is maintained, yet we demonstrate a change in the processing of mature Igf2. Female knockout mice have diminished glucose-stimulated insulin secretion, whereas the insulin response in males is not different to wild type.

Gi/o protein-coupled receptor inhibition of beta-cell electrical excitability and insulin secretion depends on Na+/K+ ATPase activation

Gi/o-coupled somatostatin or α2-adrenergic receptor activation stimulated β-cell NKA activity, resulting in islet Ca2+ fluctuations. Furthermore, intra-islet paracrine activation of β-cell Gi/o-GPCRs and NKAs by δ-cell somatostatin secretion slowed Ca2+ oscillations, which decreased insulin secretion. β-cell membrane potential hyperpolarization resulting from Gi/o-GPCR activation was dependent on NKA phosphorylation by Src tyrosine kinases. Whereas, β-cell NKA function was inhibited by cAMP-dependent PKA activity. These data reveal that NKA-mediated β-cell membrane potential hyperpolarization is the primary and conserved mechanism for Gi/o-GPCR control of electrical excitability, Ca2+ handling, and insulin secretion.

The damage-independent evolution of ageing by selective destruction

Ageing is widely believed to reflect the accumulation of molecular damage due to energetic costs of maintenance, as proposed in disposable soma theory (DST). Here we use agent-based modelling to describe an alternative theory by which ageing could undergo positive selection independent of energetic costs. We suggest that the selective advantage of aberrant cells with fast growth might necessitate a mechanism of counterselection we name selective destruction that specifically removes the faster cells from tissues, preventing the morbidity and mortality risks they pose. The resulting survival advantage of slower mutants could switch the direction of selection, allowing them to outcompete both fast mutants and wildtype cells, causing them to spread and induce ageing in the form of a metabolic slowdown. Selective destruction could therefore provide a proximal cause of ageing that is both consistent with the gene expression hallmarks of ageing, and independent of accumulating damage. Furthermore, negligible senescence would acquire a new meaning of increased basal mortality.

Consequences for Pancreatic β-Cell Identity and Function of Unregulated Transcript Processing

Mounting evidence suggests a role for alternative splicing (AS) of transcripts in the normal physiology and pathophysiology of the pancreatic β-cell. In the apparent absence of RNA repair systems, RNA decay pathways are likely to play an important role in controlling the stability, distribution and diversity of transcript isoforms in these cells. Around 35% of alternatively spliced transcripts in human cells contain premature termination codons (PTCs) and are targeted for degradation via nonsense-mediated decay (NMD), a vital quality control process. Inflammatory cytokines, whose levels are increased in both type 1 (T1D) and type 2 (T2D) diabetes, stimulate alternative splicing events and the expression of NMD components, and may or may not be associated with the activation of the NMD pathway. It is, however, now possible to infer that NMD plays a crucial role in regulating transcript processing in normal and stress conditions in pancreatic β-cells. In this review, we describe the possible role of Regulated Unproductive Splicing and Translation (RUST), a molecular mechanism embracing NMD activity in relationship to AS and translation of damaged transcript isoforms in these cells. This process substantially reduces the abundance of non-functional transcript isoforms, and its dysregulation may be involved in pancreatic β-cell failure in diabetes.

Mitigating Perspectives of Asiatic Acid in the Renal Derangements of Streptozotocin-Nicotinamide Induced Diabetic Rats

BACKGROUND

The present study was conducted to evaluate the mitigating effects of Asiatic Acid (AA), on the changes in carbohydrate metabolism, insulin signaling molecules and renal function markers in Streptozotocin (STZ)-Nicotinamide (NAD) induced diabetic rats.

METHODS

AA (20 mg/kg BW) was supplemented orally to the diabetic rats for 42 days. The levels of plasma glucose, Hemoglobin (Hb), glycosylated hemoglobin (HbA1c) insulin and renal function markers, carbohydrate metabolic enzymes in the kidney and insulin signaling molecules in skeletal muscle were measured.

RESULTS

The administration of AA elicited a significant decrease in the levels of plasma glucose, insulin resistance, HbA1c, urea, uric acid, creatinine, glycogen, glycogen synthase, glucose-6- phosphatase, and fructose-1,6-bisphosphatase and a significant increase of body weight development, insulin, Hb, hexokinase, and glycogen phosphorylase and mRNA expressions of insulin signaling molecule like insulin receptor 1, insulin receptor 2 and glucose transporter-4 in the STZ-NAD induced diabetic rats. Further, the protective effect of AA was evidenced by its histological annotation of the kidney tissues.

CONCLUSION

Hence, this study concluded that AA can protect against renal dysfunction by attenuating carbohydrate metabolic disorder and subsequently enhances glucose utilization and renal function in STZ-NAD-induced diabetic rats.

Tetraspanin-7 regulation of L-type voltage-dependent calcium channels controls pancreatic β-cell insulin secretion

KEY POINTS

Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca2+ ) handling. TSPAN-7 is enriched in pancreatic islet cells; however, the function of islet TSPAN-7 has not been identified. Here, we characterize how β-cell TSPAN-7 regulates Ca2+ handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca2+ entry, slows Ca2+ oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca2+ channels (CaV 1.2 and CaV 1.3), which reduces channel Ca2+ conductance. TSPAN-7 slows activation of CaV 1.2 and accelerates recovery from voltage-dependent inactivation; TSPAN-7 also slows CaV 1.3 inactivation kinetics. These findings strongly implicate TSPAN-7 as a key regulator in determining the set-point of glucose-stimulated Ca2+ influx and insulin secretion.

ABSTRACT

Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca2+ ) entry into pancreatic β-cells through voltage-dependent Ca2+ (CaV ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling, and thus they may also modulate GSIS. TSPAN-7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN-7 is highly expressed in β- and α-cells; however, the function of islet TSPAN-7 has not been determined. Here, we show that TSPAN-7 knockdown (KD) increases glucose-stimulated Ca2+ influx into mouse and human β-cells. Additionally, mouse β-cell Ca2+ oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN-7 on CaV channel activity was examined. TSPAN-7 KD enhanced L-type CaV currents in mouse and human β-cells. Conversely, heterologous expression of TSPAN-7 with CaV 1.2 and CaV 1.3 L-type CaV channels decreased CaV currents and reduced Ca2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN-7 and L-type CaV channels because TSPAN-7 coimmunoprecipitated with both CaV 1.2 and CaV 1.3 from primary human β-cells and from a heterologous expression system. Finally, TSPAN-7 KD in human β-cells increased basal (5.6 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN-7 modulation of β-cell L-type CaV channels is a key determinant of β-cell glucose-stimulated Ca2+ entry and thus the set-point of GSIS.

