Suppression of SOCS3 expression in the pancreatic beta-cell leads to resistance to type 1 diabetes

Type 1 diabetes results from the selective destruction of insulin-producing pancreatic beta-cells during islet inflammation, which involves inflammatory cytokines and free radicals. However, mechanisms for protecting beta-cells from destruction have not been clarified. In this study, we define the role of SOCS3 on beta-cell destruction using beta-cell-specific SOCS3-conditional knockout (cKO) mice. The beta-cell-specific SOCS3-deficient mice were resistant to the development of diabetes caused by streptozotocin (STZ), a genotoxic methylating agent, which has been used to trigger beta-cell destruction. The islets from cKO mice demonstrated hyperactivation of STAT3 and higher induction of Bcl-xL than did islets from WT mice, and SOCS3-deficient beta-cells were more resistant to apoptosis induced by STZ in vitro than were WT beta-cells. These results suggest that enhanced STAT3 signaling protects beta-cells from destruction induced by a genotoxic stress and that STAT3/SOCS3 can be a potential therapeutic target for the treatment of type 1 diabetes.     

Animal models of type 2 diabetes with reduced pancreatic beta-cell mass

Type 2 diabetes is increasingly viewed as a disease of insulin deficiency due not only to intrinsic pancreatic beta-cell dysfunction but also to reduction of beta-cell mass. It is likely that, in diabetes-prone subjects, the regulated beta-cell turnover that adapts cell mass to body's insulin requirements is impaired, presumably on a genetic basis. We still have a limited knowledge of how and when this derangement occurs and what might be the most effective therapeutic strategy to preserve beta-cell mass. The animal models of type 2 diabetes with reduced beta-cell mass described in this review can be extremely helpful (a) to have insight into the mechanisms underlying the defective growth or accelerated loss of beta-cells leading to the beta-cell mass reduction; (b) to investigate in prospective studies the mechanisms of compensatory adaptation and subsequent failure of a reduced beta-cell mass. Furthermore, these models are of invaluable importance to test the effectiveness of potential therapeutic agents that either stimulate beta-cell growth or inhibit beta-cell death.     

Combined treatment with lisofylline and exendin-4 reverses autoimmune diabetes

Type 1 diabetes mellitus (T1DM) is an autoimmune disease leading to near complete pancreatic beta-cell destruction. New evidence suggests that beta-cell regeneration is possible, but ongoing autoimmune damage prevents restoration of beta-cell mass. We tested the hypothesis that simultaneously blocking autoimmune cytokine damage and supplying a growth-promoting stimulus for beta-cells would provide a novel approach to reverse T1DM. Therefore, in this study we combined lisofylline to suppress autoimmunity and exendin-4 to enhance beta-cell proliferation for treating autoimmune-mediated diabetes in the non-obese diabetic (NOD) mouse model. We found that this combined therapy effectively reversed new-onset diabetes within a week of therapy, and even maintained euglycemia up to 145 days after treatment withdrawal. The therapeutic effect of this regimen was associated with improved beta-cell metabolism and insulin secretion, while reducing beta-cell apoptosis. It is possible that such combined therapy could become a new strategy to defeat T1DM in humans.     

Androgen receptor: a new player associated with apoptosis and proliferation of pancreatic beta-cell in type 1 diabetes mellitus

Androgen receptor (AR) mediates a wide range of cellular processes, such as proliferation, differentiation and apoptosis. Here we sought to identify whether AR was located in pancreatic beta-cells and investigate its functions in type 1 diabetes induced by multiple low doses of streptozotocin. Double/triple immunofluorescence, Western blot and semi-quantitative RT-PCR were carried out to determine variances of AR expression in beta-cells and correlation between AR and apoptosis/proliferation of beta-cells with progress of diabetes. In addition, in vitro primary beta-cells from control mice were cultured for 3 days or 6 days with compound stimulation in order to further identify effect of AR on beta-cell apoptosis and proliferation. AR expression in beta-cells peaked in control and 1-day diabetic mice, gradually and significantly decreased, even disappeared in diabetic mice with progress of diabetes. TUNEL-positive beta-cells were concomitant with overexpression of AR, and Ki67-positive beta-cells showed extremely weak, even negative AR staining. In vitro, AR could mediate beta-cell apoptosis, and AR antagonist flutamide contributed to beta-cell proliferation. In conclusion, AR is abundantly expressed in pancreatic beta-cell cytoplasm of control mice. With progress of type 1 diabetes, decrement of AR expression in diabetic mice contributes to prohibit beta-cells from apoptosis, and is strongly associated with beta-cell proliferation.     

