Glucagon in type 2 diabetes: Friend or foe?

Hyperglucagonemia is one of the 'ominous' eight factors underlying the pathogenesis of type 2 diabetes (T2D). Glucagon is a peptide hormone involved in maintaining glucose homoeostasis by increasing hepatic glucose output to counterbalance insulin action. Long neglected, the introduction of dual and triple agonists exploiting glucagon signalling pathways has rekindled the interest in this hormone beyond its classic effect on glycaemia. Glucagon can promote weight loss by regulating food intake, energy expenditure, and brown and white adipose tissue functions through mechanisms still to be fully elucidated, thus its role in T2D pathogenesis should be further investigated. Moreover, the role of glucagon in the development of T2D micro- and macro-vascular complications is elusive. Mounting evidence suggests its beneficial effect in non-alcoholic fatty liver disease, while few studies postulated its favourable role in peripheral neuropathy and retinopathy. Contrarily, glucagon receptor agonism might induce renal changes resembling diabetic nephropathy, and data concerning glucagon actions on the cardiovascular system are conflicting. This review aims to summarise the available findings on the role of glucagon in the pathogenesis of T2D and its complications. Further experimental and clinical data are warranted to better understand the implications of glucagon signalling modulation with new antidiabetic drugs.

The p66Shc Redox Protein and the Emerging Complications of Diabetes.

Diabetes mellitus is a chronic metabolic disease, the prevalence of which is constantly increasing worldwide. It is often burdened by disabling comorbidities that reduce the quality and expectancy of life of the affected individuals. The traditional complications of diabetes are generally described as macrovascular complications (e.g., coronary heart disease, peripheral arterial disease, and stroke), and microvascular complications (e.g., diabetic kidney disease, retinopathy, and neuropathy). Recently, due to advances in diabetes management and the increased life expectancy of diabetic patients, a strong correlation between diabetes and other pathological conditions (such as liver diseases, cancer, neurodegenerative diseases, cognitive impairments, and sleep disorders) has emerged. Therefore, these comorbidities have been proposed as emerging complications of diabetes. P66Shc is a redox protein that plays a role in oxidative stress, apoptosis, glucose metabolism, and cellular aging. It can be regulated by various stressful stimuli typical of the diabetic milieu and is involved in various types of organ and tissue damage under diabetic conditions. Although its role in the pathogenesis of diabetes remains controversial, there is strong evidence regarding the involvement of p66Shc in the traditional complications of diabetes. In this review, we will summarize the evidence supporting the role of p66Shc in the pathogenesis of diabetes and its complications, focusing for the first time on the emerging complications of diabetes.

The impact of obesity and dyslipidemia on Remdesivir effectiveness in hospitalized patients with SARS-CoV-2-related pneumonia: An observational study.

BACKGROUND AND AIMS: Remdesivir (GS-5734), an inhibitor of the viral RNA-dependent, RNA polymerase was early identified as a promising therapeutic candidate against COVID-19. Our aim was to evaluate the impact of several metabolic parameters on Remdesivir effectiveness among hospitalized COVID-19 patients. METHODS AND RESULTS: We conducted an observational study on patients with SARS-CoV-2-related pneumonia admitted between May 2020 and September 2021 to the COVID-19 Units of Internal Medicine, Pneumology and Intensive Care of Garibaldi Hospital, Catania, Italy, and treated with Remdesivir. The "Ordinal Scale For Clinical Improvement" was used to assess patients' clinical improvement within 28 days of hospitalization. Short-term mortality rate was also evaluated. A total of 142 patients with SARS-CoV-2-related pneumonia were studied. The prevalence of obesity (20.7% vs. 41.9%, p = 0.03), the average BMI (27.1 ± 4.4 vs. 31.1 ± 6.1, p < 0.01) and the mean LDL-C levels (78 ± 19 mg/dl vs. 103 ± 18 mg/dl, p = 0.03) were significantly lower in early-improved (EI) compared to not-improved (NI) individuals. Obesity was negatively associated to clinical improvement after Remdesivir (OR 0.48, 95%CI 0.17-0.97, p = 0.04). Both obesity (OR 2.82, 95% CI 1.05-7.71, p = 0.04) and dyslipidemia (OR 2.78, 95%CI 1.17-7.16, p = 0.03) were significantly related to patients' mortality. Dyslipidemic subjects experienced a slower clinical improvement than non-dyslipidemic ones (Long-Rank p = 0.04). CONCLUSION: Our study showed that unfavorable metabolic conditions such as obesity and dyslipidemia could predict a worse clinical response to Remdesivir as well as the mortality in hospitalized COVID-19 patients. Further prospective and larger-scale studies are needed to confirm these preliminary findings.

