Anatomical versus functional beta-cell mass in experimental diabetes

The ability of pancreatic beta-cell mass to vary according to insulin requirements is an important component of optimal long-term control of glucose homeostasis. It is generally assumed that alteration of this property largely contributes to the impairment of insulin secretion in type 2 diabetes. However, data in humans are scarce and it is impossible to correlate beta-cell mass and function with the various stages of the disease. Thus, the importance of animal models is obvious. In rodents, increased beta-cell mass associated with an increase in the function of individual beta-cells contributes to the adaptation of the insulin response to insulin resistance in late pregnancy and in obesity. A reduction in beta-cell mass always corresponds to an alteration in insulin secretory capacity of islet tissue (Zucker diabetic fatty and Goto-Kakisaki rats, db/db mice). During regenerative processes following experimental reduction of beta-cell mass [partial pancreatectomy, streptozocin (STZ) injection], beta-cell mass increase is not associated with a corresponding improvement of beta-cell function, thus indicating that regenerative beta-cells did not achieve functional maturity. The main lesson from experimental diabetes is therefore that beta-cell mass cannot always predict functional capacity of the beta-cell tissue and that the functional beta-cell mass rather than the anatomical beta-cell mass must be taken into account at all times.     

Islet amyloid polypeptide and type 2 diabetes

Type 2 diabetes is associated with progressive beta-cell failure manifest as a decline in insulin secretion and increasing hyperglycemia. A growing body of evidence suggests that beta-cell failure in type 2 diabetes correlates with the formation of pancreatic islet amyloid deposits, indicating that islet amyloid may have an important role in beta-cell loss in this disease. Islet amyloid polypeptide (IAPP; amylin), the major component of islet amyloid, is co-secreted with insulin from beta-cells. In type 2 diabetes, this peptide aggregates to form amyloid fibrils that are toxic to beta-cells. The mechanism(s) responsible for islet amyloid formation in type 2 diabetes is still unclear but it appears that an increase in the secretion of IAPP, per se, is not sufficient. Other factors, such as impairment in the processing of proIAPP, the IAPP precursor, have been proposed to contribute to the development of islet amyloid deposits. Inhibitors of islet amyloid fibril formation might prevent the progression to beta-cell failure in type 2 diabetes and should therefore be considered as a therapeutic approach to treat this disease.     

Exercise and dexamethasone oppositely modulate beta-cell function and survival via independent pathways in 90% pancreatectomized rats

Long-term dexamethasone (DEX) treatment is well known for its ability to increase insulin resistance in liver and adipose tissues leading to hyperinsulinemia. On the other hand, exercise enhances peripheral insulin sensitivity. However, it is not clear whether DEX and/or exercise affect beta-cell mass and function in diabetic rats, and whether their effects can be associated with the modulation of the insulin/IGF-I signaling cascade in pancreatic beta-cells. After an 8-week study, whole body glucose disposal rates in 90% pancreatectomized (Px) and sham-operated male rats decreased with a high dose treatment of DEX (0.1mg DEX/kg body weight/day)(HDEX) treatment, while disposal rates increased with exercise. First-phase insulin secretion was decreased and delayed by DEX via the impairment of the glucose-sensing mechanism in beta-cells, while exercise reversed the impairment of first-phase insulin secretion caused by DEX, suggesting ameliorated beta-cell functions. However, exercise and DEX did not alter second-phase insulin secretion except for the fact that HDEX decreased insulin secretion at 120 min during hyperglycemic clamp in Px rats. Unlike beta-cell functions, DEX and exercise exhibited increased pancreatic beta-cell mass in two different pathways. Only exercise, through increased proliferation and decreased apoptosis, increased beta-cell mass via hyperplasia, which resulted from an enhanced insulin/IGF-I signaling cascade by insulin receptor substrate 2 induction. By contrast, DEX expanded beta-cell mass via hypertrophy and neogenesis from precursor cells, rather than increasing proliferation and decreasing apoptosis. In conclusion, the improvement of beta-cell function and survival via the activation of an insulin/IGF-I signaling cascade due to exercise has a crucial role in preventing the development and progression of type 2 diabetes.     