Guava leaves extract ameliorates STZ induced diabetes mellitus via activation of PI3K/AKT signaling in skeletal muscle of rats

Blood glucose homeostasis and insulin signaling pathway regulation take a vital role in the management of diabetes mellitus. Our present was designed to explore the mechanism of the blood homeostasis, regulation of oxidative stress and insulin signaling pathway by guava leaf extract (GLE). Diabetes mellitus was induced in male albino Wistar by streptozotocin (STZ) (Single dose-40 mg/kg b.w.). As an extension STZ rats received GLE (GLE; 200 mg/kg b.w). At the end of the study the lipid peroxidation products, antioxidants, insulin signaling genes were analyzed. Treatment with GLE resulted in decreased plasma and skeletal muscle lipid peroxidation markers, increased antioxidants, and improved insulin signaling genes. GLE treatment helps to maintain blood homeostasis alleviates oxidative stress and regulates the insulin signaling genes in skeletal muscle. Overall the results suggest GLE treatment regulates blood glucose, inhibits oxidative stress, and importantly it regulates insulin signaling pathway genes in skeletal muscle. Further studies on the GLE role in other important pathways can add additional strength to the claim that GLE is a strong anti-diabetic candidate.

Steamed Ginger May Enhance Insulin Secretion through KATP Channel Closure in Pancreatic β-Cells Potentially by Increasing 1-Dehydro-6-Gingerdione Content

Ginger (Zingiber officinale Roscoe) and its active compounds (gingerols, shogaols and paradols) have been reported as having beneficial functions for several diseases, including diabetes. In this study, we revealed that the steaming process could enhance the anti-diabetic potential of ginger. To confirm the anti-diabetic effect of steamed ginger extract (GG03), we assessed pancreatic islets impaired by alloxan in zebrafish and demonstrated anti-hyperglycemic efficacy in a mouse model. The EC50 values of ginger extract (GE) and GG03 showed that the efficacy of GG03 was greater than that of GE. In addition, LC50 values demonstrated that GG03 had lower toxicity than GE, and the comparison of the Therapeutic Index (TI) proved that GG03 is a safer functional food. Furthermore, our data showed that GG03 significantly lowered hyperglycemia in a diabetic mouse model. HPLC was performed to confirm the change in the composition of steamed ginger. Interestingly, GG03 showed a 375% increase in 1-dehydro-6-gingerdione (GD) compared with GE. GD has not yet been studied much pharmacologically. Thus, we identified the protective effects of GD in the damaged pancreatic islets of diabetic zebrafish. We further assessed whether the anti-diabetic mechanism of action of GG03 and GD involves insulin secretion. Our results suggest that GG03 and GD might stimulate insulin secretion by the closure of KATP channels in pancreatic β-cells.

SGLT inhibitors as antidiabetic agents: a comprehensive review

Diabetes is one of the most common disorders that substantially contributes to an increase in global health burden. As a metabolic disorder, diabetes is associated with various medical conditions and diseases such as obesity, hypertension, cardiovascular diseases, and atherosclerosis. In this review, we cover the scientific studies on sodium/glucose cotransporter (SGLT) inhibitors published during the last decade. Our focus on providing an exhaustive overview of SGLT inhibitors enabled us to present their chemical classification for the first time.

Human Fetal Bone Marrow-Derived Mesenchymal Stem Cells Promote the Proliferation and Differentiation of Pancreatic Progenitor Cells and the Engraftment Function of Islet-Like Cell Clusters

Pancreatic progenitor cells (PPCs) are the primary source for all pancreatic cells, including beta-cells, and thus the proliferation and differentiation of PPCs into islet-like cell clusters (ICCs) opens an avenue to providing transplantable islets for diabetic patients. Meanwhile, mesenchymal stem cells (MSCs) can enhance the development and function of different cell types of interest, but their role on PPCs remains unknown. We aimed to explore the mechanism-of-action whereby MSCs induce the in vitro and in vivo PPC/ICC development by means of our established co-culture system of human PPCs with human fetal bone marrow-derived MSCs. We examined the effect of MSC-conditioned medium on PPC proliferation and survival. Meanwhile, we studied the effect of MSC co-culture enhanced PPC/ICC function in vitro and in vivo co-/transplantation. Furthermore, we identified IGF1 as a critical factor responsible for the MSC effects on PPC differentiation and proliferation via IGF1-PI3K/Akt and IGF1-MEK/ERK1/2, respectively. In conclusion, our data indicate that MSCs stimulated the differentiation and proliferation of human PPCs via IGF1 signaling, and more importantly, promoted the in vivo engraftment function of ICCs. Taken together, our protocol may provide a mechanism-driven basis for the proliferation and differentiation of PPCs into clinically transplantable islets.

Effects of four weeks intermittent hypoxia intervention on glucose homeostasis, insulin sensitivity, GLUT4 translocation, insulin receptor phosphorylation, and Akt activity in skeletal muscle of obese mice with type 2 diabetes

AIMS

The aims of this study were to determine the effects of four weeks of intermittent exposure to a moderate hypoxia environment (15% oxygen), and compare with the effects of exercise in normoxia or hypoxia, on glucose homeostasis, insulin sensitivity, GLUT4 translocation, insulin receptor phosphorylation, Akt-dependent GSK3 phosphorylation and Akt activity in skeletal muscle of obese mice with type 2 diabetes.

METHODS

C57BL/6J mice that developed type 2 diabetes with a high-fat-diet (55% fat) (fasting blood glucose, FBG = 13.9 ± 0.69 (SD) mmol/L) were randomly allocated into diabetic control (DC), rest in hypoxia (DH), exercise in normoxia (DE), and exercise in hypoxia (DHE) groups (n = 7, each), together with a normal-diet (4% fat) control group (NC, FBG = 9.1 ± 1.11 (SD) mmol/L). The exercise groups ran on a treadmill at intensities of 75-90% VO2max. The interventions were applied one hour per day, six days per week for four weeks. Venous blood samples were analysed for FBG, insulin (FBI) and insulin sensitivity (QUICKI) pre and post the intervention period. The quadriceps muscle samples were collected 72 hours post the last intervention session for analysis of GLUT4 translocation, insulin receptor phosphorylation, Akt expression and phosphorylated GSK3 fusion protein by western blot. Akt activity was determined by the ratio of the phosphorylated GSK3 fusion protein to the total Akt protein.