Oxidative stress and the JNK pathway are involved in the development of type 1 and type 2 diabetes

Failure of pancreatic beta-cells is the common characteristic of type 1 and type 2 diabetes. Type 1 diabetes mellitus is induced by destruction of pancreatic beta-cells which is mediated by an autoimmune mechanism and consequent inflammatory process. Various inflammatory cytokines and oxidative stress are produced during this process, which has been proposed to play an important role in mediating beta-cell destruction. The JNK pathway is also activated by such cytokines and oxidative stress, and is involved in beta-cell destruction. Type 2 diabetes is the most prevalent and serious metabolic disease, and beta-cell dysfunction and insulin resistance are the hallmark of type 2 diabetes. Under diabetic conditions, chronic hyperglycemia gradually deteriorates beta-cell function and aggravates insulin resistance. This process is called "glucose toxicity". Under such conditions, oxidative stress is provoked and the JNK pathway is activated, which is likely involved in pancreatic beta-cells dysfunction and insulin resistance. In addition, oxidative stress and activation of the JNK pathway are also involved in the progression of atherosclerosis which is often observed under diabetic conditions. Taken together, it is likely that oxidative stress and subsequent activation of the JNK pathway are involved in the pathogenesis of type 1 and type 2 diabetes.     

Beta-cells in type 2 diabetes: a loss of function and mass

Type 2 diabetes mellitus manifests itself in individuals who lose the ability to produce sufficient amounts of insulin to maintain normoglycaemia in the face of insulin resistance. The ability to secrete adequate amounts of insulin depends on beta-cell function and mass. Chronic hyperglycaemia is detrimental to pancreatic beta-cells, causing impaired insulin secretion and playing an essential role in the regulation of beta-cell turnover. This paper will address the effect of chronically elevated glucose levels on beta-cell turnover and function. In previous studies we have shown that elevated glucose concentrations induce apoptosis in human beta-cells due to an interaction between constitutively expressed Fas ligand and upregulated Fas. Human beta-cells produce interleukin (IL)-1beta in response to high glucose concentrations, independently of an immune-mediated process. This was antagonized by the IL-1 receptor antagonist (IL-1Ra), a naturally occurring anti-inflammatory cytokine also found in the beta-cell. Therefore the balance of IL-1beta and IL-1Ra may play a crucial role in the pathogenesis of diabetes. Inhibition of glucotoxicity represents a promising therapeutic stratagem in diabetes therapy to preserve functional beta-cell mass.     

Impact of endoplasmic reticulum stress pathway on pancreatic beta-cells and diabetes mellitus

Diabetes is caused by impaired insulin secretion in pancreatic beta-cells and peripheral insulin resistance. Overload of pancreatic beta-cells leads to beta-cell exhaustion and finally to the development of diabetes. Reduced beta-cell mass is evident in type 2 diabetes, and apoptosis is implicated in this process. One characteristic feature of beta-cells is highly developed endoplasmic reticulum (ER) due to a heavy engagement in insulin secretion. The ER serves several important functions, including post-translational modification, folding, and assembly of newly synthesized secretory proteins, and its proper function is essential to cell survival. Various conditions can interfere with ER function and these conditions are called ER stress. Recently, we found that nitric oxide (NO)-induced apoptosis in beta-cells is mediated by the ER-stress pathway. NO causes ER stress and leads to apoptosis through induction of ER stress-associated apoptosis factor CHOP. The Akita mouse with a missense mutation (Cys96Tyr) in the insulin 2 gene has hyperglycemia and a reduced beta-cell mass. This mutation disrupts a disulfide bond between A and B chains of insulin and may induce its conformational change. In the development of diabetes in Akita mice, mRNAs for an ER chaperone Bip and CHOP were induced in the pancreas. Overexpression of the mutant insulin in mouse MIN6 beta-cells induced CHOP expression and led to apoptosis. Targeted disruption of the CHOP gene did not delay the onset of diabetes in the homozygous Akita mice, but it protected islet cells from apoptosis and delayed the onset of diabetes in the heterozygous Akita mice. We conclude that ER overload in beta-cells causes ER stress and leads to apoptosis via CHOP induction. These results highlight the importance of chronic ER stress in beta-cell apoptosis in type 2 diabetes, and suggest a new target to the management of the disease.     