Molecular control of cell cycle progression in the pancreatic beta-cell

Type 1 and type 2 diabetes both result from inadequate production of insulin by the beta-cells of the pancreatic islet. Accordingly, strategies that lead to increased pancreatic beta-cell mass, as well as retained or enhanced function of islets, would be desirable for the treatment of diabetes. Although pancreatic beta-cells have long been viewed as terminally differentiated and irreversibly arrested, evidence now indicates that beta-cells can and do replicate, that this replication can be enhanced by a variety of maneuvers, and that beta-cell replication plays a quantitatively significant role in maintaining pancreatic beta-cell mass and function. Because beta-cells have been viewed as being unable to proliferate, the science of beta-cell replication is undeveloped. In the past several years, however, this has begun to change at a rapid pace, and many laboratories are now focused on elucidating the molecular details of the control of cell cycle in the beta-cell. In this review, we review the molecular details of cell cycle control as they relate to the pancreatic beta-cell. Our hope is that this review can serve as a common basis and also a roadmap for those interested in developing novel strategies for enhancing beta-cell replication and improving insulin production in animal models as well as in human pancreatic beta-cells.     

Relationship between beta-cell mass and diabetes onset

Regulation of blood glucose concentrations requires an adequate number of beta-cells that respond appropriately to blood glucose levels. beta-Cell mass cannot yet be measured in humans in vivo, necessitating autopsy studies, although both pre- and postmorbid changes may confound this approach. Autopsy studies report deficits in beta-cell mass ranging from 0 to 65% in type 2 diabetes (T2DM), and approximately 70-100% in type 1 diabetes (T1DM), and, when evaluated, increased beta-cell apoptosis in both T1DM and T2DM. A deficit of beta-cell mass of approximately 50% in animal studies leads to impaired insulin secretion (when evaluated directly in the portal vein) and induction of insulin resistance. We postulate three phases for diabetes progression. Phase 1: selective beta-cell cytotoxicity (autoimmune in T1DM, unknown in T2DM) leading to impaired beta-cell function and gradual loss of beta-cell mass through apoptosis. Phase 2: decompensation of glucose control when the pattern of portal vein insulin secretion is sufficiently impaired to cause hepatic insulin resistance. Phase 3: adverse consequences of glucose toxicity accelerate beta-cell dysfunction and insulin resistance. The relative contribution of beta-cell loss versus beta-cell dysfunction to diabetes onset remains an area of controversy. However, because cytotoxicity sufficient to induce beta-cell apoptosis predictably disturbs beta-cell function, it is naive to attempt to distinguish the relative contributions of these linked processes to diabetes onset.     

Restoring a functional beta-cell mass in diabetes

Type 1 and type 2 diabetes have often been presented as disease forms that profoundly differ in the presence and pathogenic significance of a reduced beta-cell mass. We review evidence indicating that the beta-cell mass in type 1 diabetes is usually not decreased by at least 90% at clinical onset, and remains often detectable for years after diagnosis at age above 15 years. Clinical and experimental evidence also exists for a reduced beta-cell mass in type 2 diabetes where it can be the cause for and/or the consequence of dysregulated beta-cell functions. With beta-cell mass defined as number of beta-cells, these views face the limitation of insufficient data and methods for human organs. Because beta-cells can occur under different phenotypes that vary with age and with environmental conditions, we propose to use the term functional beta-cell mass as an assessment of a beta-cell population by the number of beta-cells and their phenotype or functional state. Assays exist to measure functional beta-cell mass in isolated preparations. We selected a glucose-clamp test to evaluate functional beta-cell mass in type 1 patients at clinical onset and in type 1 recipients following intraportal islet cell transplantation. Comparison of the data with those in non-diabetic controls helps targeting and monitoring of therapeutic interventions.     