Adipose Tissue Secretion Pattern Influences ß-Cell Wellness in the Transition from Obesity to Type 2 Diabetes.

The dysregulation of the ß-cell functional mass, which is a reduction in the number of ß-cells and their ability to secure adequate insulin secretion, represents a key mechanistic factor leading to the onset of type 2 diabetes (T2D). Obesity is recognised as a leading cause of ß-cell loss and dysfunction and a risk factor for T2D. The natural history of ß-cell failure in obesity-induced T2D can be divided into three steps: (1) ß-cell compensatory hyperplasia and insulin hypersecretion, (2) insulin secretory dysfunction, and (3) loss of ß-cell mass. Adipose tissue (AT) secretes many hormones/cytokines (adipokines) and fatty acids that can directly influence ß-cell function and viability. As this secretory pattern is altered in obese and diabetic patients, it is expected that the cross-talk between AT and pancreatic ß-cells could drive the maintenance of the ß-cell integrity under physiological conditions and contribute to the reduction in the ß-cell functional mass in a dysmetabolic state. In the current review, we summarise the evidence of the ability of the AT secretome to influence each step of ß-cell failure, and attempt to draw a timeline of the alterations in the adipokine secretion pattern in the transition from obesity to T2D that reflects the progressive deterioration of the ß-cell functional mass.

Adipose Tissue Inflammation and Pulmonary Dysfunction in Obesity.

Obesity is a chronic disease caused by an excess of adipose tissue that may impair health by altering the functionality of various organs, including the lungs. Excessive deposition of fat in the abdominal area can lead to abnormal positioning of the diaphragm and consequent reduction in lung volume, leading to a heightened demand for ventilation and increased exposure to respiratory diseases, such as chronic obstructive pulmonary disease, asthma, and obstructive sleep apnoea. In addition to mechanical ventilatory constraints, excess fat and ectopic deposition in visceral depots can lead to adipose tissue dysfunction, which promotes metabolic disorders. An altered adipokine-secretion profile from dysfunctional adipose tissue in morbid obesity fosters systemic, low-grade inflammation, impairing pulmonary immune response and promoting airway hyperresponsiveness. A potential target of these adipokines could be the NLRP3 inflammasome, a critical component of the innate immune system, the harmful pro-inflammatory effect of which affects both adipose and lung tissue in obesity. In this review, we will investigate the crosstalk between adipose tissue and the lung in obesity, highlighting the main inflammatory mediators and novel therapeutic targets in preventing pulmonary dysfunction.

Impact of Dysfunctional Adipose Tissue Depots on the Cardiovascular System.

Obesity with its associated complications represents a social, economic and health problem of utmost importance worldwide. Specifically, obese patients carry a significantly higher risk of developing cardiovascular disease compared to nonobese individuals. Multiple molecular mechanisms contribute to the impaired biological activity of the distinct adipose tissue depots in obesity, including secretion of proinflammatory mediators and reactive oxygen species, ultimately leading to an unfavorable impact on the cardiovascular system. This review summarizes data relating to the contribution of the main adipose tissue depots, including both remote (i.e., intra-abdominal, hepatic, skeletal, pancreatic, renal, and mesenteric adipose fat), and cardiac (i.e., the epicardial fat) adipose locations, on the cardiovascular system. Finally, we discuss both pharmacological and non-pharmacological strategies aimed at reducing cardiovascular risk through acting on adipose tissues, with particular attention to the epicardial fat.

