Beta-cell ABCA1 influences insulin secretion, glucose homeostasis and response to thiazolidinedione treatment

High density lipoproteins and type 2 diabetes: Emerging concepts in their relationship

Patients with type 2 diabetes mellitus (T2DM) frequently exhibit macrovascular complications of atherosclerotic cardiovascular (CV) disease. High density lipoproteins (HDL) are protective against atherosclerosis. Low levels of HDL cholesterol (HDL-C) independently contribute to CV risk. Patients with T2DM not only exhibit low HDL-C, but also dysfunctional HDL. Furthermore, low concentration of HDL may increase the risk for the development of T2DM through a decreased β cell survival and secretory function. In this paper, we discuss emerging concepts in the relationship of T2DM with HDL.

ABCA1 in adipocytes regulates adipose tissue lipid content, glucose tolerance, and insulin sensitivity

Adipose tissue contains one of the largest reservoirs of cholesterol in the body. Adipocyte dysfunction in obesity is associated with intracellular cholesterol accumulation, and alterations in cholesterol homeostasis have been shown to alter glucose metabolism in cultured adipocytes. ABCA1 plays a major role in cholesterol efflux, suggesting a role for ABCA1 in maintaining cholesterol homeostasis in the adipocyte. However, the impact of adipocyte ABCA1 on adipose tissue function and glucose metabolism is unknown. Our aim was to determine the impact of adipocyte ABCA1 on adipocyte lipid metabolism, body weight, and glucose metabolism in vivo. To address this, we used mice lacking ABCA1 specifically in adipocytes (ABCA1(-ad/-ad)). When fed a high-fat, high-cholesterol diet, ABCA1(-ad/-ad) mice showed increased cholesterol and triglyceride stores in adipose tissue, developed enlarged fat pads, and had increased body weight. Associated with these phenotypic changes, we observed significant changes in the expression of genes involved in cholesterol and glucose homeostasis, including ldlr, abcg1, glut-4, adiponectin, and leptin. ABCA1(-ad/-ad) mice also demonstrated impaired glucose tolerance, lower insulin sensitivity, and decreased insulin secretion. We conclude that ABCA1 in adipocytes influences adipocyte lipid metabolism, body weight, and whole-body glucose homeostasis.

Death and dysfunction of transplanted β-cells: lessons learned from type 2 diabetes?

β-Cell replacement by islet transplantation is a potential curative therapy for type 1 diabetes. Despite advancements in islet procurement and immune suppression that have increased islet transplant survival, graft function progressively declines, and many recipients return to insulin dependence within a few years posttransplant. The progressive loss of β-cell function in islet transplants seems unlikely to be explained by allo- and autoimmune-mediated mechanisms alone and in a number of ways resembles β-cell failure in type 2 diabetes. That is, both following transplantation and in type 2 diabetes, islets exhibit decreased first-phase glucose-stimulated insulin secretion, impaired proinsulin processing, inflammation, formation of islet amyloid, signs of oxidative and endoplasmic reticulum stress, and β-cell death. These similarities suggest common mechanisms may underlie loss of insulin production in both type 2 diabetes and islet transplantation and point to the potential for therapeutic approaches used in type 2 diabetes that target the β-cell, such as incretin-based therapies, as adjuncts for immunosuppression in islet transplantation.

Stress and the inflammatory process: a major cause of pancreatic cell death in type 2 diabetes

