Adipose differentiation-related protein regulates lipids and insulin in pancreatic islets

Physiological Fatty Acid-Stimulated Insulin Secretion and Redox Signaling Versus Lipotoxicity

Significance: Type 2 diabetes as a world-wide epidemic is characterized by the insulin resistance concomitant to a gradual impairment of β-cell mass and function (prominently declining insulin secretion) with dysregulated fatty acids (FAs) and lipids, all involved in multiple pathological development. Recent Advances: Recently, redox signaling was recognized to be essential for insulin secretion stimulated with glucose (GSIS), branched-chain keto-acids, and FAs. FA-stimulated insulin secretion (FASIS) is a normal physiological event upon postprandial incoming chylomicrons. This contrasts with the frequent lipotoxicity observed in rodents. Critical Issues: Overfeeding causes FASIS to overlap with GSIS providing repeating hyperinsulinemia, initiates prediabetic states by lipotoxic effects and low-grade inflammation. In contrast the protective effects of lipid droplets in human β-cells counteract excessive lipids. Insulin by FASIS allows FATP1 recruitment into adipocyte plasma membranes when postprandial chylomicrons come late at already low glycemia. Future Directions: Impaired states of pancreatic β-cells and peripheral organs at prediabetes and type 2 diabetes should be revealed, including the inter-organ crosstalk by extracellular vesicles. Details of FA/lipid molecular physiology are yet to be uncovered, such as complex phenomena of FA uptake into cells, postabsorptive inactivity of G-protein-coupled receptor 40, carnitine carrier substrate specificity, the role of carnitine-O-acetyltransferase in β-cells, and lipid droplet interactions with mitochondria. Antioxid. Redox Signal. 42, 566-622.

Intrapancreatic fat deposition and nutritional treatment: the role of various dietary approaches

Ectopic fat accumulation in various organs and tissues, such as the liver, muscle, kidney, heart, and pancreas, is related to impaired capacity of adipose tissue to accumulate triglycerides, as a consequence of overnutrition and an unhealthy lifestyle. Ectopic fat promotes organ dysfunction and is a key factor in the development and progression of cardiometabolic diseases. Interest in intrapancreatic fat deposition (IPFD) has developed in the last few years, particularly in relation to improvement in methodological techniques for detection of fat in the pancreas, and to growing evidence for the role that IPFD might have in glucose metabolism disorders and cardiometabolic disease. Body weight reduction represents the main option for reducing fat, and the evidence consistently shows that hypocaloric diets are effective in reducing IPFD. Changes in diet composition, independently of changes in energy intake, might offer a more feasible and safe alternative treatment to energy restriction. This current narrative review focused particularly on the possible beneficial role of the diet and its nutrient content, in hypocaloric and isocaloric conditions, in reducing IPFD in individuals with high cardiometabolic risk, highlighting the possible effects of differences in calorie quantity and calorie quality. This review also describes plausible mechanisms by which the various dietary approaches could modulate IPFD.

Lipid droplet protein Perilipin 2 is critical for the regulation of insulin secretion through beta cell lipophagy and glucagon expression in pancreatic islets

Knockdown (KD) of lipid droplet (LD) protein perilipin 2 (PLIN2) in beta cells impairs glucose-stimulated insulin secretion (GSIS) and mitochondrial function. Here, we addressed a pathway responsible for compromised mitochondrial integrity in PLIN2 KD beta cells. In PLIN2 KD human islets, mitochondria were fragmented in beta cells but not in alpha cells. Glucagon but not insulin level was elevated. While the formation of early LDs followed by fluorescent fatty acids (FA) analog Bodipy C12 (C12) was preserved, C12 accumulated in mitochondria over time in PLIN2 KD INS-1 cells. A lysosomal acid lipase inhibitor Lali2 prevented C12 transfer to mitochondria, mitochondrial fragmentation, and the impairment of GSIS. Direct interactions between LD-lysosome and lysosome-mitochondria were increased in PLIN2 KD INS-1 cells. Thus, FA released from LDs by microlipophagy cause mitochondrial changes and impair GSIS in PLIN2 KD beta cells. Interestingly, glucolipotoxic condition (GLT) caused C12 accumulation and mitochondrial fragmentation similar to PLIN2 KD in beta cells. Moreover, Lali2 reversed mitochondrial fragmentation and improved GSIS in human islets under GLT. In summary, PLIN2 regulates microlipophagy to prevent excess FA flux to mitochondria in beta cells. This pathway also contributes to GSIS impairment when LD pool expands under nutrient load in beta cells.

Lipid Dynamics in Pancreatic β-Cells: Linking Physiology to Diabetes Onset

Significance: Glucose-induced lipid metabolism is essential for preserving functional β-cells, and its disruption is linked to type 2 diabetes (T2D) development. Lipids are an integral part of the cells playing an indispensable role as structural components, energy storage molecules, and signals. Recent Advances: Glucose presence significantly impacts lipid metabolism in β-cells, where fatty acids are primarily synthesized de novo and/or are transported from the bloodstream. This process is regulated by the glycerolipid/free fatty acid cycle, which includes lipogenic and lipolytic reactions producing metabolic coupling factors crucial for insulin secretion. Disrupted lipid metabolism involving oxidative stress and inflammation is a hallmark of T2D. Critical Issues: Lipid metabolism in β-cells is complex involving multiple simultaneous processes. Exact compartmentalization and quantification of lipid metabolism and its intermediates, especially in response to glucose or chronic hyperglycemia, are essential. Current research often uses non-physiological conditions, which may not accurately reflect in vivo situations. Future Directions: Identifying and quantifying individual steps and their signaling, including redox, within the complex fatty acid and lipid metabolic pathways as well as the metabolites formed during acute versus chronic glucose stimulation, will uncover the detailed mechanisms of glucose-stimulated insulin secretion. This knowledge is crucial for understanding T2D pathogenesis and identifying pharmacological targets to prevent this disease. Antioxid. Redox Signal. 41, 865-889.

Metabolic Roles of HIF1, c-Myc, and p53 in Glioma Cells

The metabolic reprogramming that promotes tumorigenesis in glioblastoma is induced by dynamic alterations in the hypoxic tumor microenvironment, as well as in transcriptional and signaling networks, which result in changes in global genetic expression. The signaling pathways PI3K/AKT/mTOR and RAS/RAF/MEK/ERK stimulate cell metabolism, either directly or indirectly, by modulating the transcriptional factors p53, HIF1, and c-Myc. The overexpression of HIF1 and c-Myc, master regulators of cellular metabolism, is a key contributor to the synthesis of bioenergetic molecules that mediate glioma cell transformation, proliferation, survival, migration, and invasion by modifying the transcription levels of key gene groups involved in metabolism. Meanwhile, the tumor-suppressing protein p53, which negatively regulates HIF1 and c-Myc, is often lost in glioblastoma. Alterations in this triad of transcriptional factors induce a metabolic shift in glioma cells that allows them to adapt and survive changes such as mutations, hypoxia, acidosis, the presence of reactive oxygen species, and nutrient deprivation, by modulating the activity and expression of signaling molecules, enzymes, metabolites, transporters, and regulators involved in glycolysis and glutamine metabolism, the pentose phosphate cycle, the tricarboxylic acid cycle, and oxidative phosphorylation, as well as the synthesis and degradation of fatty acids and nucleic acids. This review summarizes our current knowledge on the role of HIF1, c-Myc, and p53 in the genic regulatory network for metabolism in glioma cells, as well as potential therapeutic inhibitors of these factors.

