mTORC2 Signaling: A Path for Pancreatic β Cell's Growth and Function

The mechanistic target of rapamycin (mTOR) signaling pathway is an evolutionary conserved pathway that senses signals from nutrients and growth factors to regulate cell growth, metabolism and survival. mTOR acts in two biochemically and functionally distinct complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), which differ in terms of regulatory mechanisms, substrate specificity and functional outputs. While mTORC1 signaling has been extensively studied in islet/β-cell biology, recent findings demonstrate a distinct role for mTORC2 in the regulation of pancreatic β-cell function and mass. mTORC2, a key component of the growth factor receptor signaling, is declined in β cells under diabetogenic conditions and in pancreatic islets from patients with type 2 diabetes. β cell-selective mTORC2 inactivation leads to glucose intolerance and acceleration of diabetes as a result of reduced β-cell mass, proliferation and impaired glucose-stimulated insulin secretion. Thereby, many mTORC2 targets, such as AKT, PKC, FOXO1, MST1 and cell cycle regulators, play an important role in β-cell survival and function. This indicates mTORC2 as important pathway for the maintenance of β-cell homeostasis, particularly to sustain proper β-cell compensatory response in the presence of nutrient overload and metabolic demand. This review summarizes recent emerging advances on the contribution of mTORC2 and its associated signaling on the regulation of glucose metabolism and functional β-cell mass under physiological and pathophysiological conditions in type 2 diabetes.     

自噬在2 型糖尿病和胰岛β 细胞中作用研究进展

胰岛素分泌缺陷和胰岛素抵抗是2型糖尿病发病的主要机制之一。高血糖和脂质代谢紊乱,是糖尿病发病机制中最重要的获得性因素。最近研究表明自噬在维持内环境稳定,胰岛β细胞数量分泌能力,及对抗胰岛素抵抗等方面有重要作用。本文就自噬的基本概念及在糖尿病发病和维持β细胞功能方面作一综述。     

Adipose tissue derived-factors impaired pancreatic β-cell function in diabetes

Inflammatory factors produced and secreted by adipose tissue, in particular peri-pancreatic adipose tissue (P-WAT), may influence pancreatic β-cell dysfunction. Using the ZDF Rat model of diabetes, we show the presence of infiltrating macrophage (ED1 staining) on pancreatic tissue and P-WAT in the pre-diabetes stage of the disease. Then, when the T2D is installed, infiltrating cells decreased. Meanwhile, the P-WAT conditioned-medium composition, in terms of inflammatory factors, varies during the onset of the T2D. Using chemiarray technology, we observed an over expression of CXCL-1, -2, -3, CCL-3/MIP-1α and CXCL-5/LIX and TIMP-1 in the 9?weeks old obese ZDF pre-diabetic rat model. Surprisingly, the expression profile of these factors decreased when animals become diabetic (12?weeks obese ZDF rats). The expression of these inflammatory proteins is highly associated with inflammatory infiltrate. P-WAT conditioned-medium from pre-diabetes rats stimulates insulin secretion, cellular proliferation and apoptosis of INS-1 cells. However, inhibition of conditioned-medium chemokines acting via CXCR2 receptor do not change cellular proliferation apoptosis and insulin secretion of INS-1 cells induced by P-WAT conditioned-medium. Taken together, these results show that among the secreted chemokines, increased expression of CXCL-1, -2, -3 and CXCL-5/LIX in P-WAT conditioned-medium is concomitant with the onset of the T2D but do not exerted a direct effect on pancreatic β-cell dysfunction.     

AS1907417, a novel GPR119 agonist, as an insulinotropic and β-cell preservative agent for the treatment of type 2 diabetes