RESULTS

The FBG of the DH, DE and DHE groups returned to normal level (FBG = 9.4 ± 1.50, 8.86 ± 0.94 and 9.0 ± 1.13 (SD) mmol/L for DH, DE and DHE respectively, P < 0.05), with improved insulin sensitivity compared to DC (P < 0.05), after the four weeks treatment, while the NC and DC showed no significant changes, as analysed by general linear model with repeated measures. All three interventions resulted in a significant increase of GLUT4 translocation to cell membrane compared to the DC group (P < 0.05). The DE and DH showed a similar level of insulin receptor phosphorylation compared with NC that was significantly lower than the DC (P < 0.05) post intervention. The DH and DHE groups showed a significantly higher Akt activity compared to the DE, DC and NC (P < 0.05) post intervention, as analysed by one-way ANOVA.

CONCLUSIONS

This study produced new evidence that intermittent exposure to mild hypoxia (0.15 FiO2) for four weeks resulted in normalisation of FBG, improvement in whole body insulin sensitivity, and a significant increase of GLUT4 translocation in the skeletal muscle, that were similar to the effects of exercise intervention during the same time period, in mice with diet-induced type 2 diabetes. However, exercise in hypoxia for four weeks did not have additive effects on these responses. The outcomes of the research may contribute to the development of effective, alternative and complementary interventions for management of hyperglycaemia and type 2 diabetes, particularly for individuals with limitations in participation of physical activity.

Waist-to-hip ratio is the most relevant obesity index at each phase of insulin secretion among obese patients

We aimed to explore the relationship between different obesity indices and insulin secretion at each phase among obese subjects and to find out the most relevant obesity index. Height, weight, waist circumstance, and hip circumstance were obtained among 419 obese subjects to calculate body mass index (BMI), waist-to-hip ratio (WHR), waist-to-height ratio, body adiposity index (BAI), conicity index, abdominal volume index and a body shape index (ABSI). Fasting plasma glucose and fasting insulin were detected to calculate HOMA-β. Early and late insulin secretion indices: ΔI₃₀/ΔG₃₀ and DI_60-120 were calculated according to the result of a 75-g oral glucose tolerance test among the 235 subjects not meeting the standard of diabetes. Pearson correlation analysis and multiple linear regression analysis were used. BMI (β = 0.022, p = 0.000) and WHR (β = -1.557, p = 0.000) were independent correlation factors with HOMA-β. In 235 OGTT subjects, WHR was independently and negatively associated with ΔI₃₀/ΔG₃₀ and DI_60-120 (β = -1.187, p = 0.026; β = -1.241, p = 0.001, respectively). ABSI was independently and negatively associated with ΔI₃₀/ΔG₃₀ (β = -17.249, p = 0.012). WHR was the best and consistently correlated factor with insulin secretion at each phase among obese subjects from Hunan Province in China.

Flavonoid 8-O-Glucuronides from the Aerial Parts of Malva verticillata and Their Recovery Effects on Alloxan-Induced Pancreatic Islets in Zebrafish

Malva verticillata (Cluster mallow), a leafy vegetable that has been popular in East Asia for a long time, has also been used in herbal teas and medicines. The aqueous fraction of the aerial parts of Malva verticillata, exhibiting a very high quantity of flavonoids compared to the EtOAc and n-BuOH fractions, exhibited significant recovery effects on pancreatic islets damaged by alloxan in zebrafish larvae. Thus, the bioactive components responsible for this anti-diabetic activity were investigated. A new flavonoid glucuronide (1) and five known flavonoids were isolated from the aqueous fraction. Based on several spectroscopic methods, compound 1 was identified to be nortangeretin-8-O-β-D-glucuronide, and was named malvaflavone A. The A-ring of compound 1 had a 5,6,7,8-tetrahydroxy moiety, which rarely occurs in plant systems. Also 8-O-glucuronide attached to the flavonoid moiety was rarely occurred in plant system. Compounds 1, 3, 4, and 6 significantly improved the pancreatic islet size in zebrafish at 0.1 μM, and compounds 1 and 6 were found to block β-cell K⁺ channels in experiments with diazoxide. In ABTS, ORAC, and SOD assays, compounds 1-5 exhibited high anti-oxidant activities compared with quercetin and BHA (positive controls), indicating that the 8-O-glucuronide attached to the flavonoid moiety is a key structure for the expression of anti-oxidant activity. This is the first report of the isolation of compounds 1-6 from M. verticillata as well evaluated for anti-diabetic and anti-oxidant ativities.

Pancreatic Beta Cell Death: Novel Potential Mechanisms in Diabetes Therapy

PURPOSE OF REVIEW

Describing the diverse molecular mechanisms (particularly immunological) involved in the death of the pancreatic beta cell in type 1 and type 2 diabetes mellitus.

RECENT FINDINGS

Beta cell death is the final event in a series of mechanisms that, up to date, have not been entirely clarified; it represents the pathophysiological mechanism in the natural history of diabetes mellitus. These mechanisms are not limited to an apoptotic process only, which is characteristic of the immune-mediated insulitis in type 1 diabetes mellitus. They also include the action of proinflammatory cytokines, the production of reactive oxygen species, DNA fragmentation (typical of necroptosis in type 1 diabetic patients), excessive production of islet amyloid polypeptide with the consequent endoplasmic reticulum stress, disruption in autophagy mechanisms, and protein complex formation, such as the inflammasome, capable of increasing oxidative stress produced by mitochondrial damage.

SUMMARY

Necroptosis, autophagy, and pyroptosis are molecular mechanisms that modulate the survival of the pancreatic beta cell, demonstrating the importance of the immune system in glucolipotoxicity processes and the potential role for immunometabolism as another component of what once known as the "ominous octet."

Gut-like ectodermal tissue in a sea anemone challenges germ layer homology

Cnidarians (for example, sea anemones and jellyfish) develop from an outer ectodermal and inner endodermal germ layer, whereas bilaterians (for example, vertebrates and flies) additionally have a mesodermal layer as intermediate germ layer. Currently, cnidarian endoderm (that is, 'mesendoderm') is considered homologous to both bilaterian endoderm and mesoderm. Here we test this hypothesis by studying the fate of germ layers, the localization of gut cell types, and the expression of numerous 'endodermal' and 'mesodermal' transcription factor orthologues in the anthozoan sea anemone Nematostella vectensis. Surprisingly, we find that the developing pharyngeal ectoderm and its derivatives display a transcription-factor expression profile (foxA, hhex, islet, soxB1, hlxB9, tbx2/3, nkx6 and nkx2.2) and cell-type combination (exocrine and insulinergic) reminiscent of the developing bilaterian midgut, and, in particular, vertebrate pancreatic tissue. Endodermal derivatives, instead, display cell functions and transcription-factor profiles similar to bilaterian mesoderm derivatives (for example, somatic gonad and heart). Thus, our data supports an alternative model of germ layer homologies, where cnidarian pharyngeal ectoderm corresponds to bilaterian endoderm, and the cnidarian endoderm is homologous to bilaterian mesoderm.