脂肪细胞对胰岛β细胞功能的内分泌调节作用

脂肪因子包括脂肪细胞分泌的多种活性因子,它们通过内分泌方式调节胰岛β细胞的胰岛素分泌、基因表达以及细胞凋亡等多方面的功能。本文提出脂肪因子影响胰岛β细胞功能主要通过三条相互联系的途径而实现。第一是调节β细胞内葡萄糖和脂肪的代谢;第二是影响β细胞离子通道的活性;第三是改变β细胞本身的胰岛素敏感性。脂肪细胞的内分泌功能是一个动态过程,在不同的代谢状态下,各脂肪因子的分泌发生不同变化。从正常代谢状态发展到肥胖以及2型糖尿病的过程中,脂肪因子参与了胰岛β细胞功能障碍的发生与发展。     

Pancreatic beta-cells: From generation to regeneration

The pancreas is composed of two main compartments consisting of endocrine and exocrine tissues. The majority of the organ is exocrine and responsible for the synthesis of digestive enzymes and for their transport via an intricate ductal system into the duodenum. The endocrine tissue represents less than 2% of the organ and is organized into functional units called islets of Langerhans, comprising alpha-, beta-, delta-, epsilon- and PP-cells, producing the hormones glucagon, insulin, somatostatin, ghrelin and pancreatic polypeptide (PP), respectively. Insulin-producing beta-cells play a central role in the control of the glucose homeostasis. Accordingly, absolute or relative deficiency in beta-cells may ultimately lead to type 1 and/or type 2 diabetes, respectively. One major goal of diabetes research is therefore to understand the molecular mechanisms controlling the development of beta-cells during pancreas morphogenesis, but also those underlying the regeneration of adult injured pancreas, and assess their significance for future cell-based therapy. In this review, we will therefore present new insights into beta-cell development with focus on beta-cell regeneration.     

Diminished beta-cell replication contributes to reduced beta-cell mass in fetal sheep with intrauterine growth restriction

Human fetuses with severe intrauterine growth restriction (IUGR) have less pancreatic endocrine tissue and exhibit beta-cell dysfunction, which may limit beta-cell function in later life and contribute to their increased incidence of noninsulin-dependent diabetes mellitus. Three factors, replication, apoptosis, and neoformation, contribute to fetal beta-cell mass. We studied an ovine model of IUGR to understand whether nutrient deficits lead to decreased rates of fetal pancreatic beta-cell replication, increased rates of apoptosis, or lower rates of differentiation. At 90% of term gestation, IUGR fetal and pancreatic weights were 58% and 59% less than pair-fed control, respectively. We identified a selective impairment of beta-cell mass compared with other pancreatic cell types in IUGR fetuses. Insulin and insulin mRNA contents were less than other pancreatic endocrine hormones in IUGR fetuses, as were pancreatic insulin positive area (42%) and beta-cell mass (76%). Pancreatic beta-cell apoptosis was not different between treatments. beta-cell capacity for cell cycling, determined by proliferating cell nuclear antigen (PCNA) immunostaining, was not different between treatment groups. However, the percentage of beta-cells actually undergoing mitosis was 72% lower in IUGR fetuses. These results indicate that in utero nutrient deficits decrease the population of pancreatic beta-cells by lengthening G1, S, and G2 stages of interphase and decreasing mitosis near term. Diminished beta-cell mass in IUGR infants at birth, if not adequately compensated for after birth, may contribute to insufficient insulin production in later life and, thus, a predisposition to noninsulin-dependent diabetes.     