G-protein-coupled receptor (GPR) 119 is involved in glucose-stimulated insulin secretion (GSIS) and represents a promising target for the treatment of type 2 diabetes as it is highly expressed in pancreatic β-cells. Although a number of oral GPR119 agonists have been developed, their inability to adequately directly preserve β-cell function limits their effectiveness. Here, we evaluated the therapeutic potential of a novel small-molecule GPR119 agonist, AS1907417, which represents a modified form of a 2,4,6-tri-substituted pyrimidine core agonist, AS1269574, we previously identified. The exposure of HEK293 cells expressing human GPR119, NIT-1 cells expressing human insulin promoter, and the pancreatic β-cell line MIN-6-B1 to AS1907417, enhanced intracellular cAMP, GSIS, and human insulin promoter activity, respectively. In in vivo experiments involving fasted normal mice, a single dose of AS1907417 improved glucose tolerance, but did not affect plasma glucose or insulin levels. Twice-daily doses of AS1907417 for 4 weeks in diabetic db/db, aged db/db mice, ob/ob mice, and Zucker diabetic fatty rats reduced hemoglobin A1c levels by 1.6%, 0.8%, 1.5%, and 0.9%, respectively. In db/db mice, AS1907417 improved plasma glucose, plasma insulin, pancreatic insulin content, lipid profiles, and increased pancreatic insulin and pancreatic and duodenal homeobox 1 (PDX-1) mRNA levels. These data demonstrate that novel GPR119 agonist AS1907417 not only effectively controls glucose levels, but also preserves pancreatic β-cell function. We therefore propose that AS1907417 represents a new type of antihyperglycemic agent with promising potential for the effective treatment of type 2 diabetes.     

GABAergic regulation of pancreatic islet cells: Physiology and antidiabetic effects

Diabetes occurs when pancreatic β-cell death exceeds β-cell growth, which leads to loss of β-cell mass. An effective therapy must have two actions: promotion of β-cell replication and suppression of β-cell death. Previous studies have established an important role for γ-aminobutyric acid (GABA) in islet-cell hormone homeostasis, as well as the maintenance of the β-cell mass. GABA exerts paracrine actions on α cells in suppressing glucagon secretion, and it has autocrine actions on β cells that increase insulin secretion. Multiple studies have shown that GABA increases the mitotic rate of β cells. In mice, following β-cell depletion with streptozotocin, GABA therapy can restore the β-cell mass. Enhanced β-cell replication appears to depend on growth and survival pathways involving Akt activation. Some studies have also suggested that it induces transdifferentiation of α cells into β cells, but this has been disputed and requires further investigation. In addition to proliferative effects, GABA protects β cells against injury and markedly reduces their apoptosis under a variety of conditions. The antiapoptotic effects depend at least in part on the enhancement of sirtuin-1 and Klotho activity, which both inhibit activation of the NF-κB inflammatory pathway. Importantly, in xenotransplanted human islets, GABA therapy stimulates β-cell replication and insulin secretion. Thus, the intraislet GABAergic system is a target for the amelioration of diabetes therapy, including β-cell survival and regeneration. GABA (or GABAergic drugs) can be combined with other antidiabetic drugs for greater effect.     

Bcl-2 proteins in diabetes: mitochondrial pathways of β-cell death and dysfunction

Diabetes is a metabolic disease affecting nearly 300 million individuals worldwide. Both types of diabetes (1 and 2) are characterized by loss of functional pancreatic β-cell mass causing different degrees of insulin deficiency. The Bcl-2 family has a double-edged effect in diabetes. These proteins are crucial controllers of the mitochondrial pathway of β-cell apoptosis induced by pro-inflammatory cytokines or lipotoxicity. In parallel, some Bcl-2 members also regulate glucose metabolism and β-cell function. In this review, we describe the role of Bcl-2 proteins in β-cell homeostasis and death. We focus on how these proteins interact, their contribution to the crosstalk between endoplasmic reticulum stress and mitochondrial permeabilization, their context-dependent usage following different pro-apoptotic stimuli, and their role in β-cell physiology.     

Lentinan protects against pancreatic β-cell failure in chronic ethanol consumption-induced diabetic mice via enhancing β-cell antioxidant capacity

Chronic ethanol consumption is a well-established independent risk factor for type 2 diabetes mellitus (T2DM). Recently, increasing studies have confirmed that excessive heavy ethanol exerts direct harmful effect on pancreatic β-cell mass and function, which may be a mechanism of pancreatic β-cell failure in T2DM. In this study, we evaluated the effect of Lentinan (LNT), an active ingredient purified from the bodies of Lentinus edodes, on pancreatic β-cell apoptosis and dysfunction caused by ethanol and the possible mechanisms implicated. Functional studies reveal that LNT attenuates chronic ethanol consumption-induced impaired glucose metabolism in vivo. In addition, LNT ameliorates chronic ethanol consumption-induced β-cell dysfunction, which is characterized by reduced insulin synthesis, defected insulin secretion and increased cell apoptosis. Furthermore, mechanistic assays suggest that LNT enhances β-cell antioxidant capacity and ameliorates ethanol-induced oxidative stress by activating Nrf-2 antioxidant pathway. Our results demonstrated that LNT prevents ethanol-induced pancreatic β-cell dysfunction and apoptosis, and therefore may be a potential pharmacological agent for preventing pancreatic β-cell failure associated with T2DM and stress-induced diabetes.     