Crucial role of insulin receptor substrate-2 in compensatory beta-cell hyperplasia in response to high fat diet-induced insulin resistance

In type 2 diabetes, there is a defect in the regulation of functional beta-cell mass to overcome high-fat (HF) diet-induced insulin resistance. Many signals and pathways have been implicated in beta-cell function, proliferation and apoptosis. The co-ordinated regulation of functional beta-cell mass by insulin signalling and glucose metabolism under HF diet-induced insulin-resistant conditions is discussed in this article. Insulin receptor substrate (IRS)-2 is one of the two major substrates for the insulin signalling. Interestingly, IRS-2 is involved in the regulation of beta-cell proliferation, as has been demonstrated using knockout mice models. On the other hand, in an animal model for human type 2 diabetes with impaired insulin secretion because of insufficiency of glucose metabolism, decreased beta-cell proliferation was observed in mice with beta-cell-specific glucokinase haploinsufficiency (Gck(+/) (-)) fed a HF diet without upregulation of IRS-2 in beta-cells, which was reversed by overexpression of IRS-2 in beta-cells. As to the mechanism underlying the upregulation of IRS-2 in beta-cells, glucose metabolism plays an important role independently of insulin, and phosphorylation of cAMP response element-binding protein triggered by calcium-dependent signalling is the critical pathway. Downstream from insulin signalling via IRS-2 in beta-cells, a reduction in FoxO1 nuclear exclusion contributes to the insufficient proliferative response of beta-cells to insulin resistance. These findings suggest that IRS-2 is critical for beta-cell hyperplasia in response to HF diet-induced insulin resistance.     

Induction of beta-cell rest by a Kir6.2/SUR1-selective K(ATP)-channel opener preserves beta-cell insulin stores and insulin secretion in human islets cultured at high (11 mM) glucose

In health, most insulin is secreted in pulses. Type 2 diabetes mellitus (TTDM) is characterized by impaired pulsatile insulin secretion with a defect in insulin pulse mass. It has been suggested that this defect is partly due to chronic overstimulation of beta-cells imposed by insulin resistance and hyperglycemia, which results in depletion of pancreatic insulin stores. It has been reported that in TTDM overnight inhibition of insulin secretion (induction of beta-cell rest) leads to quantitative normalization of pulsatile insulin secretion upon subsequent stimulation. Recently, decreased orderliness of insulin secretion has been recognized as another attribute of impaired insulin secretion in TTDM. In the current studies we sought to address at the level of the isolated islet whether chronic elevated glucose concentrations induce both defects involved in impaired insulin secretion in TTDM: deficiency and decreased orderliness of insulin secretion. We use the concept of beta-cell rest, induced by a novel beta-cell selective K(ATP)-channel opener (KCO), NN414 (6-chloro-3-(1-methylcyclopropyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide), to test whether preservation of insulin stores leads to normalization of both processes in response to glucose stimulation. Human islets were isolated from three cadaveric organ donors and studied in perifusion experiments and static incubation. Acute activation of K(ATP)-channels suppressed insulin secretion from perifused human islets by approximately 90% (P < 0.0001). This KCO also inhibited glucagon secretion in a dose-dependent manner (P = 0.01). Static incubation at 11 and 16 vs. 4 mM glucose for 96 h decreased islet insulin stores by approximately 80% and 85% (P < 0.0001, respectively). In subsequent perifusion experiments, total insulin secretion ( approximately 30%; P < 0.01) from these islets and insulin pulse mass ( approximately 40%; P < 0.05) were both decreased (11 vs. 4 mM). The inhibition of insulin secretion during static incubation with KCO reduced the loss of islet insulin stores in a dose-dependent manner (P < 0.0001) and resulted in increased total insulin secretion (2.6-fold; P < 0.01) and insulin pulse mass (2.5-fold; P < 0.05) during subsequent perifusion. The orderliness of insulin secretion was significantly reduced after chronic incubation of human islets at 11 mM glucose (P = 0.04), but induction of beta-cell rest at 11 mM failed to normalize the regularity of insulin secretion during subsequent perifusion. We conclude that physiological increased glucose concentrations (11 mM), which are frequently observed in diabetes, lead to a loss of islet insulin stores and defective pulsatile insulin secretion as well as reduced orderliness of insulin secretion. Induction of beta-cell rest by selective activation of beta-cell K(ATP)-channels preserves insulin stores and pulsatile insulin secretion without restoring the orderliness of insulin secretion. Therefore, the concept of beta-cell rest may provide a strategy to protect beta-cells from chronic overstimulation and to improve islet function. Impaired glucose-regulated insulin secretion in TTDM may, however, partially involve mechanisms that are distinct from insulin stores and insulin secretion rates.     

Cell therapy for type 2 diabetes: is it desirable and can we get it?

