Evidence for a sustained increase in clonal beta-cell basal intracellular Ca2+ levels after incubation in the presence of newly diagnosed Type-1 diabetic patient sera. Possible role in serum-induced inhibition of insulin secretion

A cell-based assay for the detection of pathogenic anti-voltage-gated calcium channel autoantibodies in immunoglobulin G from patients with type 1 diabetes

Recent studies have postulated the presence of functional autoantibodies (Abs) against L-type voltage gated calcium channels (VGCCs) in the serum of patients with type 1 diabetes, with various proposed physiological consequences, both islet cell associated and extra-glandular. Arguably, the most potentially damaging effect reported for these Abs is induction of apoptosis in pancreatic beta (β) cells, yet a convincing pathogenic mechanism remains to be demonstrated. In the current study, we report an assay of reactive oxygen species (ROS) stress induction in the rat insulinoma cell line Rin A12, as determined by 2', 7'-Dichlorofluorescein diacetate (DCF-DA) fluorescence detection by flow cytometry. We demonstrate that incubation of Rin A12 cells with immunoglobulin G (IgG) containing anti-VGCC activity from patients with T1D mediates a significant increase in ROS, with subsequent induction of apoptosis, as determined by positivity for annexin V expression. Neither T1D patient-derived IgG lacking anti-VGCC activity or IgG from healthy donors altered ROS or annexin V expression, indicating the new assay is specific for the detection of functional anti-VGCC Abs. Subsequent screening of IgG samples derived from individual patients indicated a prevalence of approximately 75% in a cohort of 20 patients with T1D. The new cell-based assay provides, for the first time, experimental evidence supporting a plausible pathophysiological mechanism underlying anti-VGCC Ab-mediated apoptosis induction in β cells. Additionally, the assay is a considerable advance on previously published methods for detecting and characterising the functional activity of anti-VGCC Abs in patient-derived samples.

Protective Role of Complement C3 Against Cytokine-Mediated β-Cell Apoptosis

Type 1 diabetes is a chronic autoimmune disease characterized by pancreatic islet inflammation and β-cell destruction by proinflammatory cytokines and other mediators. Based on RNA sequencing and protein-protein interaction analyses of human islets exposed to proinflammatory cytokines, we identified complement C3 as a hub for some of the effects of cytokines. The proinflammatory cytokines interleukin-1β plus interferon-γ increase C3 expression in rodent and human pancreatic β-cells, and C3 is detected by histology in and around the islets of diabetic patients. Surprisingly, C3 silencing exacerbates apoptosis under both basal condition and following exposure to cytokines, and it increases chemokine expression upon cytokine treatment. C3 exerts its prosurvival effects via AKT activation and c-Jun N-terminal kinase inhibition. Exogenously added C3 also protects against cytokine-induced β-cell death and partially rescues the deleterious effects of inhibition of endogenous C3. These data suggest that locally produced C3 is an important prosurvival mechanism in pancreatic β-cells under a proinflammatory assault.

The role of the complement system in metabolic organs and metabolic diseases. Semin Immunol. 2013;25:47-53. [PMID: 23684628 DOI: 10.1016/j.smim.2013.04.003]

Emerging evidence points to a close crosstalk between metabolic organs and innate immunity in the course of metabolic disorders. In particular, cellular and humoral factors of innate immunity are thought to contribute to metabolic dysregulation of the adipose tissue or the liver, as well as to dysfunction of the pancreas; all these conditions are linked to the development of insulin resistance and diabetes mellitus. A central component of innate immunity is the complement system. Interestingly, the classical view of complement as a major system of host defense that copes with infections is changing to that of a multi-functional player in tissue homeostasis, degeneration, and regeneration. In the present review, we will discuss the link between complement and metabolic organs, focusing on the pancreas, adipose tissue, and liver and the diverse effects of complement system on metabolic disorders.

The central role of calcium in the effects of cytokines on beta-cell function: implications for type 1 and type 2 diabetes

The appropriate regulation of intracellular calcium is a requirement for proper cell function and survival. This review focuses on the effects of proinflammatory cytokines on calcium regulation in the insulin-producing pancreatic beta-cell and how normal stimulus-secretion coupling, organelle function, and overall beta-cell viability are impacted. Proinflammatory cytokines are increasingly thought to contribute to beta-cell dysfunction not only in type 1 diabetes (T1D), but also in the progression of type 2 diabetes (T2D). Cytokine-induced disruptions in calcium handling result in reduced insulin release in response to glucose stimulation. Cytokines can alter intracellular calcium levels by depleting calcium from the endoplasmic reticulum (ER) and by increasing calcium influx from the extracellular space. Depleting ER calcium leads to protein misfolding and activation of the ER stress response. Disrupting intracellular calcium may also affect organelles, including the mitochondria and the nucleus. As a chronic condition, cytokine-induced calcium disruptions may lead to beta-cell death in T1D and T2D, although possible protective effects are also discussed. Calcium is thus central to both normal and pathological cell processes. Because the tight regulation of intracellular calcium is crucial to homeostasis, measuring the dynamics of calcium may serve as a good indicator of overall beta-cell function.