Type 2 diabetes (T2D) is a complex metabolic disorder characterized by hyperglycemia in the context of insulin resistance, which precedes insulin deficiency as a result of β-cell failure. Accumulating evidence indicates that β-cell loss in T2D results as a response to the combination of oxidative stress and endoplasmic reticulum (ER) stress. Failure of the ER's adaptive capacity and further activation of the unfolded protein response may trigger macroautophagy (hereafter referred as autophagy) as a process of self-protection and inflammation. Many studies have shown that inflammation plays a very important role in the pathogenesis of T2D. Inflammatory mechanisms and cytokine production activated by stress via the inflammasome may further alter the normal structure of β-cells by inducing pancreatic islet cell apoptosis. Thus, the combination of oxidative and ER stress, together with autophagy insufficiency and inflammation, may contribute to β-cell death or dysfunction in T2D. Therapeutic approaches aimed at ameliorating stress and inflammation may therefore prove to be promising targets for the development of new diabetes treatment methods. Here, we discuss different mechanisms involved in stress and inflammation, and the role of antioxidants, endogenous and chemical chaperones, and autophagic pathways, which may shift the tendency from ER stress and apoptosis toward cell survival. Strategies targeting cell survival can be essential for relieving ER stress and reestablishing homeostasis, which may diminish inflammation and prevent pancreatic β-cell death associated with T2D.

Impaired β cell function in Chinese newly diagnosed type 2 diabetes mellitus with hyperlipidemia

The objective is to explore the effects of hyperlipidemia on β cell function in newly diagnosed type 2 diabetes mellitus (T2DM). 208 patients were enrolled in the study and were divided into newly diagnosed T2DM with hyperlipidemia (132 patients) and without hyperlipidemia (76 patients). Demographic data, glucose levels, insulin levels, lipid profiles, homeostasis model assessment for β cell function index (HOMA-β ), homeostasis model assessment for insulin resistance index (HOMA-IR), and quantitative insulin-sensitivity check index (QUICKI) were compared between the two groups. We found that comparing with those of normal lipid levels, the subjects of newly diagnosed T2DM with hyperlipidemia were younger, and had declined HOMA-β . However, the levels of HOMA-β were comparable regardless of different lipid profiles (combined hyperlipidemia, hypertriglyceridemia, and hypercholesterolemia). Multiple stepwise linear regression analysis showed that high fasting plasma glucose (FPG), decreased fasting insulin level (FINS), and high triglyceride (TG) were independent risk factors of β cell dysfunction in newly diagnosed T2DM. Therefore, the management of dyslipidemia, together with glucose control, may be beneficial for T2DM with hyperlipidemia.

Regulation of ABCA1 protein expression and function in hepatic and pancreatic islet cells by miR-145

OBJECTIVE

The ATP-binding cassette transporter A1 (ABCA1) protein maintains cellular cholesterol homeostasis in several different tissues. In the liver, ABCA1 is crucial for high-density lipoprotein biogenesis, and in the pancreas ABCA1 can regulate insulin secretion. In this study, our aim was to identify novel microRNAs that regulate ABCA1 expression in these tissues.

APPROACH AND RESULTS

We combined multiple microRNA prediction programs to identify 8 microRNAs that potentially regulate ABCA1. A luciferase reporter assay demonstrated that 5 of these microRNAs (miR-148, miR-27, miR-144, miR-145, and miR-33a/33b) significantly repressed ABCA1 3'-untranslated region activity with miR-145 resulting in one of the larger decreases. In hepatic HepG2 cells, miR-145 can regulate both ABCA1 protein expression levels and cholesterol efflux function. In murine islets, an increase in miR-145 expression decreased ABCA1 protein expression, increased total islet cholesterol levels, and decreased glucose-stimulated insulin secretion. Inhibiting miR-145 produced the opposite effect of increasing ABCA1 protein levels and improving glucose-stimulated insulin secretion. Finally, increased glucose levels in media significantly decreased miR-145 levels in cultured pancreatic beta cells. These findings suggest that miR-145 is involved in glucose homeostasis and is regulated by glucose concentration.

CONCLUSIONS

Our studies demonstrate that miR-145 regulates ABCA1 expression and function, and inhibiting this microRNA represents a novel strategy for increasing ABCA1 expression, promoting high-density lipoprotein biogenesis in the liver, and improving glucose-stimulated insulin secretion in islets.