ADIPOPHILIN PEPTIDE (ADPH 129-137) IS NOT A TARGET ANTIGEN FOR CD8+ T-CELLS IN PATIENTS WITH OBESITY

Context

In obesity, the infiltration of leukocytes into adipose tissue seems to play a key role in the development of inflammation and insulin resistance. Over-expression of adipophilin (ADPH) in adipose tissue, a protein which regulates lipid droplet structure and formation, has been reported in some studies.

Objective

To investigate the role of ADPH 129-137 as a target for CD8+ T-cells in PBMCs of patients with obesity.

Subjects and Methods

PBMCs were obtained from 9 non-diabetic obese patients and 11 healthy subjects expressing the HLA-A0201 molecule. The ELISPOT assay used to monitor the presence of IFN-γ producing CD8+ T-cells against a HLA class I-restricted epitope derived from Adipophilin (ADPH 129-137) and two control peptides: Flu MP58-66 and Melan-A27-35.

Results

The outcomes showed no significant difference between patient group and healthy donors in response to ADPH 129-137.

Conclusion

These results demonstrated that ADPH 129-137 peptide possibly does not act as an autoantigen in patients with obesity.

Simultaneous blockade of VEGF-B and IL-17A ameliorated diabetic kidney disease by reducing ectopic lipid deposition and alleviating inflammation response

The pathogenesis of diabetic kidney disease (DKD) is complicated. Current clinical treatments fail to achieve satisfactory efficacy in the prevention of DKD progression, it urgently needs novel and effective treatment for DKD. In this study, we firstly demonstrated that renal lipid metabolism abnormality and inflammation significantly changed in DKD conditions by mining public transcriptomic data of DKD patient samples. KEGG analysis further exhibited the critical role of vascular endothelial growth factor B (VEGF-B) and interleukin 17A (IL-17A) signal pathways in DKD progression, indicating that VEGF-B and IL-17A might be the promising targets for DKD treatment. Then the potential of a novel combination therapy, anti-VEGF-B plus anti-IL-17A antibody, was evaluated for DKD treatment. Our results demonstrated that simultaneous blockade of VEGF-B and IL-17A signaling with their neutralizing antibodies alleviated renal damage and ameliorated renal function. The therapeutic effectiveness was not only related to the reduced lipid deposition especially the neutral lipids in kidney but also associated with the decreased inflammation response. Moreover, the therapy alleviated renal fibrosis by reducing collagen deposition and the expression of fibronectin and α-SMA in kidney tissues. RNA-seq analysis indicated that differential expression genes (DEGs) in db/db mice were significantly clustered into lipid metabolism, inflammation, fibrosis and DKD pathology-related pathways, and 181 of those DEGs were significantly reversed by the combinatory treatment, suggesting the underlying mechanism of administration of anti-VEGF-B and anti-IL-17A antibodies in DKD treatment. Taken together, this study identified that renal lipid metabolism abnormality and inflammation were critically involved in the progression of DKD, and simultaneous blockade of VEGF-B and IL-17A signaling represents a potential DKD therapeutic strategy.

Lipid Droplets' Role in the Regulation of β-Cell Function and β-Cell Demise in Type 2 Diabetes

During development of type 2 diabetes (T2D), excessive nutritional load is thought to expose pancreatic islets to toxic effects of lipids and reduce β-cell function and mass. However, lipids also play a positive role in cellular metabolism and function. Thus, proper trafficking of lipids is critical for β cells to maximize the beneficial effects of these molecules while preventing their toxic effects. Lipid droplets (LDs) are organelles that play an important role in the storage and trafficking of lipids. In this review, we summarize the discovery of LDs in pancreatic β cells, LD lifecycle, and the effect of LD catabolism on β-cell insulin secretion. We discuss factors affecting LD formation such as age, cell type, species, and nutrient availability. We then outline published studies targeting critical LD regulators, primarily in rat and human β-cell models, to understand the molecular effect of LD formation and degradation on β-cell function and health. Furthermore, based on the abnormal LD accumulation observed in human T2D islets, we discuss the possible role of LDs during the development of β-cell failure in T2D. Current knowledge indicates that proper formation and clearance of LDs are critical to normal insulin secretion, endoplasmic reticulum homeostasis, and mitochondrial integrity in β cells. However, it remains unclear whether LDs positively or negatively affect human β-cell demise in T2D. Thus, we discuss possible research directions to address the knowledge gap regarding the role of LDs in β-cell failure.

Lipid Droplets Protect Human β-Cells From Lipotoxicity-Induced Stress and Cell Identity Changes

Free fatty acids (FFAs) are often stored in lipid droplet (LD) depots for eventual metabolic and/or synthetic use in many cell types, such a muscle, liver, and fat. In pancreatic islets, overt LD accumulation was detected in humans but not mice. LD buildup in islets was principally observed after roughly 11 years of age, increasing throughout adulthood under physiologic conditions, and also enriched in type 2 diabetes. To obtain insight into the role of LDs in human islet β-cell function, the levels of a key LD scaffold protein, perilipin 2 (PLIN2), were manipulated by lentiviral-mediated knockdown (KD) or overexpression (OE) in EndoCβH2-Cre cells, a human cell line with adult islet β-like properties. Glucose-stimulated insulin secretion was blunted in PLIN2KD cells and improved in PLIN2OE cells. An unbiased transcriptomic analysis revealed that limiting LD formation induced effectors of endoplasmic reticulum (ER) stress that compromised the expression of critical β-cell function and identity genes. These changes were essentially reversed by PLIN2OE or using the ER stress inhibitor, tauroursodeoxycholic acid. These results strongly suggest that LDs are essential for adult human islet β-cell activity by preserving FFA homeostasis.

Specific Deletion of CASK in Pancreatic β Cells Affects Glucose Homeostasis and Improves Insulin Sensitivity in Obese Mice by Reducing Hyperinsulinemia Running Title: β Cell CASK Deletion Reduces Hyperinsulinemia

Calcium/calmodulin-dependent serine protein kinase (CASK) is involved in the secretion of insulin vesicles in pancreatic β-cells. The present study revealed a new in vivo role of CASK in glucose homeostasis during the progression of type 2 diabetes mellitus (T2DM). A Cre-loxP system was used to specifically delete the Cask gene in mouse β-cells (βCASKKO), and the glucose metabolism was evaluated in βCASKKO mice fed a normal chow diet (ND) or a high-fat diet (HFD). ND-fed mice exhibited impaired insulin secretion in response to glucose stimulation. Transmission electron microscopy showed significantly reduced numbers of insulin granules at or near the cell membrane in the islets of βCASKKO mice. By contrast, HFD-fed βCASKKO mice showed reduced blood glucose and a partial relief of hyperinsulinemia and insulin resistance when compared to HFD-fed wildtype mice. The IRS1/PI3K/AKT signaling pathway was upregulated in the adipose tissue of HFD-βCASKKO mice. These results indicated that knockout of the Cask gene in β cells had a diverse effect on glucose homeostasis: reduced insulin secretion in ND-fed mice, but improves insulin sensitivity in HFD-fed mice. Therefore, CASK appears to function in the insulin secretion and contributes to hyperinsulinemia and insulin resistance during the development of obesity-related T2DM.