The functional mass of beta-cells is decreased in type 2 diabetes. Replacing missing beta-cells or triggering their regeneration may thus allow for improved treatment of type 2 diabetes, to the extent that this is combined with therapy for improved insulin sensitivity. Although progress has been made in deriving beta-cell-like cells from stem or precursor cells in vitro, these cannot yet be obtained in sufficient quantities or well enough differentiated to envisage their therapeutic use in beta-cell replacement therapy. Likewise, our very limited understanding of beta-cell regeneration in adult man does not yet allow for development of a valid strategy for kick-starting such a process in individuals with type 2 diabetes, whether by bona fide neogenesis or self-replication of existing beta-cells. Regardless of how beta-cell mass is restored in type 2 diabetes, it will be important to prevent any renewed decrease thereafter. Current understanding suggests that islet inflammation as well as signals from (insulin-resistant/inflamed) adipose tissue and skeletal muscle contribute towards decreased beta-cell mass in type 2 diabetes. It will likely be important to protect newly formed or implanted beta-cells from these negative influences to ensure their long-term survival.     

Beta-cell apoptosis in type 2 diabetes: quantitative and functional consequences

Type 2 diabetes, the most common form of diabetes in humans, is characterized by impaired insulin secretion paralleled by a progressive decline in beta-cell function and chronic insulin resistance. Several authors have showed that in type 2 diabetes there is a reduction of islet and/or insulin-containing cell mass or volume. Regulation of the beta-cell mass appears to involve a balance of beta-cell replication and apoptosis but, at the molecular level, pancreatic beta-cell loss by apoptosis appears to play an important role in the development of insulin deficiency and the onset and/or progression of the disease. The mechanisms favoring apoptosis in type 2 diabetic pancreatic islets and new potential therapeutic approaches to prevent beta-cell death and maintain beta-cell mass are discussed.     

Islet amyloid: a critical entity in the pathogenesis of type 2 diabetes

Islet amyloid deposition is a pathogenic feature of type 2 diabetes, and these deposits contain the unique amyloidogenic peptide islet amyloid polypeptide. Autopsy studies in humans have demonstrated that islet amyloid is associated with loss of beta-cell mass, but a direct role for amyloid in the pathogenesis of type 2 diabetes cannot be inferred from such studies. Animal studies in both spontaneous and transgenic models of islet amyloid formation have shown that amyloid forms in islets before fasting hyperglycemia and therefore does not arise merely as a result of the diabetic state. Furthermore, the extent of amyloid deposition is associated with both loss of beta-cell mass and impairment in insulin secretion and glucose metabolism, suggesting a causative role for islet amyloid in the islet lesion of type 2 diabetes. These animal studies have also shown that beta-cell dysfunction seems to be an important prerequisite for islet amyloid formation, with increased secretory demand from obesity and/or insulin resistance acting to further increase islet amyloid deposition. Recent in vitro studies suggest that the cytotoxic species responsible for islet amyloid-induced beta-cell death are formed during the very early stages of islet amyloid formation, when islet amyloid polypeptide aggregation commences. Interventions to prevent islet amyloid formation are emerging, with peptide and small molecule inhibitors being developed. These agents could thus lead to a preservation of beta-cell mass and amelioration of the islet lesion in type 2 diabetes.     

Islet amyloid in type 2 diabetes, and the toxic oligomer hypothesis

Type 2 diabetes (T2DM) is characterized by insulin resistance, defective insulin secretion, loss of beta-cell mass with increased beta-cell apoptosis and islet amyloid. The islet amyloid is derived from islet amyloid polypeptide (IAPP, amylin), a protein coexpressed and cosecreted with insulin by pancreatic beta-cells. In common with other amyloidogenic proteins, IAPP has the propensity to form membrane permeant toxic oligomers. Accumulating evidence suggests that these toxic oligomers, rather than the extracellular amyloid form of these proteins, are responsible for loss of neurons in neurodegenerative diseases. In this review we discuss emerging evidence to suggest that formation of intracellular IAPP oligomers may contribute to beta-cell loss in T2DM. The accumulated evidence permits the amyloid hypothesis originally developed for neurodegenerative diseases to be reformulated as the toxic oligomer hypothesis. However, as in neurodegenerative diseases, it remains unclear exactly why amyloidogenic proteins form oligomers in vivo, what their exact structure is, and to what extent these oligomers play a primary or secondary role in the cytotoxicity in what are now often called unfolded protein diseases.     