Irisin increases the expression of anorexigenic and neurotrophic genes in mouse brain.

BACKGROUND: Irisin, a newly discovered muscle-derived hormone, acts in different organs and tissues, improving energy homeostasis. In this study, we assessed, for the first time, the effects of intraperitoneal irisin injections on circulating levels of leptin and ghrelin, mRNA expression of the major hypothalamic appetite regulators and brain neurotrophic factors, as well as feeding behaviour in healthy mice. METHODS: Twelve male 6-week-old C57BL/6 mice were randomized into two groups and intraperitoneally injected daily with irisin (0.5 µg/g body weight) or vehicle (phosphate-buffered saline [PBS]) for 14 days. On the last day of observation, leptin and ghrelin levels were measured with an enzyme-linked immunosorbent assay (ELISA). mRNA levels of genes of interest were analysed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in brain extracts. RESULTS: Irisin administration did not change leptin or ghrelin serum concentrations. However, irisin injection increased CART, POMC, NPY, and BDNF mRNA levels, without affecting the mRNA expression of AgRP, orexin, PMCH, and UCP2. Finally, over the time frame of irisin treatment, body weight and feeding behaviour were unaltered. CONCLUSIONS: These results suggest that intraperitoneal injection of irisin, although without effects on feeding behaviour and body weight, can increase the expression of anorexigenic and neurotrophic genes in mouse brain.

The p66(Shc) redox adaptor protein is induced by saturated fatty acids and mediates lipotoxicity-induced apoptosis in pancreatic beta cells.

AIMS/HYPOTHESIS: The role of the redox adaptor protein p66(Shc) as a potential mediator of saturated fatty acid (FA)-induced beta cell death was investigated. METHODS: The effects of the FA palmitate on p66(Shc) expression were evaluated in human and murine islets and in rat insulin-secreting INS-1E cells. p66(Shc) expression was also measured in islets from mice fed a high-fat diet (HFD) and from human donors with different BMIs. Cell apoptosis was quantified by two independent assays. The role of p66(Shc) was investigated using pancreatic islets from p66 (Shc-/-) mice and in INS-1E cells with knockdown of p66(Shc) or overexpression of wild-type and phosphorylation-defective p66(Shc). Production of reactive oxygen species (ROS) was evaluated by the dihydroethidium oxidation method. RESULTS: Palmitate induced a selective increase in p66(Shc) protein expression and phosphorylation on Ser(36) and augmented apoptosis in human and mouse islets and in INS-1E cells. Inhibiting the tumour suppressor protein p53 prevented both the palmitate-induced increase in p66(Shc) expression and beta cell apoptosis. Palmitate-induced apoptosis was abrogated in islets from p66 (Shc-/-) mice and following p66 (Shc) knockdown in INS-1E cells; by contrast, overexpression of p66(Shc), but not that of the phosphorylation-defective p66(Shc) mutant, enhanced palmitate-induced apoptosis. The pro-apoptotic effects of p66(Shc) were dependent upon its c-Jun N-terminal kinase-mediated phosphorylation on Ser(36) and associated with generation of ROS. p66(Shc) protein expression and function were also elevated in islets from HFD-fed mice and from obese/overweight cadaveric human donors. CONCLUSIONS/INTERPRETATION: p53-dependent augmentation of p66(Shc) expression and function represents a key signalling response contributing to beta cell apoptosis under conditions of lipotoxicity.

Exendin-4 protects pancreatic beta cells from palmitate-induced apoptosis by interfering with GPR40 and the MKK4/7 stress kinase signalling pathway.