Evidence that low-grade systemic inflammation can induce islet dysfunction as measured by impaired calcium handling

In obesity and the early stages of type 2 diabetes (T2D), proinflammatory cytokines are mildly elevated in the systemic circulation. This low-grade systemic inflammation exposes pancreatic islets to these circulating cytokines at much lower levels than seen within the islet during insulitis. These low-dose effects have not been well described. We examined mouse islets treated overnight with a low-dose cytokine combination commonly associated with inflammation (TNF-alpha, IL-1 beta, and IFN-gamma). We then examined islet function primarily using intracellular calcium ([Ca(2+)](i)), a key component of insulin secretion and cytokine signaling. Cytokine-treated islets demonstrated several features that suggested dysfunction including excess [Ca(2+)](i) in low physiological glucose (3mM), reduced responses to glucose stimulation, and disrupted [Ca(2+)](i) oscillations. Interestingly, islets taken from young db/db mice showed similar disruptions in [Ca(2+)](i) dynamics as cytokine-treated islets. Additional studies of control islets showed that the cytokine-induced elevation in basal [Ca(2+)](i) was due to both greater calcium influx through L-type-calcium-channels and reduced endoplasmic reticulum (ER) calcium storage. Many of these cytokine-induced disruptions could be reproduced by SERCA blockade. Our data suggest that chronic low-grade inflammation produces circulating cytokine levels that are sufficient to induce beta-cell dysfunction and may play a contributing role in beta-cell failure in early T2D.

Overexpression of the malate-aspartate NADH shuttle member Aralar1 in the clonal beta-cell line BRIN-BD11 enhances amino-acid-stimulated insulin secretion and cell metabolism

In the present study, we have investigated the effects of the transduction with recombinant adenovirus AdCA-Aralar1 (aspartate–glutamate carrier 1) on the metabolism, function and secretory properties of the glucose- and amino-acid-responsive clonal insulin-secreting cell line BRIN-BD11. Aralar1 overexpression increased long-term (24 h) and acute (20 min) glucose- and amino-acid-stimulated insulin secretion, cellular glucose metabolism, L-alanine and L-glutamine consumption, cellular ATP and glutamate concentrations, and stimulated glutamate release. However, cellular triacylglycerol and glycogen contents were decreased as was lactate production. These findings indicate that increased malate–aspartate shuttle activity positively shifted β-cell metabolism, thereby increasing glycolysis capacity, stimulus–secretion coupling and, ultimately, enhancing insulin secretion. We conclude that Aralar1 is a key metabolic control site in insulin-secreting cells.

In vivo and in vitro studies of GAD-antibody positive subjects with Type 2 diabetes: A distinct sub-phenotype

The purpose of this study was to determine if immune mechanisms in GAD positive patients' contribute to the pathogenesis of a specific sub-type of Type 2 diabetes. GAD positive (n=8) and GAD negative (n=8) subjects diagnosed with Type 2 diabetes were matched for age, gender, body mass index, duration of diabetes and glycaemic control. All subjects underwent an insulin-modified frequently sampled intravenous glucose tolerance test to measure insulin sensitivity and insulin secretory function with minimal model analysis. In addition, BRIN-BD11 clonal beta-cells were supplemented with patients' sera to determine basal and alanine-stimulated insulin secretion and terminal complement complex (TCC) formation. Both groups were severely insulin resistant (0.56+/-0.17 vs. 0.99+/-0.3310(-4)min(-1)/(microUml(-1)) for GADneg and GADpos, respectively) but the GAD negative subjects had a higher basal (87+/-11 vs. 58+/-14pmoll(-1), p<0.05) and glucose-stimulated insulin secretion (DeltaAUCins 0.96+/-0.12 vs. 0.60+/-0.12pmol/(l(-1)min), p<0.05). In vivo measures of insulin secretion were negatively correlated with TCC formation, independent of antibody status. In conclusion, GAD positive subjects initially diagnosed with Type 2 diabetes are unable to compensate for insulin resistance due to more pronounced beta-cell impairment. TCC formation may be partly responsible for the insulin secretory dysfunction associated with this specific sub-type of Type 2 diabetes.

Physiological regulation of the pancreatic β-cell: functional insights for understanding and therapy of diabetes

Knowledge about the sites and actions of the numerous physiological and pharmacological factors affecting insulin secretion and pancreatic beta-cell function has been derived from the use of bioengineered insulin-producing cell lines. Application of an innovative electrofusion approach has generated novel glucose-responsive insulin-secreting cells for pharmaceutical and experimental research, including popular BRIN-BD11 beta-cells. This review gives an overview of the establishment and core characteristics of clonal electrofusion-derived BRIN-BD11 beta-cells. As discussed, BRIN-BD11 cells have facilitated studies aimed at dissecting important pathways by which nutrients and other bioactive molecules regulate the complex mechanisms regulating insulin secretion, and highlight the future potential of novel and diverse bioengineering approaches to provide a cell-based insulin-replacement therapy for diabetes. Clonal BRIN-BD11 beta-cells have been instrumental in: (a) characterization of K(ATP) channel-dependent and -independent actions of nutrients and established and emerging insulinotropic antidiabetic drugs, and the understanding of drug-induced beta-cell desensitization; (b) tracing novel metabolic and beta-cell secretory pathways, including use of state-of-the-art NMR approaches to provide new insights into the relationships between glucose and amino acid handling and insulin secretion; and (c) determination of the chronic detrimental actions of nutrients and the diabetic environment on pancreatic beta-cells, including the recent discovery that homocysteine, a risk factor for metabolic syndrome, may play a role in the progressive demise of insulin secretion and pancreatic beta-cell function in diabetes. Collectively, the studies discussed in this review highlight the importance of innovative experimental beta-cell physiology in the discovery and characterization of new and improved drugs and therapeutic strategies to help tackle the emerging diabetes epidemic.