Amniotic fluid stem cells prevent β-cell injury

Background aimsThe contribution of amniotic fluid stem cells (AFSC) to tissue protection and regeneration in models of acute and chronic kidney injuries and lung failure has been shown in recent years. In the present study, we used a chemically induced mouse model of type 1 diabetes to determine whether AFSC could play a role in modulating β-cell injury and restoring β-cell function.MethodsStreptozotocin-induced diabetic mice were given intracardial injection of AFSC; morphological and physiological parameters and gene expression profile for the insulin pathway were evaluated after cell transplantation.ResultsAFSC injection resulted in protection from β-cell damage and increased β-cell regeneration in a subset of mice as indicated by glucose and insulin levels, increased islet mass and preservation of islet structure. Moreover, β-cell preservation/regeneration correlated with activation of the insulin receptor/Pi3K/Akt signaling pathway and vascular endothelial growth factor-A expression involved in maintaining β-cell mass and function.ConclusionsOur results suggest a therapeutic role for AFSC in preserving and promoting endogenous β-cell functionality and proliferation. The protective role of AFSC is evident when stem cell transplantation is performed before severe hyperglycemia occurs, which suggests the importance of early intervention. The present study demonstrates the possible benefits of the application of a non–genetically engineered stem cell population derived from amniotic fluid for the treatment of type 1 diabetes mellitus and gives new insight on the mechanism by which the beneficial effect is achieved.     

The progression of type 2 diabetes: Partly caused by deficiency of ASP-C5L2 pathway?

Type 2 diabetes is characterized by insulin resistance and β-cell dysfunction. The pathway of acylation-stimulating protein (ASP) and its specific receptor, C5a-like receptor 2 (C5L2), involves in the effective clearance of plasma glucose and free fat acid. Abnormal ASP-C5L2 pathway may induce insulin resistance, as well as cause hyperglycemia and elevated plasma free fat acid. High levels of plasma glucose and free fat acid induce β-cell apoptosis and dysfunction. We proposed that the abnormality of ASP-C5L2 pathway contributes to progression of type 2 diabetes.     

Successful β cells islet regeneration in streptozotocin-induced diabetic baboons using ultrasound-targeted microbubble gene therapy with cyclinD2/CDK4/GLP1

Both major forms of diabetes mellitus (DM) involve β-cell destruction and dysfunction. New treatment strategies have focused on replenishing the deficiency of β-cell mass common to both major forms of diabetes by islet transplantation or β-cell regeneration. The pancreas, not the liver, is the ideal organ for islet regeneration, because it is the natural milieu for islets. Since islet mass is known to increase during obesity and pregnancy, the concept of stimulating pancreatic islet regeneration in vivo is both rational and physiologic. This paper proposes a novel approach in which non-viral gene therapy is targeted to pancreatic islets using ultrasound targeted microbubble destruction (UTMD) in a non-human primate model (NHP), the baboon. Treated baboons received a gene cocktail comprised of cyclinD2, CDK, and GLP1, which in rats results in robust and durable islet regeneration with normalization of blood glucose, insulin, and C-peptide levels. We were able to generate important preliminary data indicating that gene therapy by UTMD can achieve in vivo normalization of the intravenous (IV) glucose tolerance test (IVGTT) curves in STZ hyperglycemic-induced conscious tethered baboons. Immunohistochemistry clearly demonstrated evidence of islet regeneration and restoration of β-cell mass.     