Perilipin 2 downregulation in β cells impairs insulin secretion under nutritional stress and damages mitochondria

Perilipin 2 (PLIN2) is a lipid droplet (LD) protein in β cells that increases under nutritional stress. Downregulation of PLIN2 is often sufficient to reduce LD accumulation. To determine whether PLIN2 positively or negatively affects β cell function under nutritional stress, PLIN2 was downregulated in mouse β cells, INS1 cells, and human islet cells. β Cell–specific deletion of PLIN2 in mice on a high-fat diet reduced glucose-stimulated insulin secretion (GSIS) in vivo and in vitro. Downregulation of PLIN2 in INS1 cells blunted GSIS after 24-hour incubation with 0.2 mM palmitic acid. Downregulation of PLIN2 in human pseudoislets cultured at 5.6 mM glucose impaired both phases of GSIS, indicating that PLIN2 is critical for GSIS. Downregulation of PLIN2 decreased specific OXPHOS proteins in all 3 models and reduced oxygen consumption rates in INS1 cells and mouse islets. Moreover, we found that PLIN2-deficient INS1 cells increased the distribution of a fluorescent oleic acid analog to mitochondria and showed signs of mitochondrial stress, as indicated by susceptibility to fragmentation and alterations of acyl-carnitines and glucose metabolites. Collectively, PLIN2 in β cells has an important role in preserving insulin secretion, β cell metabolism, and mitochondrial function under nutritional stress.

High Fat Rodent Models of Type 2 Diabetes: From Rodent to Human

Poor dietary habits contribute to increased incidences of obesity and related co-morbidities, such as type 2 diabetes (T2D). The biological, genetic, and pathological implications of T2D, are commonly investigated using animal models induced by a dietary intervention. In spite of significant research contributions, animal models have limitations regarding the translation to human pathology, which leads to questioning their clinical relevance. Important considerations include diet-specific effects on whole organism energy balance and glucose and insulin homeostasis, as well as tissue-specific changes in insulin and glucose tolerance. This review will examine the T2D-like phenotype in rodents resulting from common diet-induced models and their relevance to the human disease state. Emphasis will be placed on the disparity in percentages and type of dietary fat, the duration of intervention, and whole organism and tissue-specific changes in rodents. An evaluation of these models will help to identify a diet-induced rodent model with the greatest clinical relevance to the human T2D pathology. We propose that a 45% high-fat diet composed of approximately one-third saturated fats and two-thirds unsaturated fats may provide a diet composition that aligns closely to average Western diet macronutrient composition, and induces metabolic alterations mirrored by clinical populations.

Genetic deletion of miR-204 improves glycemic control despite obesity in db/db mice

MicroRNAs (miRs) are small non-coding RNAs that regulate the target gene expression. A change in miR profile in the pancreatic islets during diabetes is known, and multiple studies have demonstrated that miRs influence the pancreatic β-cell function. The miR-204 is highly expressed in the β-cells and reported to regulate insulin synthesis. Here we investigated whether the absence of miR-204 rescues the impaired glycemic control and obesity in the genetically diabetic (db/db) mice. We found that the db/db mice overexpressed miR-204 in the islets. The db/db mice lacking miR-204 (db/db-204^-/-) initially develops hyperglycemia and obesity like the control (db/db) mice but later displayed a gradual improvement in glycemic control despite remaining obese. The db/db-204^-/- mice had a lower fasting blood glucose and higher serum insulin level compared to the db/db mice. A homeostatic model assessment (HOMA) suggests the improvement of β-cell function contributes to the improvement in glycemic control in db/db-204^-/- mice. Next, we examined the cellular proliferation and endoplasmic reticulum (ER) stress and found an increased frequency of proliferating cells (PCNA + ve) and a decreased CHOP expression in the islets of db/db-204^-/- mice. Next, we determined the effect of systemic miR-204 inhibition in improving glycemic control in the high-fat diet (HFD)-fed insulin-resistant mice. MiR-204 inhibition for 6 weeks improved the HFD-triggered impairment in glucose disposal. In conclusion, the absence of miR-204 improves β-cell proliferation, decreases islet ER stress, and improves glycemic control with limited change in body weight in obese mice.

Adipose Triglyceride Lipase Is a Key Lipase for the Mobilization of Lipid Droplets in Human β-Cells and Critical for the Maintenance of Syntaxin 1a Levels in β-Cells

Lipid droplets (LDs) are frequently increased when excessive lipid accumulation leads to cellular dysfunction. Distinct from mouse β-cells, LDs are prominent in human β-cells. However, the regulation of LD mobilization (lipolysis) in human β-cells remains unclear. We found that glucose increases lipolysis in nondiabetic human islets but not in islets in patients with type 2 diabetes (T2D), indicating dysregulation of lipolysis in T2D islets. Silencing adipose triglyceride lipase (ATGL) in human pseudoislets with shRNA targeting ATGL (shATGL) increased triglycerides (TGs) and the number and size of LDs, indicating that ATGL is the principal lipase in human β-cells. In shATGL pseudoislets, biphasic glucose-stimulated insulin secretion (GSIS), and insulin secretion to 3-isobutyl-1-methylxanthine and KCl were all reduced without altering oxygen consumption rate compared with scramble control. Like human islets, INS1 cells showed visible LDs, glucose-responsive lipolysis, and impairment of GSIS after ATGL silencing. ATGL-deficient INS1 cells and human pseudoislets showed reduced SNARE protein syntaxin 1a (STX1A), a key SNARE component. Proteasomal degradation of Stx1a was accelerated likely through reduced palmitoylation in ATGL-deficient INS1 cells. Therefore, ATGL is responsible for LD mobilization in human β-cells and supports insulin secretion by stabilizing STX1A. The dysregulated lipolysis may contribute to LD accumulation and β-cell dysfunction in T2D islets.

The clinicopathological significance of the adipophilin and fatty acid synthase expression in salivary duct carcinoma

Salivary duct carcinoma (SDC) is an aggressive, uncommon tumor histologically comparable to high-grade mammary ductal carcinoma. SDCs are usually androgen receptor (AR)–positive and often HER2-positive. Recently, therapies targeting these molecules for SDC have attracted attention. Lipid metabolism changes have been described in association with biological behavior in various cancers, although no such relationship has yet been reported for SDC. We therefore analyzed the clinicopathological relevance of the immunohistochemical expression of adipophilin (ADP) and fatty acid synthase (FASN), representative lipid metabolism–related proteins, in 147 SDCs. ADP and FASN were variably immunoreactive in most SDCs (both 99.3%), and the ADP and FASN expression was negatively correlated (P = 0.014). ADP-positive (≥ 5%) SDCs more frequently exhibited a prominent nuclear pleomorphism and high-Ki-67 labeling index than those ADP-negative (P = 0.013 and 0.011, respectively). In contrast, a high FASN score, calculated by the staining proportion and intensity, (≥ 120) was correlated with the high expression of AR and FOXA1 (P < 0.001 and = 0.003, respectively). The ADP and FASN expression differed significantly among the subtypes based on biomarker immunoprofiling, as assessed by the AR, HER2, and Ki-67 status (P = 0.017 and 0.003, respectively). A multivariate analysis showed that ADP-positive expression was associated with a shorter overall and progression-free survival (P = 0.018 and 0.003, respectively). ADP was associated with an aggressive histopathology and unfavorable prognosis, and FASN may biologically interact with the AR signaling pathway in SDC. ADP may, therefore, be a new prognostic indicator and therapeutic target in SDC.