Glucagon-Like Peptide-1 and Its Class B G Protein-Coupled Receptors: A Long March to Therapeutic Successes

The glucagon-like peptide (GLP)-1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) that mediates the action of GLP-1, a peptide hormone secreted from three major tissues in humans, enteroendocrine L cells in the distal intestine, α cells in the pancreas, and the central nervous system, which exerts important actions useful in the management of type 2 diabetes mellitus and obesity, including glucose homeostasis and regulation of gastric motility and food intake. Peptidic analogs of GLP-1 have been successfully developed with enhanced bioavailability and pharmacological activity. Physiologic and biochemical studies with truncated, chimeric, and mutated peptides and GLP-1R variants, together with ligand-bound crystal structures of the extracellular domain and the first three-dimensional structures of the 7-helical transmembrane domain of class B GPCRs, have provided the basis for a two-domain-binding mechanism of GLP-1 with its cognate receptor. Although efforts in discovering therapeutically viable nonpeptidic GLP-1R agonists have been hampered, small-molecule modulators offer complementary chemical tools to peptide analogs to investigate ligand-directed biased cellular signaling of GLP-1R. The integrated pharmacological and structural information of different GLP-1 analogs and homologous receptors give new insights into the molecular determinants of GLP-1R ligand selectivity and functional activity, thereby providing novel opportunities in the design and development of more efficacious agents to treat metabolic disorders.

Incretin effect of Urena lobata leaves extract on structure and function of rats islet β-cells

This study aims to determine the incretin effects of Urena lobata leaves extract on the structure and function of rats islet β-cells. This study utilizes male Sprague-Dawley rats divided into 2 control group and 3 test group (n = 5). Diabetic rats were induced with High Fructose Diet (HFD) and single dose intraperitoneal streptozotocin 25 mg/kg bw. Aqueous leaves extract of U. lobata was prepared by decoction methods and administrated orally with doses of 250, 500, and 1000 mg/kg bw for 4 weeks then incretin effect was evaluated by measuring serum GLP-1, insulin, and blood glucose levels. Histology of islet β-cells was evaluated using photomicroscopy by analyzing size, shape, and number. Data were analyzed using ANOVA test followed by LSD test and p ≤ 0.05 is considered significant. Oral administration of aqueous extract U. lobata leaves at doses of 250, 500, and 1000 mg/kg body weight were able to prolong GLP-1 bioavailability by 3-fold, 5-fold, and 7-fold respectively when compared to the diabetic group whereas blood glucose level were decreased about 30%, 35%, and 40% respectively (p < 0.05). Extract at doses of 500 and 1000 mg/kg bw also increased insulin level by 4-fold and 8-fold respectively compared to the diabetic group and the islet β-cells were repaired. The active compound in U. lobata leaves extract are suggested to prevent degradation of GLP-1 by inhibition of DPP-4 activity. Aqueous extract of U. lobata also improved the structure and function of islet β-cells by increasing of GLP-1 bioavailability.

Dietary Reversal Ameliorates Short- and Long-Term Memory Deficits Induced by High-fat Diet Early in Life

A high-fat diet (HFD), one of the major factors contributing to metabolic syndrome, which is associated with an increased risk of neurodegenerative diseases, leads to insulin resistance and cognitive impairment. It is not known whether these alterations are improved with dietary intervention. To investigate the long-term impact of a HFD on hippocampal insulin signaling and memory, C57BL6 mice were placed into one of three groups based on the diet: a standard diet (control), a HFD, or a HFD for 16 weeks and then the standard diet for 8 weeks (HF₁₆). HFD-induced impairments in glucose tolerance and hippocampal insulin signaling occurred concurrently with deficits in both short- and long-term memory. Furthermore, these conditions were improved with dietary intervention; however, the HFD-induced decrease in insulin receptor expression in the hippocampus was not altered with dietary intervention. Our results demonstrate that memory deficits due to the consumption of a HFD at an early age are reversible.

Beneficial effects of growth hormone-releasing hormone agonists on rat INS-1 cells and on streptozotocin-induced NOD/SCID mice

Agonists of growth hormone-releasing hormone (GHRH) have been previously reported to promote growth, function, and engraftment of islet cells following transplantation. Here we evaluated recently synthesized GHRH agonists on the proliferation and biological functions of rat pancreatic β-cell line (INS-1) and islets. In vitro treatment of INS-1 cells with GHRH agonists increased cell proliferation, the expression of cellular insulin, insulin-like growth factor-1 (IGF1), and GHRH receptor, and also stimulated insulin secretion in response to glucose challenge. Exposure of INS-1 cells to GHRH agonists, MR-356 and MR-409, induced activation of ERK and AKT pathways. Agonist MR-409 also significantly increased the levels of cellular cAMP and the phosphorylation of cAMP response element binding protein (CREB) in INS-1 cells. Treatment of rat islets with agonist, MR-409 significantly increased cell proliferation, islet size, and the expression of insulin. In vivo daily s.c. administration of 10 μg MR-409 for 3 wk dramatically reduced the severity of streptozotocin (STZ)-induced diabetes in nonobese diabetic severe combined immunodeficiency (NOD/SCID) mice. The maximal therapeutic benefits with respect to the efficiency of engraftment, ability to reach normoglycemia, gain in body weight, response to high glucose challenge, and induction of higher levels of serum insulin and IGF1 were observed when diabetic mice were transplanted with rat islets preconditioned with GHRH agonist, MR-409, and received additional treatment with MR-409 posttransplantation. This study provides an improved approach to the therapeutic use of GHRH agonists in the treatment of diabetes mellitus.