Protection of pancreatic beta-cells: Is it feasible?

Hyperglycemia, which is the biochemical hallmark of type 2 diabetes, mainly results from insulin resistance and beta-cell dysfunction. However, the latter is crucial in the development of the disease because diabetes cannot occur without an impairment of insulin secretion. Beta-cell failure is also responsible for progressive loss of metabolic control in type 2 diabetic patients and the eventual need for insulin treatment.An impairment of beta-cell function can be detected in several ways and can be observed already in pre-diabetic individuals. Histopathology studies documented that beta-cell volume is reduced in pre-diabetes and, to a greater extent, in type 2 diabetes mainly because the apoptotic rate of beta-cells is increased whereas neogenesis is intact.All anti-diabetic agents can improve, directly or indirectly, beta-cell function. However, only PPAR-gamma agonists and incretin-mimetic agents seem to have favorable effects on beta-cell morphology and volume.Many trials showed that type 2 diabetes can be prevented but few of them directly addressed the issue of beta-cell protection by the intervention used in the study. It is reasonable to conclude that in these trials diabetes prevention, which was based on the use of lifestyle changes (diet and/or exercise) or different drugs (tolbutamide, acarbose, metformin, glitazones, bezafibrate, orlistat, angiotensin converting enzyme inhibitors, angiotensin II receptor blockers or pravastatin), depended also, or mainly, on a protection of the beta-cells but in most studies data on insulin secretion are not available or are insufficient to draw firm conclusions. The mechanisms of beta-cell protection in these trials, if any, remain unknown. They could be various and likely included reduced glucotoxicity, lipotoxicity, insulin resistance, inflammation, oxidant stress and/or apoptosis, an amelioration of islet blood flow and/or favorable changes in cation balance within the islets.Contrasting the decline and the eventual failure of beta-cells is crucial in preventing type 2 diabetes as well as in changing the natural history of the disease, when it occurs. The protection can be achieved in several ways but any strategy should include a change in lifestyle in order to generate a healthier islet milieu. Among anti-diabetic drugs, PPAR-gamma agonists and incretin-mimetic agents are the most promising in the protection. Among other drugs, inhibitors of the renin–angiotensin system might play a significant role.The increased worldwide diffusion of type 2 diabetes and the progressive loss of metabolic control in affected patients are clear demonstrations that the strategies to protect the beta-cells implemented so far, if any, were largely inadequate. Anti-diabetic agents targeting the intimate mechanisms of beta-cell damage might change the scenario in the near future.     

Streptozotocin,an O-GlcNAcase inhibitor,blunts insulin and growth hormone secretion

Type 2 diabetes mellitus results from a complex interaction between nutritional excess and multiple genes. Whereas pancreatic beta-cells normally respond to glucose challenge by rapid insulin release (first phase insulin secretion), there is a loss of this acute response in virtually all of the type 2 diabetes patients with significant fasting hyperglycemia. Our previous studies demonstrated that irreversible intracellular accumulation of a glucose metabolite, protein O-linked N-acetylglucosamine modification (O-GlcNAc), is associated with pancreatic beta-cell apoptosis. In the present study, we show that streptozotocin (STZ), a non-competitive chemical blocker of O-GlcNAcase, induces an insulin secretory defect in isolated rat islet cells. In contrast, transgenic mice with down-regulated glucose to glucosamine metabolism in beta-cells exhibited an enhanced insulin secretion capacity. Interestingly, the STZ blockade of O-GlcNAcase activity is also associated with a growth hormone secretory defect and impairment of intracellular secretory vesicle trafficking. These results provide evidence for the roles of O-GlcNAc in the insulin secretion and possible involvement of O-GlcNAc in general glucose-regulated hormone secretion pathways.     