Connecting pancreatic islet lipid metabolism with insulin secretion and the development of type 2 diabetes

Obesity is the major contributing factor for the increased prevalence of type 2 diabetes (T2D) in recent years. Sustained positive influx of lipids is considered to be a precipitating factor for beta cell dysfunction and serves as a connection between obesity and T2D. Importantly, fatty acids (FA), a key building block of lipids, are a double-edged sword for beta cells. FA acutely increase glucose-stimulated insulin secretion through cell-surface receptor and intracellular pathways. However, chronic exposure to FA, combined with elevated glucose, impair the viability and function of beta cells in vitro and in animal models of obesity (glucolipotoxicity), providing an experimental basis for the propensity of beta cell demise under obesity in humans. To better understand the two-sided relationship between lipids and beta cells, we present a current view of acute and chronic handling of lipids by beta cells and implications for beta cell function and health. We also discuss an emerging role for lipid droplets (LD) in the dynamic regulation of lipid metabolism in beta cells and insulin secretion, along with a potential role for LD under nutritional stress in beta cells, and incorporate recent advancement in the field of lipid droplet biology.

Type 2 diabetes is associated with suppression of autophagy and lipid accumulation in β-cells

Both type 2 diabetes (T2D) and obesity are characterized by excessive hyperlipidaemia and subsequent lipid droplet (LD) accumulation in adipose tissue. To investigate whether LDs also accumulate in β-cells of T2D patients, we assessed the expression of PLIN2, a LD-associated protein, in non-diabetic (ND) and T2D pancreata. We observed an up-regulation of PLIN2 mRNA and protein in β-cells of T2D patients, along with significant changes in the expression of lipid metabolism, apoptosis and oxidative stress genes. The increased LD buildup in T2D β-cells was accompanied by inhibition of nuclear translocation of TFEB, a master regulator of autophagy and by down-regulation of lysosomal biomarker LAMP2. To investigate whether LD accumulation and autophagy were influenced by diabetic conditions, we used rat INS-1 cells to model the effects of hyperglycaemia and hyperlipidaemia on autophagy and metabolic gene expression. Consistent with human tissue, both LD formation and PLIN2 expression were enhanced in INS-1 cells under hyperglycaemia, whereas TFEB activation and autophagy gene expression were significantly reduced. Collectively, these results suggest that lipid clearance and overall homeostasis is markedly disrupted in β-cells under hyperglycaemic conditions and interventions ameliorating lipid clearance could be beneficial in reducing functional impairments in islets caused by glucolipotoxicity.

Roles of Perilipins in Diseases and Cancers

Perilipins, an ancient family of lipid droplet-associated proteins, are embedded in a phospho-lipid monolayer of intracellular lipid droplets. The core of lipid droplets is composed of neutral fat, which mainly includes triglyceride and cholesterol ester. Perilipins are closely related to the function of lipid droplets, and they mediate lipid metabolism and storage. Therefore, perilipins play an important role in the development of obesity, diabetes, cancer, hepatic diseases, atherosclerosis, and carcinoma, which are caused by abnormal lipid metabolism. Accumulation of lipid droplets is a common phenomenon in tumor cells. Available data on the pathophysiology of perilipins and the relationship of perilipins with endocrine metabolic diseases and cancers are summarized in this mini-review. The research progress on this family offers novel insights into the therapeutic strategies for these diseases.

Kaempferol alleviates palmitic acid-induced lipid stores, endoplasmic reticulum stress and pancreatic β-cell dysfunction through AMPK/mTOR-mediated lipophagy

Kaempferol, a natural flavonoid, has the beneficial effects of preserving pancreatic β-cell mass and function, but its action on β-cell lipid metabolism still remains elusive. Recently, autophagy has been reported to play a major role in lipid metabolism in various cell types, but its role in pancreatic β-cell's lipid metabolism is rarely reported. Here, we investigated the role of kaempferol-induced autophagy in inhibition of lipid stores, ER stress and β-cell dysfunction in palmitic acid-challenged RIN-5F cells and isolated pancreatic islets. The lipid-lowering effect of kaempferol was determined by Oil Red O staining, triglyceride assay, BODIPY labeling, RT-PCR and immunoblot analysis of PLIN2 (the lipid droplet coat protein) expression. Further, the involvement of AMPK/mTOR-mediated lipophagy was established by pharmacological and genetic inhibitors of autophagy and AMPK. The co-localization studies of lipid droplets with autophagosomes/lysosomes by BODIPY-MDC-LysoTracker co-staining, LC3/BODIPY labeling and LC3/PLIN2 double immunolabeling further strengthened the findings. Kaempferol treatment exhibited decreased lipid stores and increased co-localization of lipid droplets with autophagosomes and lysosomes in palmitic acid-challenged β-cells. Moreover, inhibition of autophagy led to decreased co-localization and increased lipid droplets accumulation. Kaempferol-induced alleviation of ER stress and β-cell dysfunctions was established by immunoblot analysis of CHOP-10 (a key mediator of cell death in response to ER stress) and insulin content/secretion analysis respectively. Together, these findings suggest that kaempferol prevents ectopic lipid accumulation and ER stress, thus restoring β-cell function through AMPK-mediated lipophagy. The current data implies that kaempferol may be a potential therapeutic candidate to prevent obesity-linked diabetic complications.

Fatty Acid-Induced Lipotoxicity in Pancreatic Beta-Cells During Development of Type 2 Diabetes

Type 2 diabetes is caused by chronic insulin resistance and progressive decline in beta-cell function. Optimal beta-cell function and mass is essential for glucose homeostasis and beta-cell impairment leads to the development of diabetes. Elevated levels of circulating fatty acids (FAs) and disturbances in lipid metabolism regulation are associated with obesity, and they are major factors influencing the increase in the incidence of type 2 diabetes. Chronic free FA (FFA) treatment induces insulin resistance and beta-cell dysfunction; therefore, reduction of elevated plasma FFA levels might be an important therapeutic target in obesity and type 2 diabetes. Lipid signals via receptors, and intracellular mechanisms are involved in FFA-induced apoptosis. In this paper, we discuss lipid actions in beta cells, including effects on metabolic pathways and stress responses, to help further understand the molecular mechanisms of lipotoxicity-induced type 2 diabetes.