Metabolism of plasma cholesterol and lipoprotein parameters are related to a higher degree of insulin sensitivity in high HDL-C healthy normal weight subjects

Background

We have searched if plasma high density lipoprotein-cholesterol (HDL-C) concentration interferes simultaneously with whole-body cholesterol metabolism and insulin sensitivity in normal weight healthy adult subjects.

Methods

We have measured the activities of several plasma components that are critically influenced by insulin and that control lipoprotein metabolism in subjects with low and high HDL-C concentrations. These parameters included cholesteryl ester transfer protein (CETP), phospholipid transfer protein (PLTP), lecithin cholesterol acyl transferase (LCAT), post-heparin lipoprotein lipase (LPL), hepatic lipase (HL), pre-beta-₁HDL, and plasma sterol markers of cholesterol synthesis and intestinal absorption.

Results

In the high-HDL-C group, we found lower plasma concentrations of triglycerides, alanine aminotransferase, insulin, HOMA-IR index, activities of LCAT and HL compared with the low HDL-C group; additionally, we found higher activity of LPL and pre-beta-₁HDL concentration in the high-HDL-C group. There were no differences in the plasma CETP and PLTP activities.

Conclusions

These findings indicate that in healthy hyperalphalipoproteinemia subjects, several parameters that control the metabolism of plasma cholesterol and lipoproteins are related to a higher degree of insulin sensitivity.

MicroRNAs in cardiovascular health: from order to disorder

In the last decade, microRNAs (miRNAs) have revolutionized how we understand metabolism and disease. These small, 20- to 22-nucleotide RNA molecules fine-tune gene expression and can often coordinate multiple genes in a single pathway. Given the multifactorial nature of cardiovascular disease, it is perhaps not surprising that miRNAs have been shown to orchestrate many aspects of disease development, from modulating metabolic risk factors over a lifetime (eg, cholesterol and hormones) to controlling the response to an acute cardiovascular event (eg, inflammation and hypoxia). In this review, we discuss how miRNAs exert control over metabolic pathways that maintain vascular health and, when these pathways go awry, how miRNAs can be targeted for therapeutic modulation.

Myeloid cell-specific ABCA1 deletion does not worsen insulin resistance in HF diet-induced or genetically obese mouse models

Obesity-associated low-grade chronic inflammation plays an important role in the development of insulin resistance. The membrane lipid transporter ATP-binding cassette transporter A1 (ABCA1) promotes formation of nascent HDL particles. ABCA1 also dampens macrophage inflammation by reducing cellular membrane cholesterol and lipid raft content. We tested the hypothesis that myeloid-specific ABCA1 deletion may exacerbate insulin resistance by increasing the obesity-associated chronic low-grade inflammation. Myeloid cell-specific ABCA1 knockout (MSKO) and wild-type (WT) mice developed obesity, insulin resistance, mild hypercholesterolemia, and hepatic steatosis to a similar extent with a 45% high-fat (HF) diet feeding or after crossing into the ob/ob background. Resident peritoneal macrophages and stromal vascular cells from obese MSKO mice accumulated significantly more cholesterol. Relative to chow, HF diet markedly induced macrophage infiltration and inflammatory cytokine expression to a similar extent in adipose tissue of WT and MSKO mice. Among pro-inflammatory cytokines examined, only IL-6 was highly upregulated in MSKO-ob/ob versus ob/ob mouse peritoneal macrophages, indicating a nonsignificant effect of myeloid ABCA1 deficiency on obesity-associated chronic inflammation. In conclusion, myeloid-specific ABCA1 deficiency does not exacerbate obesity-associated low-grade chronic inflammation and has minimal impact on the pathogenesis of insulin resistance in both HF diet-induced and genetically obese mouse models.