Synergistic Potentials of Coffee on Injured Pancreatic Islets and Insulin Action via KATP Channel Blocking in Zebrafish

Pancreatic islets (PIs) are damaged under diabetic conditions, resulting in decreased PI size. This study examined the regenerative effects of coffee and its components (caffeine, CFI; trigonelline, TRG; chlorogenic acid, CGA) on zebrafish larval PIs and β-cells damaged by administration of alloxan (AX). In addition, the influence of coffee and its active components on KATP channels was investigated using diazoxide (DZ) as a KATP channel activator. PI size and fluorescence intensity were significantly increased in the coffee-treated group relative to the no-treatment group (P < 0.0001). In addition, coffee exerted significant regenerative effects on pancreatic β-cells (p = 0.006). Treatment with TRG and CGA rescued PI damage, and the combination of TRG/CGA had a synergistic effect. In conclusion, the results indicate that coffee has beneficial effects on AX-damaged PIs and may also be useful as a blocker of pancreatic β-cell K(+) channels.

Aged insulin granules display reduced microtubule-dependent mobility and are disposed within actin-positive multigranular bodies

Insulin secretion is key for glucose homeostasis. Insulin secretory granules (SGs) exist in different functional pools, with young SGs being more mobile and preferentially secreted. However, the principles governing the mobility of age-distinct SGs remain undefined. Using the time-reporter insulin-SNAP to track age-distinct SGs we now show that their dynamics can be classified into three components: highly dynamic, restricted, and nearly immobile. Young SGs display all three components, whereas old SGs are either restricted or nearly immobile. Both glucose stimulation and F-actin depolymerization recruit a fraction of nearly immobile young, but not old, SGs for highly dynamic, microtubule-dependent transport. Moreover, F-actin marks multigranular bodies/lysosomes containing aged SGs. These data demonstrate that SGs lose their responsiveness to glucose stimulation and competence for microtubule-mediated transport over time while changing their relationship with F-actin.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these diseases.

A peptide from Porphyra yezoensis stimulates the proliferation of IEC-6 cells by activating the insulin-like growth factor I receptor signaling pathway

Porphyra yezoensis (P. yezoensis) is the most noteworthy red alga and is mainly consumed in China, Japan and Korea. In the present study, the effects of a P. yezoensis peptide (PY-PE) on cell proliferation and the associated signaling pathways were examined in IEC-6 rat intestinal epithelial cells. First, the MTS assay showed that PY-PE induced cell proliferation in a dose-dependent manner. Subsequently, the mechanism behind the proliferative activity induced by PY-PE was determined. The insulin-like growth factor-I receptor (IGF-IR) signaling pathway was the main focus as it plays an important role in the regulation of cell growth and proliferation. PY-PE increased the protein and mRNA expression of IGF-IR, insulin receptor substrate-1, Shc and PY-99. In addition, PY-PE stimulated extracellular signal-regulated kinase phosphorylation and phosphatidylinositol 3-kinase/Akt activation but inhibited p38 and c-Jun N-terminal kinase phosphorylation. Furthermore, PY-PE treatment increased protein and mRNA expression levels of activator protein-1, which regulates cell proliferation and survival, in the nuclear fraction. These results have significant implications for understanding the role of cell proliferation signaling pathways in intestinal epithelial cells.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Regulation of pancreatic islet beta-cell mass by growth factor and hormone signaling

Dysfunction and destruction of pancreatic islet beta cells is a hallmark of diabetes. Better understanding of cellular signals in beta cells will allow development of therapeutic strategies for diabetes, such as preservation and expansion of beta-cell mass and improvement of beta-cell function. During the past several decades, the number of studies analyzing the molecular mechanisms, including growth factor/hormone signaling pathways that impact islet beta-cell mass and function, has increased exponentially. Notably, somatolactogenic hormones including growth hormone (GH), prolactin (PRL), and insulin-like growth factor-1 (IGF-1) and their receptors (GHR, PRLR, and IGF-1R) are critically involved in beta-cell growth, survival, differentiation, and insulin secretion. In this chapter, we focus more narrowly on GH, PRL, and IGF-1 signaling, and GH-IGF-1 cross talk. We also discuss how these signaling aspects contribute to the regulation of beta-cell proliferation and apoptosis. In particular, our novel findings of GH-induced formation of GHR-JAK2-IGF-1R protein complex and synergistic effects of GH and IGF-1 on beta-cell signaling, proliferation, and antiapoptosis lead to a new concept that IGF-1R may serve as a proximal component of GH/GHR signaling.

Imatinib mesilate-induced phosphatidylinositol 3-kinase signalling and improved survival in insulin-producing cells: role of Src homology 2-containing inositol 5'-phosphatase interaction with c-Abl

AIMS/HYPOTHESIS

It is not clear how small tyrosine kinase inhibitors, such as imatinib mesilate, protect against diabetes and beta cell death. The aim of this study was to determine whether imatinib, as compared with the non-cAbl-inhibitor sunitinib, affects pro-survival signalling events in the phosphatidylinositol 3-kinase (PI3K) pathway.

METHODS

Human EndoC-βH1 cells, murine beta TC-6 cells and human pancreatic islets were used for immunoblot analysis of insulin receptor substrate (IRS)-1, Akt and extracellular signal-regulated kinase (ERK) phosphorylation. Phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] plasma membrane concentrations were assessed in EndoC-βH1 and MIN6 cells using evanescent wave microscopy. Src homology 2-containing inositol 5'-phosphatase 2 (SHIP2) tyrosine phosphorylation and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) serine phosphorylation, as well as c-Abl co-localisation with SHIP2, were studied in HEK293 and EndoC-βH1 cells by immunoprecipitation and immunoblot analysis. Gene expression was assessed using RT-PCR. Cell viability was measured using vital staining.

RESULTS

Imatinib stimulated ERK(thr202/tyr204) phosphorylation in a c-Abl-dependent manner. Imatinib, but not sunitinib, also stimulated IRS-1(tyr612), Akt(ser473) and Akt(thr308) phosphorylation. This effect was paralleled by oscillatory bursts in plasma membrane PI(3,4,5)P3 levels. Wortmannin induced a decrease in PI(3,4,5)P3 levels, which was slower in imatinib-treated cells than in control cells, indicating an effect on PI(3,4,5)P3-degrading enzymes. In line with this, imatinib decreased the phosphorylation of SHIP2 but not of PTEN. c-Abl co-immunoprecipitated with SHIP2 and its binding to SHIP2 was largely reduced by imatinib but not by sunitinib. Imatinib increased total β-catenin levels and cell viability, whereas sunitinib exerted negative effects on cell viability.

CONCLUSIONS/INTERPRETATION

Imatinib inhibition of c-Abl in beta cells decreases SHIP2 activity, which results in enhanced signalling downstream of PI3 kinase.