Identification of a novel GPR119 agonist, AS1269574, with in vitro and in vivo glucose-stimulated insulin secretion

The G protein-coupled receptor 119 (GPR119) is highly expressed in pancreatic β-cells. On activation, this receptor enhances the effect of glucose-stimulated insulin secretion (GSIS) via the elevation of intracellular cAMP concentrations. Although GPR119 agonists represent promising oral antidiabetic agents for the treatment of type 2 diabetes therapy, they suffer from the inability to adequately directly preserve β-cell function. To identify a new structural class of small-molecule GPR119 agonists with both GSIS and the potential to preserve β-cell function, we screened a library of synthetic compounds and identified a candidate molecule, AS1269574, with a 2,4,6-tri-substituted pyrimidine core. Here, we examined the preliminary in vitro and in vivo effects of AS1269574 on insulin secretion and glucose tolerance. AS1269574 had an EC₅₀ value of 2.5 μM in HEK293 cells transiently expressing human GPR119 and enhanced insulin secretion in the mouse pancreatic β-cell line MIN-6 only under high-glucose (16.8 mM) conditions. This contrasted with the action of the sulfonylurea glibenclamide, which also induced insulin secretion under low-glucose conditions (2.8 mM). In in vivo studies, a single administration of AS1269574 to normal mice reduced blood glucose levels after oral glucose loading based on the observed insulin secretion profiles. Significantly, AS1269574 did not affect fed and fasting plasma glucose levels in normal mice. Taken together, these results suggest that AS1269574 represents a novel structural class of small molecule, orally administrable GPR119 agonists with GSIS and promising potential for the treatment of type 2 diabetes.     

miR-375, a microRNA related to diabetes

miR-375 is an important small non-coding RNA that is specifically expressed in islet cells of the pancreas. miR-375 is required for normal pancreatic genesis and influences not only β-cell mass but also α-cell mass. miR-375 is also important to glucose-regulated insulin secretion through the regulation of the expression of Mtpn and Pdk1 genes. When human embryonic stem cells (hESCs) differentiate into endodermal lineages, miR-375 is highly expressed in the definitive endoderm, which suggests that miR-375 may have a distinct role in early development. miR-375 plays an important role in the complex regulatory network of pancreatic development, which could be regulated by pancreatic genes, such as NeuroD1, Ngn3, Pdx1 and Hnf6; additionally, miR-375 regulates genes related to pancreas development, cell growth and proliferation and insulin secretion genes to exert its function. Because of the special role of miR-375, it may be a potential target to treat diabetes. Antagonising miR-375 may enhance the effects of exendin-4 in patients, and controlling the expression of miR-375 could assist mature hESCs-derived β-cells.     

Suppression of NADPH oxidase 2 substantially restores glucose-induced dysfunction of pancreatic NIT-1 cells

Defects in insulin secretion by pancreatic cells and/or decreased sensitivity of target tissues to insulin action are the key features of type 2 diabetes. It has been shown that excessive generation of reactive oxygen species (ROS) is linked to glucose-induced β-cell dysfunction. However, cellular mechanisms involved in ROS generation in β-cells and the link between ROS and glucose-induced β-cell dysfunction are poorly understood. Here, we demonstrate a key role of NADPH oxidase 2 (NOX2)-derived ROS in the deterioration of β-cell function induced by a high concentration of glucose. Sprague-Dawley rats were fed a high-fat diet for 24 weeks to induce diabetes. Diabetic rats showed increased glucose levels and elevated ROS generation in blood, but decreased insulin content in pancreatic β-cells. In vitro, increased ROS levels in pancreatic NIT-1 cells exposed to high concentrations of glucose (33.3 mmol·L(-1)) were associated with elevated expression of NOX2. Importantly, decreased glucose-induced insulin expression and secretion in NIT-1 cells could be rescued via siRNA-mediated NOX2 reduction. Furthermore, high glucose concentrations led to apoptosis of β-cells by activation of p38MAPK and p53, and dysfunction of β-cells through phosphatase and tensih homolog (PTEN)-dependent Jun N-terminal kinase (JNK) activation and protein kinase B (AKT/PKB) inhibition, which induced the translocation of forkhead box O1 and pancreatic duodenal homeobox-1, followed by reduced insulin expression and secretion. In conclusion, NOX2-derived ROS could play a critical role in high glucose-induced β-cell dysfunction through PTEN-dependent JNK activation and AKT inhibition.     

Targeting hIAPP fibrillation: A new paradigm to prevent β-cell death?