GLP-1 receptor signaling: effects on pancreatic beta-cell proliferation and survival

Type 2 diabetes is a metabolic disorder characterized by insulin resistance as well as a progressive deterioration of pancreatic beta-cell mass and function. Glucagon-like peptide 1 (GLP-1), an incretin hormone secreted by intestinal L cells, is a promising therapeutic agent in the treatment of diabetes. GLP-1 analogs and enhancers constitute a novel class of anti-diabetes medications which address both the insulin secretion defect as well as the decline in beta-cell mass. GLP-1 improves glucose-stimulated insulin secretion, restores glucose competence in glucose-resistant beta-cells, and stimulates insulin gene expression and biosynthesis. Furthermore, GLP-1 acts as a growth factor by promoting beta-cell proliferation, survival and neogenesis. This review focuses on the molecular mechanisms by which GLP-1 signaling induces beta-cell mass expansion.     

Evidence for a circulating islet cell growth factor in insulin-resistant states

Insulin resistance is a feature of many common disorders including obesity and type 2 diabetes mellitus. In these disorders, the beta-cells compensate for the insulin resistance for long periods of time with an increase in secretory capacity, an increase in beta-cell mass, or both. To determine whether the beta-cell response might relate to a circulating growth factor, we have transplanted normal islets under the kidney capsule of normoglycemic insulin-resistant mice with two different models of insulin resistance: lean mice that have a double heterozygous deletion of the insulin receptor and insulin receptor substrate-1 (DH) or the obese, hyperglycemic ob/ob mice. In the grafts transplanted into both hosts, there was a marked increase in beta-cell mitotic activity and islet mass that was comparable with that observed in the endogenous pancreas. By contrast, islets of the DH mouse transplanted into normal mice showed reduced mitotic index. These data suggest the insulin resistance is associated with a circulating islet cell growth factor that is independent of glucose and obesity.     

Growth hormone signaling in pancreatic beta-cells--calcium handling regulated by growth hormone

Deficiency in insulin secretion is a fundamental part in the pathogenesis of all forms of diabetes, determined by impaired secretory function and inadequate beta-cell mass. Growth hormone (GH) is a multifunctional hormone, involved in cellular metabolism, mitogenesis and differentiation. In pancreatic islets, GH is involved in maintaining beta-cell mass, stimulating islet hormone production and insulin secretion, and, therefore, plays a role in maintaining normal insulin sensitivity and glucose homeostasis. The intracellular events that convey the GH signal into various cellular responses remain incompletely understood. In this review, we discuss GH signaling in the beta-cells, with emphasis on Ca(2+) handling and insulin secretion regulated by human GH (hGH). hGH-stimulated rise in [Ca(2+)](i) is dependent on extracellular Ca(2+) and is mediated by Ca(2+)-induced Ca(2+) release (CICR) in the beta-cell. This process is triggered by hGH-stimulated activation of the non-receptor tyrosine kinases JAK2 and c-Src, which causes tyrosine phosphorylation of RyRs, resulting in sensitization of CICR. The rise in [Ca(2+)](i) elicited by hGH is associated with an enhanced insulin secretion, effects that are mediated mainly through the prolactin receptor. These mechanisms indicate that a rise in [Ca(2+)](i) elicited by activation of PRLR is JAK2-dependent and is associated with enhanced insulin secretion. In contrast, GH receptor-mediated increase in [Ca(2+)](i) is JAK2-independent and is dissociated from insulin secretion.     

Transgenic mice overproducing islet amyloid polypeptide have increased insulin storage and secretion in vitro

Summary To determine whether chronic overproduction of islet amyloid polypeptide alters beta-cell function, we studied a line of transgenic mice which overexpress islet amyloid polypeptide in their beta-cells. At 3 months of age, these transgenic mice had greater pancreatic content of both islet amyloid polypeptide and insulin. Further, basal and glucose-stimulated secretion of both islet amyloid polypeptide and insulin were also elevated in the perfused pancreas of the transgenic animals. These findings demonstrate that chronic overproduction and secretion of islet amyloid polypeptide are associated with increased insulin storage and enhanced secretion of insulin in vitro. This increase in insulin storage and secretion may be due to a direct effect of islet amyloid polypeptide on the beta-cell or a betacell adaptation to islet amyloid polypeptide-induced insulin resistance.Abbreviations IAPP Islet amyloid polypeptide - bp base pair - TFA trifluoroacetic acid - IRI immunoreactive insulin - SLI somatostatin-like immunoreactivity - IAPP-LI IAPP-like immunoreactivity