Antidiabetic and Hypolipidemic Activities of Curculigo latifolia Fruit:Root Extract in High Fat Fed Diet and Low Dose STZ Induced Diabetic Rats

Curculigo latifolia fruit is used as alternative sweetener while root is used as alternative treatment for diuretic and urinary problems. The antidiabetic and hypolipidemic activities of C. latifolia fruit:root aqueous extract in high fat diet (HFD) and 40 mg streptozotocin (STZ) induced diabetic rats through expression of genes involved in glucose and lipid metabolisms were investigated. Diabetic rats were treated with C. latifolia fruit:root extract for 4 weeks. Plasma glucose, insulin, adiponectin, lipid profiles, alanine aminotransferase (ALT), gamma glutamyltransferase (GGT), urea, and creatinine levels were measured before and after treatments. Regulations of selected genes involved in glucose and lipid metabolisms were determined. Results showed the significant (P < 0.05) increase in body weight, high density lipoprotein (HDL), insulin, and adiponectin levels and decreased glucose, total cholesterol (TC), triglycerides (TG), low density lipoprotein (LDL), urea, creatinine, ALT, and GGT levels in diabetic rats after 4 weeks treatment. Furthermore, C. latifolia fruit:root extract significantly increased the expression of IRS-1, IGF-1, GLUT4, PPAR α , PPAR γ , AdipoR1, AdipoR2, leptin, LPL, and lipase genes in adipose and muscle tissues in diabetic rats. These results suggest that C. latifolia fruit:root extract exerts antidiabetic and hypolipidemic effects through altering regulation genes in glucose and lipid metabolisms in diabetic rats.

Plasma membrane events associated with the meiotic divisions in the amphibian oocyte: insights into the evolution of insulin transduction systems and cell signaling

BACKGROUND

Insulin and its plasma membrane receptor constitute an ancient response system critical to cell growth and differentiation. Studies using intact Rana pipiens oocytes have shown that insulin can act at receptors on the oocyte surface to initiate resumption of the first meiotic division. We have reexamined the insulin-induced cascade of electrical and ion transport-related plasma membrane events using both oocytes and intact plasma membranes in order to characterize the insulin receptor-steroid response system associated with the meiotic divisions.

RESULTS

[(125)I]Insulin binding (K(d) = 54 ± 6 nM) at the oocyte plasma membrane activates membrane serine protease(s), followed by the loss of low affinity ouabain binding sites, with a concomitant 3-4 fold increase in high affinity ouabain binding sites. The changes in protease activity and ouabain binding are associated with increased Na(+)/Ca2(+) exchange, increased endocytosis, decreased Na(+) conductance resulting in membrane hyperpolarization, increased 2-deoxy-D-glucose uptake and a sustained elevation of intracellular pH (pHi). Hyperpolarization is largely due to Na(+)-channel inactivation and is the main driving force for glucose uptake by the oocyte via Na(+)/glucose cotransport. The Na(+) sym- and antiporter systems are driven by the Na(+) free energy gradient generated by Na(+)/K(+)-ATPase. Shifts in α and/or β Na(+)-pump subunits to caveolar (lipid raft) membrane regions may activate Na/K-ATPase and contribute to the Na(+) free energy gradient and the increase in both Na(+)/glucose co-transport and pHi.

CONCLUSIONS

Under physiological conditions, resumption of meiosis results from the concerted action of insulin and progesterone at the cell membrane. Insulin inactivates Na(+) channels and mobilizes fully functional Na(+)-pumps, generating a Na(+) free energy gradient which serves as the energy source for several membrane anti- and symporter systems.

Activated expression of the chemokine Mig after chemotherapy contributes to chemotherapy-induced bone marrow suppression and lethal toxicity

Alterations in gene expression after chemotherapy may potentially help to identify mediators that induce suppression or regeneration in bone marrow. This paper reports our observation that the expression of the chemokine monokine induced by IFN-γ (Mig) and its receptor CXCR3 was significantly activated in mice after treatment with the chemotherapeutic agent 5-fluorouracil (5-FU). The neutralization of antibodies against the activated Mig increased the survival rate and accelerated BM recovery after chemotherapy. In addition, elevation of Mig plasma levels after 5-FU treatment corresponded with increased mortality. The cell cycle-inhibiting effect of the prophylactic administration of Mig protected hematopoietic progenitor cells (HPCs) from 1-β-d-arabinofuranosylcytosine in spleen colony assays and enhanced the irradiated recipients' survival. In CXCR3(-/-) mice, Mig did not propagate BM suppression, indicating that the suppressive effect of Mig is dependent on CXCR3. On the one hand, Mig stimulated p70 S6K and Erk1/2 pathways in mesenchymal stroma cells, inhibiting mesenchymal stroma cell-dependent HPC expansion. Moreover, Mig suppressed the STAT5 pathway in HPCs, inhibiting leukocyte differentiation. Our results strongly suggest that Mig contributes to the acute lethal toxicity arising from 5-FU administration. Neutralization of Mig may offer new strategies to alleviate BM toxicity with potentially dramatic implications for chemotherapy.

RNA-binding protein HuD controls insulin translation

Although expression of the mammalian RNA-binding protein HuD was considered to be restricted to neurons, we report that HuD is present in pancreatic β cells, where its levels are controlled by the insulin receptor pathway. We found that HuD associated with a 22-nucleotide segment of the 5' untranslated region (UTR) of preproinsulin (Ins2) mRNA. Modulating HuD abundance did not alter Ins2 mRNA levels, but HuD overexpression decreased Ins2 mRNA translation and insulin production, and conversely, HuD silencing enhanced Ins2 mRNA translation and insulin production. Following treatment with glucose, HuD rapidly dissociated from Ins2 mRNA and enabled insulin biosynthesis. Importantly, HuD-knockout mice displayed higher insulin levels in pancreatic islets, while HuD-overexpressing mice exhibited lower insulin levels in islets and in plasma. In sum, our results identify HuD as a pivotal regulator of insulin translation in pancreatic β cells.

AVE8134, a novel potent PPARα agonist, improves lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats

AIM

AVE8134 is a structurally novel potent PPARα agonist. The aim of this study is to investigate the efficacy of AVE8134 on lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats.

METHODS

A cell based PPAR Gal4 transactivation assay was constructed for testing the activities of AVE8134 at 3 different PPAR isoforms in vitro. Transgenic human Apo A1 (hApo A1) mice and insulin-resistant ZDF rats were used to evaluate the effects of AVE8134 in vivo.

