Aims/hypothesis Chronic inflammation in type 2 diabetes is proposed to affect islets as well as insulin target organs. However, the nature of islet inflammation and its effects on islet function in type 2 diabetes remain unclear. Moreover, the immune cell profiles of human islets in healthy and type 2 diabetic conditions are undefined. We aimed to investigate the correlation between proinflammatory cytokine expression, islet leucocyte composition and insulin secretion in type 2 diabetic human islets. Methods Human islets from organ donors with or without type 2 diabetes were studied. First and second phases of glucose-stimulated insulin secretion were determined by perifusion. The expression of inflammatory markers was obtained by quantitative PCR. Immune cells within human islets were analysed by FACS. Results Type 2 diabetic islets, especially those without first-phase insulin secretion, displayed higher CCL2 and TNFa expression than healthy islets. CD45+ leucocytes were elevated in type 2 diabetic islets, to a greater extent in moderately functional type 2 diabetic islets compared with poorly functional ones, and corresponded with elevated ALOX12 but not with CCL2 or TNFa expression. T and B lymphocytes and CD11c+ cells were detectable within both non-diabetic and type 2 diabetic islet leucocytes. Importantly, the proportion of B cells was significantly elevated within type 2 diabetic islets. Conclusions/interpretation Elevated total islet leucocyte content and proinflammatory mediators correlated with islet dysfunction, suggesting that heterogeneous insulitis occurs during the development of islet dysfunction in type 2 diabetes. In addition, the altered B cell content highlights a potential role for the adaptive immune response in islet dysfunction.

The excess accumulation of lipids in islets is thought to contribute to the development of diabetes in obesity by impairing ␤-cell function. However, lipids also serve a nutrient function in islets, and fatty acids acutely increase insulin secretion. A better understanding of lipid metabolism in islets will shed light on complex effects of lipids on ␤-cells. Adipose differentiation-related protein (ADFP) is localized on the surface of lipid droplets in a wide range of cells and plays an important role in intracellular lipid metabolism. We found that ADFP was highly expressed in murine ␤-cells. Moreover, islet ADFP was increased in mice on a high-fat diet (3.5-fold of control) and after fasting (2.5-fold of control), revealing dynamic changes in ADFP in response to metabolic cues. ADFP expression was also increased by addition of fatty acids in human islets. The downregulation of ADFP in MIN6 cells by antisense oligonucleotide (ASO) suppressed the accumulation of triglycerides upon fatty acid loading (56% of control) along with a reduction in the mRNA levels of lipogenic genes such as diacylglycerol O-acyltransferase-2 and fatty acid synthase. Fatty acid uptake, oxidation, and lipolysis were also reduced by downregulation of ADFP. Moreover, the reduction of ADFP impaired the ability of palmitate to increase insulin secretion. These findings demonstrate that ADFP is important in regulation of lipid metabolism and insulin secretion in ␤-cells. MIN6 cells; oleic acid; palmitic acid; high-fat diet; fasting THE CURRENT EPIDEMIC OF OBESITY is feared to increase the prevalence of type 2 diabetes due to its contribution to insulin resistance (38). However, excess adiposity and dyslipidemia commonly seen in obesity may have an additional role in the development of diabetes by directly damaging ␤-cells (45). Indeed, prolonged exposure to elevated levels of fatty acids impairs insulin secretion in vivo and ex vivo, a phenomenon termed "lipotoxicity" (16,47,50,56). Ceramide generation, an increase in reactive oxygen species, 12/15-lipoxygenase activation, and atypical protein kinase C activation are some of the mechanisms proposed for islet dysfunction and lipid-induced apoptosis (5, 44, 51, 52). On the other hand, lipids serve a nutrient function and are the principal energy source in islets deprived of exogenous nutrients (34, 54). Not only does acute exposure to fatty acids augment insulin secretion ex vivo, but a rise in fatty acids also facilitates glucose-stimulated insulin secretion (GSIS) after fasting in vivo (10). The activation of cell surface fatty acid receptor G protein-coupled receptor 40 (GPR40) plays a significant role in the augmentation of insulin secretion by fatty acids. However, cellular uptake of fatty acids is also believed to contribute to the insulin secretion by provision of lipid metabolites, including long-chain acyl-CoA and diacylglycerides (25,43,65). Thus, fatty acids play complex and seemingly contradictory roles in ␤-cells, which calls for a better understanding of fatty acid uptake, storage...