Gastric mucosal devitalization reduces adiposity and improves lipid and glucose metabolism in obese rats

BACKGROUND AND AIMS

The gastric mucosa is an endocrine organ that regulates satiation pathways by expression of orexigenic and anorexigenic hormones. Vertical sleeve gastrectomy (VSG) excludes gastric mucosa and reduces gastric volume. Our study aimed to investigate the independent effects of altering gastric mucosa on obesity and its related comorbidities.

METHODS

Gastric mucosa devitalization (GMD) of 70% of the stomach was achieved by argon plasma coagulation in a high-fat diet rat model and was compared with VSG and sham surgery. In an 8-week follow-up study, we quantified body weight, visceral adiposity, insulin resistance index, cholesterol profiles, and free fatty acid profiles by enzyme-linked immunosorbent assay (ELISA). Following a 2-hour oral glucose tolerance test, the kinetics of ghrelin, glucagon-like peptide-1, peptide YY, and serum and liver bile acid levels were measured. Liver lipid content was quantified by ELISA.

RESULTS

GMD resulted in significant reductions in body weight, visceral and subcutaneous adipose tissue, and hepatic steatosis as well as an improvement in lipid metabolism. GMD resulted in significant reductions in food intake and intestinal malabsorption of free fatty acids, both contributing to improved body composition and metabolic profile. Mechanistically, GMD resulted in a significant reduction in serum palmitate levels as well as an increase in serum and liver bile acid levels, known to alter glucose and lipid metabolism. Similar changes were noted when VSG rats were compared with sham surgery rats.

CONCLUSIONS

Devitalization of gastric mucosa, independent of altering gastric volume, was able to reduce obesity-related comorbidities. The gastric mucosa may be a potential target for treating obesity and its associated comorbidities.

Cardiac overexpression of perilipin 2 induces dynamic steatosis: prevention by hormone-sensitive lipase

Cardiac intracellular lipid accumulation (steatosis) is a pathophysiological phenomenon observed in starvation and diabetes mellitus. Perilipin 2 (PLIN2) is a lipid droplet (LD)-associated protein expressed in nonadipose tissues, including the heart. To explore the pathophysiological function of myocardial PLIN2, we generated transgenic (Tg) mice by cardiac-specific overexpression of PLIN2. Tg hearts showed accumulation of numerous small LDs associated with mitochondrial chains and high cardiac triacylglycerol (TAG) content [8-fold greater than wild-type (WT) mice]. Despite massive steatosis, cardiac uptake of glucose, fatty acids and VLDL, systolic function, and expression of metabolic genes were comparable in the two genotypes, and no morphological changes were observed by electron microscopy in the Tg hearts. Twenty-four hours of fasting markedly reduced steatosis in Tg hearts, whereas WT mice showed accumulation of LDs. Although activity of adipose triglyceride lipase in heart homogenate was comparable between WT and Tg mice, activity of hormone-sensitive lipase (HSL) was 40-50% less in Tg than WT mice under both feeding and fasting conditions, suggesting interference of PLIN2 with HSL. Mice generated through crossing of PLIN2-Tg mice and HSL-Tg mice showed cardiac-specific HSL overexpression and complete lack of steatosis. The results suggest that cardiac PLIN2 plays an important pathophysiological role in the development of dynamic steatosis and that the latter was prevented by upregulation of intracellular lipases, including HSL.

PLIN2 is a Key Regulator of the Unfolded Protein Response and Endoplasmic Reticulum Stress Resolution in Pancreatic β Cells

Progressive pancreatic β cell failure underlies the transition of impaired glucose tolerance to overt diabetes; endoplasmic reticulum (ER) stress expedites β cell failure in this situation. ER stress can be elicited by lipotoxicity and an increased demand for insulin in diabetes. We previously reported that the lipid droplet protein perilipin 2 (PLIN2) modulates lipid homeostasis in the liver. Here, we show that PLIN2 modulates the unfolded protein response (UPR) and ER stress in pancreatic β cells. PLIN2 expression goes up when β cells are exposed to a lipid load or to chemical ER stress inducers. Downregulation of PLIN2 ameliorates the effects of fatty acid- and chemical-induced ER stress, whereas PLIN2 overexpression exacerbates them. Diabetic Akita mice, which carry a heterozygous C96Y Ins2 mutation, exhibit elevated PLIN2 expression and ER stress in their β cells. Genetic ablation of Plin2 in Akita mice leads to mitigation of ER stress, forestalling β cell apoptosis, partially restoring β cell mass, and ameliorating diabetes. Mechanistic experiments showed that PLIN2 downregulation is associated with enhanced autophagic flux and accelerated ER stress resolution. In sum, we have identified a crucial role for PLIN2 in modulating autophagy, ER stress resolution, and β cell apoptosis and survival.

The perilipin 2 (PLIN2) gene Ser251Pro missense mutation is associated with reduced insulin secretion and increased insulin sensitivity in Italian obese subjects

BACKGROUND

Perilipin 2 (PLIN2), a member of the family of perilipin lipid droplets coating proteins, is very widely expressed. The Ser251Pro (rs35568725) missense mutation in exon 6 of PLIN2 gene was previously associated with increased lipid accumulation, decreased lipolysis and increased number of small lipid droplets per cell. Furthermore, the Pro251 mutation was associated with decreased plasma triglyceride and very low density lipoprotein concentrations in population studies. The aim of this study was to evaluate the effect of the Ser251Pro mutation of PLIN2 gene in a cohort with a higher predisposition to obesity-associated metabolic alterations, such as insulin resistance, decreased insulin-secretion, hyperglycaemia, and dyslipidaemia.

METHODS

A large cohort (N = 1692) of Italian obese subjects (mean body mass index = 41 kg/m(2) ) was genotyped for the Ser251Pro mutation. All participants underwent oral glucose tolerance tests (OGTT), with measurement of glucose and insulin levels. Indices of insulin resistance and of insulin secretion were also calculated. Clinical and biochemical parameters were collected for all participants.

RESULTS

We observed that insulin concentration was significantly reduced at 120 min after the administration of glucose in Pro251 allele carriers, whereas glucose levels were similar in Pro251 allele carriers and non-carriers throughout the OGTT. Furthermore, the CIR120 index of insulin secretion was significantly lower (P < 0.035) and the ISI index of insulin-sensitivity was significantly higher (P < 0.031) in carriers of the Pro251 allele. When we analysed men and women separately to test for gender-specific associations, we observed that in women insulin levels were significantly lower in Pro251 allele carriers compared with wild-type subjects throughout the whole OGTT. In men, we confirmed a significant reduction in insulin concentration only at 120 min after the OGTT. No significant differences between genotype groups regarding triglyceride levels and anyother clinical and metabolic parameters were observed.

CONCLUSION

We observed a strong significant association between the PLIN2 Pro251 mutation and lower insulin secretion associated with an increased insulin sensitivity. Copyright © 2015 John Wiley & Sons, Ltd.