RESULTS

AVE8134 was a full PPARα dominated PPAR agonist (the values of EC(50) for human and rodent PPARα receptor were 0.01 and 0.3 μmol/L, respectively). AVE8134 was not active at PPARδ receptor. In female hApo A1 mice, AVE8134 (1-30 mg·kg(-1)·d(-1), po for 12 d) dose-dependently lowered the plasma triglycerides, and increased the serum HDL-cholesterol, hApo A1 and mouse Apo E levels. In female ZDF rats, AVE8134 (3-30 mg·kg(-1)·d(-1) for 2 weeks) improved insulin-sensitivity index. In pre-diabetic male ZDF rats (at the age of 7 weeks), AVE8134 (10 mg·kg(-1)·d(-1) for 8 weeks) produced an anti-diabetic action comparable to rosiglitazone, without the PPARγ mediated adverse effects on body weight and heart weight. In male ZDF rats (at the age of 6 weeks), AVE8134 (20 mg·kg(-1)·d(-1) for 12 weeks) increased mRNA levels of the target genes LPL and PDK4 about 20 fold in the liver, and there was no relevant effect with rosiglitazone.

CONCLUSION

AVE8134 improves lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats.

Specific glucose-induced control of insulin receptor substrate-2 expression is mediated via Ca2+-dependent calcineurin/NFAT signaling in primary pancreatic islet β-cells

OBJECTIVE

Insulin receptor substrate-2 (IRS-2) plays an essential role in pancreatic islet β-cells by promoting growth and survival. IRS-2 turnover is rapid in primary β-cells, but its expression is highly regulated at the transcriptional level, especially by glucose. The aim was to investigate the molecular mechanism on how glucose regulates IRS-2 gene expression in β-cells.

RESEARCH DESIGN AND METHODS

Rat islets were exposed to inhibitors or subjected to adenoviral vector-mediated gene manipulations and then to glucose-induced IRS-2 expression analyzed by real-time PCR and immunoblotting. Transcription factor nuclear factor of activated T cells (NFAT) interaction with IRS-2 promoter was analyzed by chromatin immunoprecipitation assay and glucose-induced NFAT translocation by immunohistochemistry.

RESULTS

Glucose-induced IRS-2 expression occurred in pancreatic islet β-cells in vivo but not in liver. Modulating rat islet β-cell Ca(2+) influx with nifedipine or depolarization demonstrated that glucose-induced IRS-2 gene expression was dependent on a rise in intracellular calcium concentration derived from extracellular sources. Calcineurin inhibitors (FK506, cyclosporin A, and a peptide calcineurin inhibitor [CAIN]) abolished glucose-induced IRS-2 mRNA and protein levels, whereas expression of a constitutively active calcineurin increased them. Specific inhibition of NFAT with the peptide inhibitor VIVIT prevented a glucose-induced IRS-2 transcription. NFATc1 translocation to the nucleus in response to glucose and association of NFATc1 to conserved NFAT binding sites in the IRS-2 promoter were demonstrated.

CONCLUSIONS

The mechanism behind glucose-induced transcriptional control of IRS-2 gene expression specific to the islet β-cell is mediated by the Ca(2+)/calcineurin/NFAT pathway. This insight into the IRS-2 regulation could provide novel therapeutic means in type 2 diabetes to maintain an adequate functional mass.

Islet transplantation: factors in short-term islet survival

Islet transplantation has the potential to cure type 1 diabetes. In recent years, the proportion of patients achieving initial insulin independence has improved, but longer term outcomes remain poor compared to those for whole pancreas transplants. This review article will discuss factors affecting islet yield and viability leading up to transplantation and in the immediate post-transplant period.

The roles of ATF3, an adaptive-response gene, in high-fat-diet-induced diabetes and pancreatic beta-cell dysfunction

Most people with type 2 diabetes (T2D) have reduced beta-cell mass, and apoptosis is a key factor for this reduction. Previously, we showed that ATF3, an adaptive-response gene, is induced by various stress signals relevant to T2D, such as high glucose and high fatty acid. Because ATF3 is proapoptotic in beta-cells, we tested the hypothesis that ATF3 plays a detrimental role and contributes to the development of T2D. We compared wild-type (WT) and ATF3 knockout (KO) mice in an animal model for T2D, high-fat diet-induced diabetes. We also used INS-1 beta-cells and primary islets to analyze the roles of ATF3 in beta-cell function, including insulin gene expression and glucose-induced insulin secretion. Surprisingly, WT mice performed better in glucose tolerance test than KO mice, suggesting a protective, rather than detrimental, role of ATF3. At 12 wk on high-fat diet, no beta-cell apoptosis was observed, and the WT and KO mice had comparable beta-cell areas. However, ATF3 deficiency significantly reduced serum insulin levels in the KO mice without affecting insulin sensitivity, suggesting reduced beta-cell function in the KO mice. Analyses using INS-1 cells and primary islets support the notion that this defect is due, at least partly, to reduced insulin gene transcription in the KO islets without detectable reduction in glucose-induced calcium influx, a critical step for insulin secretion. In conclusion, our results support a model in which, before apoptosis becomes obvious, expression of ATF3 can be beneficial by helping beta-cells to cope with higher metabolic demand.

Human insulin A-chain peptide analog(s) with in vitro biological activity

In a previous study, we showed that a synthetic human insulin 1-chain analog, named analog (3) was capable of mimicking in vitro effects of native insulin, including stimulation of cell proliferation, glucose uptake and glycogen synthesis. Here, we have synthesized three new analogs (6, 9, 12) of the human A-chain, bearing or not their N- or C-terminal residue, to determine the structural features which are responsible for their biological properties. In vitro experiments clearly demonstrated that the N-terminal part of the peptides is required for the biological activity of the molecules, suggesting its crucial role in the mechanism underlying the cellular effect. Our findings may help to better understand the mechanism of interaction between insulin and its receptor. In addition, the present data demonstrate that some mini-insulin derived from the A-chain can exert similar effects as native insulin. These small peptides may offer specific advantages over insulin in the definition of new strategies for diabetes treatment.

Palmitate activates insulin signaling pathway in pancreatic rat islets

OBJECTIVE

To investigate the action of palmitate on insulin receptor (IR) signaling pathway in rat pancreatic islets. The following proteins were studied: IR substrate-1 and -2 (IRS1 and IRS2), phosphatidylinositol 3-kinase, extracellular signal-regulated protein kinase-1 and -2 (ERK1/2), and signal transducer and activator of transcription 3 (STAT3).