Elevation of circulating fatty acids (FA) during fasting supports postprandial (PP) insulin secretion that is critical for glucose homeostasis and is impaired in diabetes. We tested our hypothesis that lipid droplet (LD) protein perilipin 5 (PLIN5) in β-cells aids PP insulin secretion by regulating intracellular lipid metabolism. We demonstrated that PLIN5 serves as an LD protein in human islets. In vivo, Plin5 and triglycerides were increased by fasting in mouse islets. MIN6 cells expressing PLIN5 (adenovirus [Ad]-PLIN5) and those expressing perilipin 2 (PLIN2) (Ad-PLIN2) had higher [3H]FA incorporation into triglycerides than Ad-GFP control, which support their roles as LD proteins. However, Ad-PLIN5 cells had higher lipolysis than Ad-PLIN2 cells, which increased further by 8-Br-cAMP, indicating that PLIN5 facilitates FA mobilization upon cAMP stimulation as seen postprandially. Ad-PLIN5 in islets enhanced the augmentation of glucose-stimulated insulin secretion by FA and 8-Br-cAMP in G-protein–coupled receptor 40 (GPR40)- and cAMP-activated protein kinase–dependent manners, respectively. When PLIN5 was increased in mouse β-cells in vivo, glucose tolerance after an acute exenatide challenge was improved. Therefore, the elevation of islet PLIN5 during fasting allows partitioning of FA into LD that is released upon refeeding to support PP insulin secretion in cAMP- and GPR40-dependent manners.

The reduction of functional β cell mass is a key feature of type 2 diabetes. Here, we studied metabolic functions and islet gene expression profiles of C57BL/6J mice with naturally occurring nicotinamide nucleotide transhydrogenase (NNT) deletion mutation, a widely used model of diet-induced obesity and diabetes. On high fat diet (HF), the mice developed obesity and hyperinsulinemia, while blood glucose levels were only mildly elevated indicating a substantial capacity to compensate for insulin resistance. The basal serum insulin levels were elevated in HF mice, but insulin secretion in response to glucose load was significantly blunted. Hyperinsulinemia in HF fed mice was associated with an increase in islet mass and size along with higher BrdU incorporation to β cells. The temporal profiles of glucose-stimulated insulin secretion (GSIS) of isolated islets were comparable in HF and normal chow fed mice. Islets isolated from HF fed mice had elevated basal oxygen consumption per islet but failed to increase oxygen consumption further in response to glucose or carbonyl cyanide-4-trifluoromethoxyphenylhydrazone (FCCP). To obtain an unbiased assessment of metabolic pathways in islets, we performed microarray analysis comparing gene expression in islets from HF to normal chow-fed mice. A few genes, for example, those genes involved in the protection against oxidative stress (hypoxia upregulated protein 1) and Pgc1α were up-regulated in HF islets. In contrast, several genes in extracellular matrix and other pathways were suppressed in HF islets. These results indicate that islets from C57BL/6J mice with NNT deletion mutation develop structural, metabolic and gene expression features consistent with compensation and decompensation in response to HF diet.

Neuropeptide Y (NPY) is highly expressed in the hypothalamus, where it regulates feeding and energy homeostasis. Interestingly, NPY and its receptors are also expressed in peripheral tissues with roles in metabolism, including pancreatic islets. In islets, NPY is known to suppress insulin secretion acutely. In addition, the role of NPY in β-cell de-differentiation has been postulated recently. Therefore, we studied transgenic mice expressing NPY under rat insulin promoter (TG) to determine the effects of chronic up-regulation of NPY on islet morphology and function. NPY levels were 25 times higher in islets of TG mice compared with wild-type (WT) littermates, whereas no differences in NPY expression were noted in the brains of TG and WT mice. Islet NPY secretion was 2.3-fold higher in TG compared with WT mice. There were no significant changes in body weight, glucose tolerance, or insulin sensitivity in TG mice fed regular rodent diet or high-fat diet (HF). Islet β-cell area was comparable between TG and WT mice both on regular rodent and HF diets, indicating that NPY overexpression is insufficient to alter β-cell maturation or the compensatory increase of β-cell area on HF. One abnormality noted was that the glucose-stimulated insulin secretion in islets isolated from TG was reduced compared with those from WT mice on HF diet. Overall, an increase in islet NPY level has little impact on islet function and is insufficient to affect glucose homeostasis in mice.

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