Liver Perilipin 5 Expression Worsens Hepatosteatosis But Not Insulin Resistance in High Fat-Fed Mice

Perilipin 5 (PLIN5) is a lipid droplet (LD) protein highly expressed in oxidative tissues, including the fasted liver. However, its expression also increases in nonalcoholic fatty liver. To determine whether PLIN5 regulates metabolic phenotypes of hepatosteatosis under nutritional excess, liver targeted overexpression of PLIN5 was achieved using adenoviral vector (Ad-PLIN5) in male C57BL/6J mice fed high-fat diet. Mice treated with adenovirus expressing green fluorescent protein (GFP) (Ad-GFP) served as control. Ad-PLIN5 livers increased LD in the liver section, and liquid chromatography with tandem mass spectrometry revealed increases in lipid classes associated with LD, including triacylglycerol, cholesterol ester, and phospholipid classes, compared with Ad-GFP liver. Lipids commonly associated with hepatic lipotoxicity, diacylglycerol, and ceramides, were also increased in Ad-PLIN5 liver. The expression of genes in lipid metabolism regulated by peroxisome proliferator-activated receptor-α was reduced suggestive of slower mobilization of stored lipids in Ad-PLIN5 mice. However, the increase of hepatosteatosis by PLIN5 overexpression did not worsen glucose homeostasis. Rather, serum insulin levels were decreased, indicating better insulin sensitivity in Ad-PLIN5 mice. Moreover, genes associated with liver injury were unaltered in Ad-PLIN5 steatotic liver compared with Ad-GFP control. Phosphorylation of protein kinase B was increased in Ad-PLIN5-transduced AML12 hepatocyte despite of the promotion of fatty acid incorporation to triacylglycerol as well. Collectively, our data indicates that the increase in liver PLIN5 during hepatosteatosis drives further lipid accumulation but does not adversely affect hepatic health or insulin sensitivity.

AMPK activation promotes lipid droplet dispersion on detyrosinated microtubules to increase mitochondrial fatty acid oxidation

Lipid droplets (LDs) are intracellular organelles that provide fatty acids (FAs) to cellular processes including synthesis of membranes and production of metabolic energy. While known to move bidirectionally along microtubules (MTs), the role of LD motion and whether it facilitates interaction with other organelles are unclear. Here we show that during nutrient starvation, LDs and mitochondria relocate on detyrosinated MT from the cell centre to adopt a dispersed distribution. In the cell periphery, LD–mitochondria interactions increase and LDs efficiently supply FAs for mitochondrial beta-oxidation. This cellular adaptation requires the activation of the energy sensor AMPK, which in response to starvation simultaneously increases LD motion, reorganizes the network of detyrosinated MTs and activates mitochondria. In conclusion, we describe the existence of a specialized cellular network connecting the cellular energetic status and MT dynamics to coordinate the functioning of LDs and mitochondria during nutrient scarcity.

Lipid droplets (LDs) supply fatty acids to cellular processes and move bidirectionally on microtubules. Here the authors show that nutrient starvation causes dispersal of mitochondria and LD to the periphery of the cell along detyrosinated microtubules and increases LD–mitochondria interactions in an AMPK-dependent manner.

Perilipin 5 regulates islet lipid metabolism and insulin secretion in a cAMP-dependent manner: implication of its role in the postprandial insulin secretion

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 [(3)H]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.

Fatty acid uptake and lipid storage induced by HIF-1α contribute to cell growth and survival after hypoxia-reoxygenation

An in vivo model of antiangiogenic therapy allowed us to identify genes upregulated by bevacizumab treatment, including Fatty Acid Binding Protein 3 (FABP3) and FABP7, both of which are involved in fatty acid uptake. In vitro, both were induced by hypoxia in a hypoxia-inducible factor-1α (HIF-1α)-dependent manner. There was a significant lipid droplet (LD) accumulation in hypoxia that was time and O2 concentration dependent. Knockdown of endogenous expression of FABP3, FABP7, or Adipophilin (an essential LD structural component) significantly impaired LD formation under hypoxia. We showed that LD accumulation is due to FABP3/7-dependent fatty acid uptake while de novo fatty acid synthesis is repressed in hypoxia. We also showed that ATP production occurs via β-oxidation or glycogen degradation in a cell-type-dependent manner in hypoxia-reoxygenation. Finally, inhibition of lipid storage reduced protection against reactive oxygen species toxicity, decreased the survival of cells subjected to hypoxia-reoxygenation in vitro, and strongly impaired tumorigenesis in vivo.

Characterisation of the genetic effects of the ADFP gene and its association with production traits in dairy goats

Adipose differentiation-related protein (ADFP) is important for regulation of lipid metabolism and insulin secretion in beta-cells. In this study, we investigated polymorphisms within the caprine ADFP gene and determined its relationship with production traits. As there was no sequence information available for the caprine ADFP gene, we generated DNA sequence data and examined the genomic organisation. The caprine ADFP gene is organised into 7 exons and 6 introns that span approximately 8.7 kbp and is transcribed into mRNA containing 1,353 bp of sequence coding for a protein of 450 amino acids. The protein sequences showed substantial similarity (71-99%) to orthologues from cattle, human and mouse. We identified polymorphisms in the sequences using DNA sequencing, PCR-RFLP and forced PCR-RFLP methods. Seven single nucleotide polymorphisms (SNPs) were identified using samples from 4 different goat populations consisting of 1408 healthy and unrelated individuals. Six haplotypes involving the 7 SNPs from the caprine ADFP gene were identified and their effects on production traits were analysed. Haplotype 6 had the highest haplotype frequency and was highly significantly associated with chest circumference and milk yield in the analysed populations. The results of this study suggest that the ADFP gene is a strong candidate gene affecting production traits and may be used for marker-assisted selection and management in Chinese dairy goat breeding programmes.

Efficacy and tolerability of an herbal formulation for weight management

The clinical effects and tolerability of a novel herbal formulation comprising the extracts of Sphaeranthus indicus and Garcinia mangostana were assessed in two similarly designed randomized, double-blind, placebo-controlled, clinical trials in 100 human subjects with a body mass index (BMI) between 30 and 40 kg/m². Participants were randomized into two groups receiving either 400 mg of herbal blend twice daily or two identical placebo capsules. All subjects received three meals (2000 kcal/day) throughout the study and walked 5 days a week for 30 min. The primary outcome was reduction in body weight. Secondary outcomes were reduction in BMI and in waist and hip circumference. Serum glycemic, lipid, and adiponectin levels were also measured. Ninety-five subjects completed the trials, and data from these two studies were pooled and analyzed. At study conclusion (8 weeks), statistically significant reductions in body weight (5.2 kg; P<.0001), BMI (2.2 kg/m²; P<.0001), as well as waist (11.9 cm; P<.0001) and hip circumferences (6.3 cm; P=.0001) were observed in the herbal group compared with placebo. An increase in serum adiponectin concentration was also found in the herbal group versus placebo (P=.0008) at study conclusion along with reductions in fasting blood glucose (12.2%, P=.01), cholesterol (13.8%, P=.002), and triglyceride (41.6%, P<.0001) concentrations. No changes were seen across organ function panels, multiple vital signs, and no major adverse events were reported. The minor adverse events were equally distributed between the two groups. Our findings suggest that the herbal blend appears to be a well-tolerated and effective ingredient for weight management.