Loss of pancreatic β-cell mass is deleterious for type 2 diabetes patients since it reduces insulin production, critical for glucose homeostasis. The main research axis developed over the last few years was to generate new pancreatic β-cells or to transplant pancreatic islets as occurring for some specific type 1 diabetes patients. We evaluate here a new paradigm consisting in preservation of β-cells by prevention of human islet amyloid polypeptide (hIAPP) oligomers and fibrils formation leading to pancreatic β-cell death. We review the hIAPP physiology and the pathology that contributes to β-cell destruction, deciphering the various cellular steps that could be involved. Recent progress in understanding other amyloidosis such as Aβ, Tau, α-synuclein or prion, involved in neurodegenerative processes linked with inflammation, has opened new research lines of investigations to preserve neuronal cells. We evaluate and estimate their transposition to the pancreatic β-cells preservation. Among them is the control of reactive oxygen species (ROS) production occurring with inflammation and the possible implication of the mitochondrial translocator protein as a diagnostic and therapeutic target. The present review also focuses on other amyloid forming proteins from molecular to physiological and physiopathological points of view that could help to better decipher hIAPP-induced β-cell death mechanisms and to prevent hIAPP fibril formation.     

Epigenetics: The missing link to understanding β-cell dysfunction in the pathogenesis of type 2 diabetes

Type 2 diabetes (T2D) is a growing health problem worldwide. While peripheral insulin resistance is common during obesity and aging in both animals and people, progression to T2D is largely due to insulin secretory dysfunction and significant apoptosis of functional β-cells, leading to an inability to compensate for insulin resistance. It is recognized that environmental factors and nutrition play an important role in the pathogenesis of diabetes. However, our knowledge surrounding molecular mechanisms by which these factors trigger β-cell dysfunction and diabetes is still limited. Recent discoveries raise the possibility that epigenetic changes in response to environmental stimuli may play an important role in the development of diabetes. In this paper, we review emerging knowledge regarding epigenetic mechanisms that may be involved in β-cell dysfunction and pathogenesis of diabetes, including the role of nutrition, oxidative stress and inflammation. We will mainly focus on the role of DNA methylation and histone modifications but will also briefly review data on miRNA effects on the pancreatic islets. Further studies aimed at better understanding how epigenetic regulation of gene expression controls β-cell function may reveal potential therapeutic targets for prevention and treatment of diabetes.     

Ultra-structural study of insulin granules in pancreatic β-cells of db/db mouse by scanning transmission electron microscopy tomography

Insulin granule trafficking is a key step in the secretion of glucose-stimulated insulin from pancreatic β-cells. The main feature of type 2 diabetes (T2D) is the failure of pancreatic β-cells to secrete sufficient amounts of insulin to maintain normal blood glucose levels. In this work, we developed and applied tomography based on scanning transmission electron microscopy (STEM) to image intact insulin granules in the β-cells of mouse pancreatic islets. Using three-dimensional (3D) reconstruction, we found decreases in both the number and the grey level of insulin granules in db/db mouse pancreatic β-cells. Moreover, insulin granules were closer to the plasma membrane in diabetic β-cells than in control cells. Thus, 3D ultra-structural tomography may provide new insights into the pathology of insulin secretion in T2D.     

IEX-1-induced cell death requires BIM and is modulated by MCL-1

MCL-1 (myeloid cell leukemia-1) is a distinguished and pivotal member of the pro-survival BCL-2 family of proteins, and we isolated IEX-1 (immediate early response gene X-1) as a MCL-1-interacting protein using the yeast two-hybrid system and confirmed their endogenous association in human cells. The underlying mechanisms by which IEX-1 affects cell survival and death are largely unknown. Ectopic expression of IEX-1-induced caspase-dependent apoptosis in 293T cells, and the response was significantly modulated by changes in the MCL-1 expression level in cells. Forced expression of IEX-1 was unable to induce cell death or to perturb mitochondrial membrane potential in BIM-depleted cells. Additionally, knockouts of NOXA or PUMA did not affect the activities of IEX-1, indicating that the pro-death action of IEX-1 specifically requires BIM. Our findings provide insight into a new regulatory circuit that controls cell death and survival by the coordinated action of MCL-1, IEX-1, and BIM.