A role for G(z) in pancreatic islet beta-cell biology

Glucose-stimulated insulin secretion and beta-cell growth are important facets of pancreatic islet beta-cell biology. As a result, factors that modulate these processes are of great interest for the potential treatment of Type 2 diabetes. Here, we present evidence that the heterotrimeric G protein G(z) and its effectors, including some previously thought to be confined in expression to neuronal cells, are present in pancreatic beta-cells, the largest cellular constituent of the islets of Langerhans. Furthermore, signaling pathways upon which G alpha(z) impacts are intact in beta-cells, and G alpha(z) activation inhibits both cAMP production and glucose-stimulated insulin secretion in the Ins-1(832/13) beta-cell-derived line. Inhibition of glucose-stimulated insulin secretion by prostaglandin E (PGE1) is pertussis-toxin insensitive, indicating that other G alpha(i) family members are not involved in this process in this beta-cell line. Indeed, overexpression of a selective deactivator of G alpha(z), the RGS domain of RGSZ1, blocks the inhibitory effect of PGE1 on glucose-stimulated insulin secretion. Finally, the inhibition of glucose-stimulated insulin secretion by PGE1 is substantially blunted by small interfering RNA-mediated knockdown of G alpha(z) expression. Taken together, these data strongly imply that the endogenous E prostanoid receptor in the Ins-1(832/13) beta-cell line couples to G(z) predominantly and perhaps even exclusively. These data provide the first evidence for G(z) signaling in pancreatic beta-cells, and identify an endogenous receptor-mediated signaling process in beta-cells that is dependent on G alpha(z) function.     

Destruction of pancreatic beta-cells by transgenic induction of prostaglandin E2 in the islets

Type 2 diabetes mellitus is characterized by insulin resistance of peripheral tissues and dysfunction of pancreatic beta-cells. Furthermore, the number of pancreatic beta-cells decreases as a secondary effect of advanced type 2 diabetes, although the molecular mechanism has not been elucidated. Recently, it has been shown that hyperglycemic conditions induce the expression of cyclooxygenase-2 in pancreatic islets and increase the downstream product prostaglandin E(2) (PGE(2)). To investigate whether high glucose-induced PGE(2) has an adverse effect on pancreatic beta-cells, we generated transgenic mice (RIP-C2mE) that express cyclooxygenase-2 and microsomal prostaglandin E synthase-1 in their beta-cells using the rat insulin-2 gene promoter (RIP). The homozygous RIP-C2mE (Tg/Tg) mice showed severe hyperglycemia from six weeks of age. Although the heterozygous RIP-C2mE (Tg/-) mice showed normal blood glucose levels throughout their lifetime, this level increased significantly compared with that of wild-type mice when glucose was loaded. The relative number of beta-cells to the total islet cell number was reduced to 54 and 14% in the RIP-C2mE (Tg/-) and (Tg/Tg) mice, respectively, whereas that in the wild-type mice was 84%. Importantly, the proliferation rate in the islets of the RIP-C2mE (Tg/Tg) mice at four weeks of age decreased significantly in comparison to that in the wild-type mice. Because beta-cells replicate not only during the postnatal period but also in the adult pancreas at a basal level, it is possible that increased PGE(2) signaling thus contributes to the reduction of the pancreatic beta-cell mass through inhibition of proliferation, thereby aggravating diabetes further.     

Insulin-producing cells derived from stem cells: recent progress and future directions