Hypoxia and metabolism in cancer

Interest in targeting metabolism has been renewed in recent years as research increases understanding of the altered metabolic profile of tumor cells compared with that of normal cells. Metabolic reprogramming allows cancer cells to survive and proliferate in the hostile tumor microenvironment. These metabolic changes support energy generation, anabolic processes, and the maintenance of redox potential, mechanisms that are all essential for the proliferation and survival of tumor cells. The metabolic switch in a number of key metabolic pathways is mainly regulated by genetic events, rendering cancer cells addicted to certain nutrients, such as glutamine. In addition, hypoxia is induced when highly proliferative tumor cells distance themselves from an oxygen supply. Hypoxia-inducible factor 1α is largely responsible for alterations in metabolism that support the survival of hypoxic tumor cells. Metabolic alterations and dependencies of cancer cells may be exploited to improve anticancer therapy. This chapter reviews the main aspects of altered metabolism in cancer cells, emphasizing recent advances in glucose, glutamine, and lipid metabolism.

Plin2 inhibits cellular glucose uptake through interactions with SNAP23, a SNARE complex protein

Although a link between excess lipid storage and aberrant glucose metabolism has been recognized for many years, little is known what role lipid storage droplets and associated proteins such as Plin2 play in managing cellular glucose levels. To address this issue, the influence of Plin2 on glucose uptake was examined using 2-NBD-Glucose and [(3)H]-2-deoxyglucose to show that insulin-mediated glucose uptake was decreased 1.7- and 1.8-fold, respectively in L cell fibroblasts overexpressing Plin2. Conversely, suppression of Plin2 levels by RNAi-mediated knockdown increased 2-NBD-Glucose uptake several fold in transfected L cells and differentiated 3T3-L1 cells. The effect of Plin2 expression on proteins involved in glucose uptake and transport was also examined. Expression of the SNARE protein SNAP23 was increased 1.6-fold while levels of syntaxin-5 were decreased 1.7-fold in Plin2 overexpression cells with no significant changes observed in lipid droplet associated proteins Plin1 or FSP27 or with the insulin receptor, GLUT1, or VAMP4. FRET experiments revealed a close proximity of Plin2 to SNAP23 on lipid droplets to within an intramolecular distance of 51 Å. The extent of targeting of SNAP23 to lipid droplets was determined by co-localization and co-immunoprecipitation experiments to show increased partitioning of SNAP23 to lipid droplets when Plin2 was overexpressed. Taken together, these results suggest that Plin2 inhibits glucose uptake by interacting with, and regulating cellular targeting of SNAP23 to lipid droplets. In summary, the current study for the first time provides direct evidence for the role of Plin2 in mediating cellular glucose uptake.

Mice exposed in situ to urban air pollution exhibit pulmonary alterations in gene expression in the lipid droplet synthesis pathways

It is clear that particulate air pollution poses a serious risk to human health; however, the underlying mechanisms are not completely understood. We investigated pulmonary transcriptional responses in mice following in-situ exposure to ambient air in a heavily industrialized urban environment. Mature C57BL/CBA male mice were caged in sheds near two working steel mills and a major highway in Hamilton, Ontario, Canada in the spring/summer of 2004. Control mice were housed in the same environment, but received only high-efficiency particle filtered air (HEPA). Whole lung tissues were collected from mice exposed for 3, 10, or for 10 weeks followed by 6 weeks recovery in the laboratory (16 weeks). DNA microarrays were used to profile changes in pulmonary gene expression. Transcriptional profiling revealed changes in the expression of genes implicated in the lipid droplet synthesis (Plin I, Dgat2, Lpl, S3-12, and Agpat2), and antioxidant defense (Ucp1) pathways in mice breathing unfiltered air. We postulate that exposure to urban air, containing an abundance of particulate matter adsorbed with polycyclic aromatic hydrocarbons, triggers lipid droplet (holding depots for lipids and malformed/excess proteins tagged for degradation) synthesis in the lungs, which may act to sequester particulates. Increased lipid droplet synthesis could lead to endogenous/stressor-induced production of reactive oxygen species and activation of antioxidant mechanisms. Further investigation into the stimulation of lipid droplet synthesis in the lung in response to air pollution and the resulting health implications is warranted.

Efficacy and tolerability of a novel herbal formulation for weight management

OBJECTIVE

To evaluate the efficacy of an herbal blend.

DESIGN AND METHODS

A randomized, double-blind, clinical trial in 60 subjects with body mass index (BMI) between 30 and 40 kg/m(2) . Participants were randomized into two groups receiving either 400 mg herbal capsules or 400 mg placebo capsules twice daily. The herbal blend comprises of extracts from Sphaeranthus indicus and Garcinia mangostana. Participants received a standard diet (2,000 kcal per day) and walked 30 min 5 days per week.

RESULTS

After 8 weeks, significant net reductions in body weight (3.74 kg; P < 0.0001), BMI (1.61 kg/m(2) ; P < 0.0001), and waist circumference (5.44 cm; P < 0.05) were observed in the herbal group compared with placebo. Additionally, a significant increase in serum adiponectin concentration was found in the herbal group versus placebo (P = 0.001). Adverse events were mild and were equally distributed between the two groups. In vitro studies in the 3T3-L1 adipocyte cell line showed that the herbal extract markedly downregulated the expression of peroxisome proliferator-activated receptor gamma, adipocyte-differentiation related protein, and cluster of differentiation 36 but increased adiponectin expression. The herbal extract also reduced the expression and the recruitment of perilipin onto the membrane of lipid droplets.

CONCLUSION

Supplementation with the herbal blend resulted in a greater degree of weight loss than placebo over 8 weeks.

The minor allele of the missense polymorphism Ser251Pro in perilipin 2 (PLIN2) disrupts an α-helix, affects lipolysis, and is associated with reduced plasma triglyceride concentration in humans

Perilipin 2 (PLIN2) is the most abundant lipid droplet (LD)-associated protein in nonadipose tissue, and its expression correlates with intracellular lipid accumulation. Here we identified a missense polymorphism, Ser251Pro, that has major effect on protein structure and function, along with an influence on human plasma triglyceride concentration. The evolutionarily conserved Ser251Pro polymorphism was identified with the ClustalW program. Structure modeling using 3D-JigSaw and the Chimera package revealed that the Pro251 allele disrupts a predicted α-helix in PLIN2. Analyses of macrophages from individuals carrying Ser251Pro variants and human embryonic kidney 293 (HEK293) cells stably transfected with either of the alleles demonstrated that the Pro251 variant causes increased lipid accumulation and decreased lipolysis. Analysis of LD size distribution in stably transfected cells showed that the minor Pro251 allele resulted in an increased number of small LDs per cell and increased perilipin 3 protein expression levels as compared with cells carrying the major Ser251 allele. Genotyping of 2113 individuals indicated that the Pro251 variant is associated with decreased plasma triglyceride and very low-density lipoprotein concentrations. Altogether, these data provide the first evidence of a polymorphism in PLIN2 that affects PLIN2 function and may influence the development of metabolic and cardiovascular diseases.