AIMS/HYPOTHESIS: The mechanisms of the protective effects of exendin-4 on NEFA-induced beta cell apoptosis were investigated. METHODS: The effects of exendin-4 and palmitate were evaluated in human and murine islets, rat insulin-secreting INS-1E cells and murine glucagon-secreting alpha-TC1-6 cells. mRNA and protein expression/phosphorylation were measured by real-time RT-PCR and immunoblotting or immunofluorescence, respectively. Small interfering (si)RNAs for Ib1 and Gpr40 were used. Cell apoptosis was quantified by two independent assays. Insulin release was assessed with an insulin ELISA. RESULTS: Exposure of human and murine primary islets and INS-1E cells, but not alpha-TC1-6 cells, to exendin-4 inhibited phosphorylation of the stress kinases, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), and prevented apoptosis in response to palmitate. Exendin-4 increased the protein content of islet-brain 1 (IB1), an endogenous JNK blocker; however, siRNA-mediated reduction of IB1 did not impair the ability of exendin-4 to inhibit JNK and prevent apoptosis. Exendin-4 reduced G-protein-coupled receptor 40 (GPR40) expression and inhibited palmitate-induced phosphorylation of mitogen-activated kinase kinase (MKK)4 and MKK7. The effects of exendin-4 were abrogated in the presence of the protein kinase A (PKA) inhibitors, H89 and KT5720. Knockdown of GPR40, as well as use of a specific GPR40 antagonist, resulted in diminished palmitate-induced JNK and p38 MAPK phosphorylation and apoptosis. Furthermore, inhibition of JNK and p38 MAPK activity prevented palmitate-induced apoptosis. CONCLUSIONS/INTERPRETATION: Exendin-4 counteracts the proapoptotic effects of palmitate in beta cells by reducing GPR40 expression and inhibiting MKK7- and MKK4-dependent phosphorylation of the stress kinases, JNK and p38 MAPK, in a PKA-dependent manner.

Type 2 Diabetes and Alzheimer's Disease: The Emerging Role of Cellular Lipotoxicity.

Type 2 diabetes (T2D) and Alzheimer's diseases (AD) represent major health issues that have reached alarming levels in the last decades. Although growing evidence demonstrates that AD is a significant comorbidity of T2D, and there is a ~1.4-2-fold increase in the risk of developing AD among T2D patients, the involvement of possible common triggers in the pathogenesis of these two diseases remains largely unknown. Of note, recent mechanistic insights suggest that lipotoxicity could represent the missing ring in the pathogenetic mechanisms linking T2D to AD. Indeed, obesity, which represents the main cause of lipotoxicity, has been recognized as a major risk factor for both pathological conditions. Lipotoxicity can lead to inflammation, insulin resistance, oxidative stress, ceramide and amyloid accumulation, endoplasmic reticulum stress, ferroptosis, and autophagy, which are shared biological events in the pathogenesis of T2D and AD. In the current review, we try to provide a critical and comprehensive view of the common molecular pathways activated by lipotoxicity in T2D and AD, attempting to summarize how these mechanisms can drive future research and open the way to new therapeutic perspectives.

The p66Shc Protein Mediates Insulin Resistance and Secretory Dysfunction in Pancreatic ß-Cells Under Lipotoxic Conditions.

We evaluated the role of the p66Shc redox adaptor protein in pancreatic ß-cell insulin resistance that develops under lipotoxic conditions and with excess body fat. Prolonged exposure to palmitate in vitro or the presence of overweight/obesity augmented p66Shc expression levels and caused an impaired ability of exogenous insulin to increase cellular insulin content and secreted C-peptide levels in INS-1E cells and human and murine islets. In INS-1E cells, p66Shc knockdown resulted in enhanced insulin-induced augmentation of insulin content and C-peptide secretion and prevented the ability of palmitate to impair these effects of insulin. Conversely, p66Shc overexpression impaired insulin-induced augmentation of insulin content and C-peptide secretion in both the absence and presence of palmitate. Under lipotoxic condition, the effects of p66Shc are mediated by a p53-induced increase in p66Shc protein levels and JNK-induced p66Shc phosphorylation at Ser36 and appear to involve the phosphorylation of the ribosomal protein S6 kinase at Thr389 and of insulin receptor substrate 1 at Ser307, resulting in the inhibition of insulin-stimulated protein kinase B phosphorylation at Ser473. Thus, the p66Shc protein mediates the impaired ß-cell function and insulin resistance induced by saturated fatty acids and excess body fat.