Type 1 diabetes is characterized by the selective destruction of pancreatic beta-cells caused by an autoimmune attack. Type 2 diabetes is a more complex pathology which, in addition to beta-cell loss caused by apoptotic programs, includes beta-cell dedifferentiation and peripheric insulin resistance. beta-Cells are responsible for insulin production, storage and secretion in accordance to the demanding concentrations of glucose and fatty acids. The absence of insulin results in death and therefore diabetic patients require daily injections of the hormone for survival. However, they cannot avoid the appearance of secondary complications affecting the peripheral nerves as well as the eyes, kidneys and cardiovascular system. These afflictions are caused by the fact that external insulin injection does not mimic the tight control that pancreatic-derived insulin secretion exerts on the body's glycemia. Restoration of damaged beta-cells by transplantation from exogenous sources or by endocrine pancreas regeneration would be ideal therapeutic options. In this context, stem cells of both embryonic and adult origin (including beta-cell/islet progenitors) offer some interesting alternatives, taking into account the recent data indicating that these cells could be the building blocks from which insulin secreting cells could be generated in vitro under appropriate culture conditions. Although in many cases insulin-producing cells derived from stem cells have been shown to reverse experimentally induced diabetes in animal models, several concerns need to be solved before finding a definite medical application. These refer mainly to the obtainment of a cell population as similar as possible to pancreatic beta-cells, and to the problems related with the immune compatibility and tumor formation. This review will summarize the different approaches that have been used to obtain insulin-producing cells from embryonic and adult stem cells, and the main problems that hamper the clinical applications of this technology.     

Unraveling the pathogenesis of type 1 diabetes with proteomics: present and future directions

Type 1 diabetes (T1D) is the result of selective destruction of the insulin-producing beta-cells in the pancreatic islets of Langerhans. T1D is due to a complex interplay between the beta-cell, the immune system, and the environment in genetically susceptible individuals. The initiating mechanism(s) behind the development of T1D are largely unknown, and no genes or proteins are specific for most T1D cases. Different pro-apoptotic cytokines, IL-1 beta in particular, are present in the islets during beta-cell destruction and are able to modulate beta-cell function and induce beta-cell death. In beta-cells exposed to IL-1 beta, a race between destructive and protective events are initiated and in susceptible individuals the deleterious events prevail. Proteins are involved in most cellular processes, and it is thus expected that their cumulative expression profile reflects the specific activity of cells. Proteomics may be useful in describing the protein expression profile and thus the diabetic phenotype. Relatively few studies using proteomics technologies to investigate the T1D pathogenesis have been published to date despite the defined target organ, the beta-cell. Proteomics has been applied in studies of differentiating beta-cells, cytokine exposed islets, dietary manipulated islets, and in transplanted islets. Although that the studies have revealed a complex and detailed picture of the protein expression profiles many functional implications remain to be answered. In conclusion, a rather detailed picture of protein expression in beta-cell lines, islets, and transplanted islets both in vitro and in vivo have been described. The data indicate that the beta-cell is an active participant in its own destruction during diabetes development. No single protein alone seems to be responsible for the development of diabetes. Rather the cumulative pattern of changes seems to be what favors a transition from dynamic stability in the unperturbed beta-cell to dynamic instability and eventually to beta-cell destruction.     

Islet amyloid polypeptide (IAPP) transgenic rodents as models for type 2 diabetes

Blood glucose concentrations are maintained by insulin secreted from beta-cells located in the islets of Langerhans. There are approximately 2000 beta-cells per islet, and approximately one million islets of Langerhans scattered throughout the pancreas. The islet in type 2 diabetes mellitus (T2D) has deficient beta-cell mass due to increased beta-cell apoptosis and islet amyloid derived from islet amyloid polypeptide (IAPP). Accumulating evidence implicates toxic IAPP oligomers in the mediation of beta-cell apoptosis in T2D. Humans, monkeys, and cats express an amyloidogenic toxic form of IAPP and spontaneously develop diabetes characterized by islet amyloid deposits. However, longitudinal studies of islet pathology in humans are impossible, and studies in nonhuman primates and cats are costly and impractical. Rodent IAPP is not amyloidogenic, thus commonly used rodent models of diabetes do not recapitulate islet pathology in humans. To investigate the diabetogenic role of human IAPP (h-IAPP), several mouse models and, more recently, a rat model transgenic for h-IAPP have been developed. Studies in these models have revealed that the toxic effect of h-IAPP on beta-cell apoptosis demonstrates a threshold-dependent effect. Specifically, increasing h-IAPP transgene expression by breeding or induction of insulin resistance leads to increased beta-cell apoptosis and diabetes. These transgenic rodent models for h-IAPP provide an opportunity to elucidate the mechanisms responsible for h-IAPP-induced beta-cell apoptosis further and to test novel approaches to the prevention and treatment of T2D.