METHODS

Immunoblotting and immunoprecipitation assays were used to evaluate the phosphorylation states of IRS1 and IRS2 (tyrosine [Tyr]), ERK1/2 (threonine 202 [Thr202]/Tyr204), and STAT3 (serine [Ser727]).

RESULTS

The exposure of rat pancreatic islets to 0.1-mmol/L palmitate for up to 30 minutes produced a significant increase of Tyr phosphorylation in IRS2 but not in IRS1. The association of phosphatidylinositol 3-kinase with IRS2 was also upregulated by palmitate. Exposure to 5.6-mmol/L glucose caused a gradual decrease in ERK1/2 (Thr202/Tyr204) and STAT3 (serine [Ser727]) phosphorylations after 30-minute incubation. The addition of palmitate (0.1 mmol/L), associated with 5.6-mmol/L glucose, abolished these latter effects of glucose after 15-minute incubation.

CONCLUSIONS

Palmitate at physiological concentration associated with 5.6-mmol/L glucose activates IR signaling pathway in pancreatic beta cells.

Epigenomic derangement of hepatic glucose metabolism by feeding of high fructose diet and its prevention by Rosiglitazone in rats

BACKGROUND

The high consumption of fructose leads to the increasing incidence of insulin resistance by several unknown mechanisms. Hepatic glucose metabolism may also be an important target of fructose-induced-metabolic alterations.

AIM

The aim of present study was to investigate alterations in hepatic glycogenolysis, glycogenesis and gluconeogenic fluxes by feeding of 21% high fructose diet and the effects of Rosiglitazone treatment to prevent these derangements in rats.

METHODS

Rats were maintained on normal chow and high fructose diet with or without Rosiglitazone for 8 weeks and various biochemical and gene expression measures were estimated.

RESULTS

The feeding of high fructose diet impaired glucose, insulin and pyruvate tolerance tests and increased blood HbA(1c), insulin, triglyceride, free fatty acids and homeostasis model assessment after 8 weeks. In addition, high fructose diet feeding increased expression of phosphoenol-pyruvatecorboxykinase, glucose-6-phosphatase, sterol regulatory element binding proteins-1 and fatty acid synthase through enhanced expression of fork-head receptor, peroxisome proliferator activated receptor-gamma-co-activator 1 and cAMP reactive element binding protein. The treatment with Rosiglitazone inhibited all these derangements, i.e. hepato-lipogenic and gluconeogenic effects of high fructose diet feeding in rats.

CONCLUSIONS

Together these findings suggest that high fructose diet induced hepatic gluconeogenic and lipogenic rate, and increased circulating triglycerides and free fatty acids, which may be the major risk factors for glucose intolerance, hyperglycemia and insulin resistance in rats. In such situations high fructose flux also induces transcriptional cascade of gluconeogenic enzymes through the modulation of various associated transcriptional factors.

Hyperinsulinemic hypoglycemia with nesidioblastosis: histologic features and growth factor expression

Hypoglycemia secondary to nesidioblastosis is rare in adults, and the pathogenesis of this condition is unknown. To determine factors leading to nesidioblastosis in adults, we analyzed 36 cases of nesidioblastosis including 27 cases of postgastric bypass nesidioblastosis and 9 cases of idiopathic nesidioblastosis in adults by immunohistochemistry using antibodies to insulin-like growth factor 1, insulin-like growth factor 2 (IGF2), insulin-like growth factor one receptor-alpha epidermal growth factor receptor, transforming growth factor-beta1 and 2, and transforming growth factor-beta receptor type 3. Fifty-two surgically excised pancreatic specimens from patients with benign exocrine tumors and no evidence of hypoglycemia were used as controls. There was increased IGF2, insulin-like growth factor receptor 1 receptor-alpha and transforming growth factor-beta receptor 3 expression in islets from nesidioblastosis patients compared to controls. Peliosis-type vascular ectasia was more common in nesidioblastosis patients compared to controls. These findings suggest that increased production of growth factors and growth factor receptors may contribute to the development of nesidioblastosis in adults.

Calorie restriction initiated at a young age activates the Akt/PKC zeta/lambda-Glut4 pathway in rat white adipose tissue in an insulin-independent manner

Calorie restriction (CR) may exert an anti-aging effect through a metabolic adaptation to limited energy intake. The present study investigated the effect of CR on insulin signaling in response to glucose load in the epididymal adipose tissue of male F344 rats at 7 and 22 months of age. Young and middle-aged rats were fed ad libitum (AL) or 30% CR diets for 4 months, underwent glucose tolerance tests and were sacrificed 15 min after an intraperitoneal glucose or saline injection to evaluate glucose-stimulated insulin response and subsequent activation of insulin signaling molecules in the adipose tissue. In the 7- and 22- month AL groups, glucose administration increased serum insulin levels and also increased phosphorylated (p) levels of the insulin receptor (IR), v-akt murine thymoma viral oncogene homolog (Akt), protein kinase C (PKC) zeta/lambda and the membrane fraction of glucose transporter 4 (mGlut4). In contrast, in the 7-month CR group, p-Akt, p-PKC zeta/lambda and mGlut4 levels were upregulated without glucose stimulation; the glucose load augmented the p-IR level but there was no additional activation of the downstream molecules. In the 22-month CR group, these unexpected findings were not observed. In summary, basal levels of insulin signaling molecules such as p-Akt, p-PKC zeta/lambda, and mGlut4 were significantly increased with a low insulin response in the 7-month CR group. The present results suggest the presence of an age-specific insulin-independent mechanism that is induced by CR to regulate energy metabolism in white adipose tissue.

The role of mTOR in the adaptation and failure of beta-cells in type 2 diabetes

Mammalian target of rapamycin (mTOR) is an important nutrient sensor that plays a critical role in cellular metabolism, growth, proliferation and apoptosis and in the cellular response to oxidative stress. In addition, mTOR-raptor complex, also called mammalian target of rapamycin complex 1 (mTORC1), generates an inhibitory feedback loop on insulin receptor substrate proteins. It was suggested that nutrient overload leads to insulin/insulin-like growth factor 1 resistance in peripheral insulin-responsive tissues and in the beta-cells through sustained activation of mTORC1. In this review, we summarize the literature on the regulation and function of mTOR, its role in the organism's response to nutrients and its potential impact on lifespan, insulin resistance and the metabolic adaptation to hyperglycaemia in type 2 diabetes. We also propose a hypothesis based on data in the literature as well as data generated in our laboratory, which assigns a central positive role to mTOR in the maintenance of beta-cell function and mass in the diabetic environment.