Mini Review: Effect of GLP-1 Receptor Agonists and SGLT-2 Inhibitors on the Growth Hormone/IGF Axis.

Accumulating evidence supports the early use of glucagon-like peptide-1 receptor agonists (GLP-1RAs) and sodium glucose transporter-2 inhibitors (SGLT-2is) for the treatment of type 2 diabetes. Indeed, these compounds exert numerous pleiotropic actions that favorably affect metabolism and diabetes comorbidities, showing an additional effect beyond glucose control. Although a substantial amount of knowledge has been generated regarding the mechanism of action of both drug classes, much remains to be understood. Growth hormone (GH) is an important driver for multiple endocrine responses involving changes in glucose and lipid metabolism, and affects several tissues and organs (e.g., bone, heart). It acts directly on several target tissues, including skeletal muscle and bone, but several effects are mediated indirectly by circulating (liver-derived) or locally produced IGF-1. In consideration of the multiple metabolic and cardiovascular effects seen in subjects treated with GLP-1RAs and SGLT-2is (e.g., reduction of hyperglycemia, weight loss, free/fat mass and bone remodeling, anti-atherosclerosis, natriuresis), it is reasonable to speculate that GH and IGF-1 may play a about a relevant role in this context. This narrative mini-review aims to describe the involvement of the GH/IGF-1/IGF-1R axis in either mediating or responding to the effects of each of the two drug classes.

Effects of Extra Virgin Olive Oil Polyphenols on Beta-Cell Function and Survival.

Extra virgin olive oil (EVOO) is a major component of the Mediterranean diet and is appreciated worldwide because of its nutritional benefits in metabolic diseases, including type 2 diabetes (T2D). EVOO contains significant amounts of secondary metabolites, such as phenolic compounds (PCs), that may positively influence the metabolic status. In this study, we investigated for the first time the effects of several PCs on beta-cell function and survival. To this aim, INS-1E cells were exposed to 10 µM of the main EVOO PCs for up to 24 h. Under these conditions, survival, insulin biosynthesis, glucose-stimulated insulin secretion (GSIS), and intracellular signaling activation (protein kinase B (AKT) and cAMP response element-binding protein (CREB)) were evaluated. Hydroxytyrosol, tyrosol, and apigenin augmented beta-cell proliferation and insulin biosynthesis, and apigenin and luteolin enhanced the GSIS. Conversely, vanillic acid and vanillin were pro-apoptotic for beta-cells, even if they increased the GSIS. In addition, oleuropein, p-coumaric, ferulic and sinapic acids significantly worsened the GSIS. Finally, a mixture of hydroxytyrosol, tyrosol, and apigenin promoted the GSIS in human pancreatic islets. Apigenin was the most effective compound and was also able to activate beneficial intracellular signaling. In conclusion, this study shows that hydroxytyrosol, tyrosol, and apigenin foster beta-cells' health, suggesting that EVOO or supplements enriched with these compounds may improve insulin secretion and promote glycemic control in T2D patients.

Irisin and Incretin Hormones: Similarities, Differences, and Implications in Type 2 Diabetes and Obesity.

Incretins are gut hormones that potentiate glucose-stimulated insulin secretion (GSIS) after meals. Glucagon-like peptide-1 (GLP-1) is the most investigated incretin hormone, synthesized mainly by L cells in the lower gut tract. GLP-1 promotes ß-cell function and survival and exerts beneficial effects in different organs and tissues. Irisin, a myokine released in response to a high-fat diet and exercise, enhances GSIS. Similar to GLP-1, irisin augments insulin biosynthesis and promotes accrual of ß-cell functional mass. In addition, irisin and GLP-1 share comparable pleiotropic effects and activate similar intracellular pathways. The insulinotropic and extra-pancreatic effects of GLP-1 are reduced in type 2 diabetes (T2D) patients but preserved at pharmacological doses. GLP-1 receptor agonists (GLP-1RAs) are therefore among the most widely used antidiabetes drugs, also considered for their cardiovascular benefits and ability to promote weight loss. Irisin levels are lower in T2D patients, and in diabetic and/or obese animal models irisin administration improves glycemic control and promotes weight loss. Interestingly, recent evidence suggests that both GLP-1 and irisin are also synthesized within the pancreatic islets, in α- and ß-cells, respectively. This review aims to describe the similarities between GLP-1 and irisin and to propose a new potential axis-involving the gut, muscle, and endocrine pancreas that controls energy homeostasis.