Signaling molecules involved in lipid-induced pancreatic beta-cell dysfunction

The increasing incidence of type 2 diabetes mellitus is partially due to the rising obesity rates and the elevated levels of free fatty acids (FFAs). It is known that FFAs are putative mediators of beta-cell dysfunction, which is characterized with impaired glucose-stimulated insulin secretion and increased apoptosis, being defined as lipotoxicity. To date, many factors and their related signal pathways have been reported to be involved in FFA-induced beta-cell dysfunction. However, the entire blueprint is still not obtained. Some essential and newfound effectors, including the sterol regulatory element-binding protein (SREBP)-1c, farnesoid X receptor (FXR), forkhead box-containing protein O (FoxO) 1, ubiquitin C-terminal hydrolase L (UCHL) 1, N-myc downstream-regulated gene (NDRG) 2, perilipin family proteins, silent information regulator 2 protein 1 (Sirt1), pituitary adenylate cyclase-activating polypeptide (PACAP), and ghrelin are described in this review, which may help to further understand the molecular network for lipotoxicity.

β-cell metabolic alterations under chronic nutrient overload in rat and human islets

The aim of this study was to assess multifactorial β-cell responses to metabolic perturbations in primary rat and human islets. Treatment of dispersed rat islet cells with elevated glucose and free fatty acids (FFAs, oleate:palmitate = 1:1 v/v) resulted in increases in the size and the number of lipid droplets in β-cells in a time- and concentration-dependent manner. Glucose and FFAs synergistically stimulated the nutrient sensor mammalian target of rapamycin complex 1 (mTORC1). A potent mTORC1 inhibitor, rapamycin (25 nM), significantly reduced triglyceride accumulation in rat islets. Importantly, lipid droplets accumulated only in β-cells but not in α-cells in an mTORC1-dependent manner. Nutrient activation of mTORC1 upregulated the expression of adipose differentiation related protein (ADRP), known to stabilize lipid droplets. Rat islet size and new DNA synthesis also increased under nutrient overload. Insulin secretion into the culture medium increased steadily over a 4-day period without any significant difference between glucose (10 mM) alone and the combination of glucose (10 mM) and FFAs (240 μM). Insulin content and insulin biosynthesis, however, were significantly reduced under the combination of nutrients compared with glucose alone. Elevated nutrients also stimulated lipid droplet formation in human islets in an mTORC1-dependent manner. Unlike rat islets, however, human islets did not increase in size under nutrient overload despite a normal response to nutrients in releasing insulin. The different responses of islet cell growth under nutrient overload appear to impact insulin biosynthesis and storage differently in rat and human islets.

Role of fatty acid transport protein 4 in oleic acid-induced glucagon-like peptide-1 secretion from murine intestinal L cells

The antidiabetic intestinal L cell hormone glucagon-like peptide-1 (GLP-1) enhances glucose-dependent insulin secretion and inhibits gastric emptying. GLP-1 secretion is stimulated by luminal oleic acid (OA), which crosses the cell membrane by an unknown mechanism. We hypothesized that L cell fatty acid transport proteins (FATPs) are essential for OA-induced GLP-1 release. Therefore, the murine GLUTag L cell model was used for immunoblotting, [(3)H]OA uptake assay, and GLP-1 secretion assay as determined by radioimmunoassay following treatment with OA ± phloretin, sulfo-N-succinimidyl oleate, or siRNA against FATP4. FATP4(-/-) and cluster-of-differentiation 36 (CD36)(-/-) mice received intraileal OA, and plasma GLP-1 was measured by sandwich immunoassay. GLUTag cells were found to express CD36, FATP1, FATP3, and FATP4. The cells demonstrated specific (3)H[OA] uptake that was dose-dependently inhibited by 500 and 1,000 μM unlabeled OA (P < 0.001). Cell viability was not altered by treatment with OA. Phloretin and sulfo-N-succinimidyl oleate, inhibitors of protein-mediated transport and CD36, respectively, also decreased [(3)H]OA uptake, as did knockdown of FATP4 by siRNA transfection (P < 0.05-0.001). OA dose-dependently increased GLP-1 secretion at 500 and 1,000 μM (P < 0.001), whereas phloretin, sulfo-N-succinimidyl oleate, and FATP4 knockdown decreased this response (P < 0.05-0.01). FATP4(-/-) mice displayed lower plasma GLP-1 at 60 min in response to intraileal OA (P < 0.05), whereas, unexpectedly, CD36(-/-) mice displayed higher basal GLP-1 levels (P < 0.01) but a normal response to intraileal OA. Together, these findings demonstrate a key role for FATP4 in OA-induced GLP-1 secretion from the murine L cell in vitro and in vivo, whereas the precise role of CD36 remains unclear.

Glycerolipid/free fatty acid cycle and islet β-cell function in health, obesity and diabetes

Pancreatic β-cells secrete insulin in response to fluctuations in blood fuel concentrations, in particular glucose and fatty acids. However, chronic fuel surfeit can overwhelm the metabolic, signaling and secretory capacity of the β-cell leading to its dysfunction and death - often referred to as glucolipotoxicity. In β-cells and many other cells, glucose and lipid metabolic pathways converge into a glycerolipid/free fatty acid (GL/FFA) cycle, which is driven by the substrates, glycerol-3-phosphate and fatty acyl-CoA, derived from glucose and fatty acids, respectively. Although the overall operation of GL/FFA cycle, consisting of lipolysis and lipogenesis, is "futile" in terms of energy expenditure, this metabolic cycle likely plays an indispensable role for various β-cell functions, in particular insulin secretion and excess fuel detoxification. In this review, we discuss the significance of GL/FFA cycle in the β-cell, its regulation and role in generating essential metabolic signals that participate in the lipid amplification arm of glucose stimulated insulin secretion and in β-cell growth. We propose the novel concept that the lipolytic segment of GL/FFA cycle is instrumental in producing signals for insulin secretion, whereas, the lipogenic segment generates signals relevant for β-cell survival/death and growth/proliferation.

Islet organogenesis, angiogenesis and innervation

The pancreas is characterized by a major component, an exocrine and ductal system involved in digestion, and a minor component, the endocrine islets represented by islet micro-organs that tightly regulate glucose homoeostasis. Pancreatic organogenesis is strictly co-ordinated by transcription factors that are expressed sequentially to yield functional islets capable of maintaining glucose homoeostasis. Angiogenesis and innervation complete islet development, equipping islets to respond to metabolic demands. Proper regulation of this triad of processes during development is critical for establishing functional islets.

Characterization of the lipid droplet proteome of a clonal insulin-producing β-cell line (INS-1 832/13)

Lipids are known to play a crucial role both in the normal control of insulin release and in the deterioration of β-cell function, as observed in type 2 diabetes. Despite this established dual role of lipids, little is known about lipid storage and handling in β-cells. Here, we isolated lipid droplets from oleate-incubated INS-1 832/13 cells and characterized the lipid droplet proteome. In a total of four rounds of droplet isolation and proteomic analysis by HPLC-MS/MS, we identified 96 proteins that were specific to droplets. The proteins fall into six categories based on function or previously observed localization: metabolism, endoplasmic reticulum/ribosomes, mitochondria, vesicle formation and transport, signaling, and miscellaneous. The protein profile reinforces the emerging picture of the lipid droplet as an active and dynamic organelle involved in lipid homeostasis and intracellular trafficking. Proteins belonging to the category mitochondria were highly represented, suggesting that the β-cell mitochondria and lipid droplets form a metabolic unit of potential relevance for insulin secretion.