Functional loss of pancreatic islets in type 2 diabetes: How can we halt it?

The loss of beta-cell functional mass is a necessary and early condition in the development of type 2 diabetes (T2D). In T2D patients, beta-cell function is already reduced by about 50% at diagnosis and further declines thereafter. Beta-cell mass is also reduced in subjects with T2D, and islets from diabetic donors are smaller compared to non-diabetic donors. Thus, beta-cell regeneration and/or preservation of the functional islet integrity should be highly considered for T2D treatment and possibly cure. To date, the available anti-diabetes drugs have been developed as "symptomatic" medications since they act to primarily reduce elevated blood glucose levels. However, a truly efficient anti-diabetes medication, capable to prevent the onset and progression of T2D, should stop beta-cell loss and/or promote the restoration of fully functional beta-cell mass, independently of reducing hyperglycemia and ameliorating glucotoxicity on the pancreatic islets. This review provides a view of the experimental and clinical evidence on the ability of available anti-diabetes drugs to exert protective effects on beta-cells, with a specific focus on human pancreatic islets and clinical trials. Potential explanations for the lack of concordance between evidence of beta-cell protection in vitro and of persistent amelioration of beta-cell function in vivo are also discussed.

Scalene muscle activity during progressive inspiratory loading under pressure support ventilation in normal humans.

We hypothesized that (1) in healthy humans subjected to intermittent positive pressure non-invasive ventilation, changes in the ventilator trigger sensitivity would be associated with increased scalene activity, (2) if properly processed - through inspiratory phase-locked averaging - surface electromyograms (EMG) of the scalenes would reliably detect and quantify this, (3) there would be a correlation between dyspnea and scalene EMG. Surface and intramuscular EMG activity of scalene muscles were measured in 10 subjects. They breathed quietly through a face mask for 10min and then were connected to a mechanical ventilator. Recordings were performed during three 15-min epochs where the subjects breathed against an increasingly negative pressure trigger (-5%, -10% and -15% of maximal inspiratory pressure). With increasing values of the inspiratory trigger, inspiratory efforts, dyspnea and the scalene activity increased significantly. The scalene EMG activity level was correlated with the esophageal pressure time product and with dyspnea intensity. Inspiration-adjusted surface EMG averaging could be useful to detect small increases of the scalene muscles activity during mechanical ventilation.

Control of tracheal cuff pressure: a pilot study using a pneumatic device.

OBJECTIVE: To evaluate the efficacy of a simple mechanical device to maintain constant endotracheal cuff pressure (Pcuff) during mechanical ventilation (large encased inflatable cuff connected to the endotracheal cuff and receiving constant pressure from a heavy mass attached to an articulated arm). DESIGN AND SETTING: Single-center, prospective, randomized, crossover, pilot study in a medical intensive care unit. PATIENTS AND PARTICIPANTS: Nine consecutive mechanically ventilated patients (age 62+/-20 years, SAPS II score 39+/-15). INTERVENTIONS: Control day: Pcuff monitored and adjusted with a manometer (Hi-Lo, Tyco Healthcare) according to current recommendations (twice a day and after each intervention on the tracheal tube); initial target Pcuff 22-28 cmH20. Prototype day: test device connected to the endotracheal cuff; same initial target. Continuous Pcuff recording during both days. Control and prototype days in random order. RESULTS: Pcuff values over 50 cmH20 were recorded in six patients during the control day (178+/-159min), never during the prototype day. During the control day, Pcuff was between 30 and 50 cmH20 for 29+/-25% of the time, vs 0.3+/-0.3% during the prototype day (p<0.01). Pcuff was between 15 and 30 cmH20 for 56+/-36% of the time during the control day, vs 95+/-14% during the prototype day p<0.01). During the control day, Pcuff was below 15 cmH20 for 15+/-17% of the time, vs 4.7+/-15% during the prototype day (p<0.05). CONCLUSIONS: The tested device successfully controlled Pcuff with minimal human resource consumption. Prospective studies are required to assess its clinical impact.

The Myokine Irisin Is Released in Response to Saturated Fatty Acids and Promotes Pancreatic ß-Cell Survival and Insulin Secretion.

This study explored the role of irisin as a new pancreatic ß-cell secretagogue and survival factor and its potential role in the communication between skeletal muscle and pancreatic ß-cells under lipotoxic conditions. Recombinant irisin stimulated insulin biosynthesis and glucose-stimulated insulin secretion (GSIS) in a PKA-dependent manner and prevented saturated fatty acid-induced apoptosis in human and rat pancreatic ß-cells, as well as in human and murine pancreatic islets, via AKT/BCL2 signaling. Treatment of myotubes with 0.5 mmol/L palmitate for 4 h, but not with oleate, promoted an increase in irisin release in the culture medium. Moreover, increased serum levels of irisin were observed in mice fed with a high-fat diet. Mouse serum rich in irisin and the conditioned medium from myotubes exposed to palmitate for 4 h significantly reduced apoptosis of murine pancreatic islets and insulin-secreting INS-1E cells, respectively, and this was abrogated in the presence of an irisin-neutralizing antibody. Finally, in vivo administration of irisin improved GSIS and increased ß-cell proliferation. In conclusion, irisin can promote ß-cell survival and enhance GSIS and may thus participate in the communication between skeletal muscle and ß-cells under conditions of excess saturated fatty acids.

Long-Term Exposure of Pancreatic ß-Cells to Palmitate Results in SREBP-1C-Dependent Decreases in GLP-1 Receptor Signaling via CREB and AKT and Insulin Secretory Response.

The effects of prolonged exposure of pancreatic ß-cells to high saturated fatty acids on glucagon-like peptide-1 (GLP-1) action were investigated. Murine islets, human pancreatic 1.1B4 cells, and rat INS-1E cells were exposed to palmitate for 24 hours. mRNA and protein expression/phosphorylation were measured by real-time RT-PCR and immunoblotting, respectively. Specific short interfering RNAs were used to knockdown expression of the GLP-1 receptor (Glp1r) and Srebf1. Insulin release was assessed with a specific ELISA. Exposure of murine islets, as well as of human and INS-1E ß-cells, to palmitate reduced the ability of exendin-4 to augment insulin mRNA levels, protein content, and release. In addition, palmitate blocked exendin-4-stimulated cAMP-response element-binding protein and v-akt murine thymoma viral oncogene homolog phosphorylation, whereas phosphorylation of MAPK-ERK kinase-1/2 and ERK-1/2 was not altered. Similarly, RNA interference-mediated suppression of Glp1r expression prevented exendin-4-induced cAMP-response element-binding protein and v-akt murine thymoma viral oncogene homolog phosphorylation, but did not impair exendin-4 stimulation of MAPK-ERK kinase-1/2 and ERK-1/2. Both islets from mice fed a high fat diet and human and INS-1E ß-cells exposed to palmitate showed reduced GLP-1 receptor and pancreatic duodenal homeobox-1 (PDX-1) and increased sterol regulatory element-binding protein (SREBP-1C) mRNA and protein levels. Furthermore, suppression of SREBP-1C protein expression prevented the reduction of PDX-1 and GLP-1 receptor levels and restored exendin-4 signaling and action. Finally, treatment of INS-1E cells with metformin for 24 h resulted in inhibition of SREBP-1C expression, increased PDX-1 and GLP-1 receptor levels, consequently, enhancement of exendin-4-induced insulin release. Palmitate impairs exendin-4 effects on ß-cells by reducing PDX-1 and GLP-1 receptor expression and signaling in a SREBP-1C-dependent manner. Metformin counteracts the impairment of GLP-1 receptor signaling induced by palmitate.