Obesity, adiposity, and dyslipidemia: a consensus statement from the National Lipid Association

The term "fat" may refer to lipids as well as the cells and tissue that store lipid (ie, adipocytes and adipose tissue). "Lipid" is derived from "lipos," which refers to animal fat or vegetable oil. Adiposity refers to body fat and is derived from "adipo," referring to fat. Adipocytes and adipose tissue store the greatest amount of body lipids, including triglycerides and free cholesterol. Adipocytes and adipose tissue are active from an endocrine and immune standpoint. Adipocyte hypertrophy and excessive adipose tissue accumulation can promote pathogenic adipocyte and adipose tissue effects (adiposopathy), resulting in abnormal levels of circulating lipids, with dyslipidemia being a major atherosclerotic coronary heart disease risk factor. It is therefore incumbent upon lipidologists to be among the most knowledgeable in the understanding of the relationship between excessive body fat and dyslipidemia. On September 16, 2012, the National Lipid Association held a Consensus Conference with the goal of better defining the effect of adiposity on lipoproteins, how the pathos of excessive body fat (adiposopathy) contributes to dyslipidemia, and how therapies such as appropriate nutrition, increased physical activity, weight-management drugs, and bariatric surgery might be expected to impact dyslipidemia. It is hoped that the information derived from these proceedings will promote a greater appreciation among clinicians of the impact of excess adiposity and its treatment on dyslipidemia and prompt more research on the effects of interventions for improving dyslipidemia and reducing cardiovascular disease risk in overweight and obese patients.

Comparison of effects of pitavastatin and atorvastatin on glucose metabolism in type 2 diabetic patients with hypercholesterolemia

AIMS/INTRODUCTION

The distinct effects of different statins on glycemic control have not been fully evaluated. In this open-label, prospective, cross-over clinical trial, we compared the effects of pitavastatin and atorvastatin on glycemic control in type 2 diabetic patients with hypercholesterolemia.

MATERIALS AND METHODS

A total of 28 Japanese type 2 diabetics with hypercholesterolemia treated with rosuvastatin (2.5 mg/day) for at least 8 weeks were recruited to this quasi-randomized cross-over study. At study entry, the patients assigned to sequence 1 received pitavastatin (2 mg/day) for 12 weeks in period 1 and atorvastatin (10 mg/day) for another 12 weeks in period 2, whereas patients assigned to sequence 2 received atorvastatin (10 mg/day) for 12 weeks in period 1 and pitavastatin (2 mg/day) for another 12 weeks in period 2. Blood samples were collected at three visits (baseline, after 12 and 24 weeks).

RESULTS

Lipid control was similar in both statins. The difference in glycated hemoglobin between pitavastatin and atorvastatin treatments was -0.18 (95% confidence interval -0.34 to -0.02; P = 0.03). Compared with atorvastatin, pitavastatin treatment significantly lowered the levels of glycoalbumin, fasting glucose and homeostasis model assessment of insulin resistance.

CONCLUSIONS

Our results showed that treatment with pitavastatin had a more favorable outcome on glycemic control in patients with type 2 diabetes compared with atorvastatin. This trial was registered with UMIN (no. 000003554).

The R230C variant of the ATP binding cassette protein A1 (ABCA1) gene is associated with a decreased response to glyburide therapy in patients with type 2 diabetes mellitus

OBJECTIVE

To test the hypothesis that persons with the R230C allele of ABCA1 show a decreased glycemic response to glyburide. This polymorphism is exclusively found in Ameri-indian populations and is associated with type 2 diabetes.

RESEARCH DESIGN AND METHODS

This is a single blind controlled study including participants with type 2 diabetes (fasting glucose levels 126-250 mg/dl and HbA1c 7%-10%) managed with metformin and a lifestyle program. Each person with the risk allele (R230C) was matched by age, gender and BMI to three others with the wild type variant (R230R). Following a four week stabilization period, only participants with a greater than 70% adherence to metformin and a stable body weight were prescribed glyburide therapy for a further 16 weeks. The main outcome variable was the glyburide dose required to achieve treatment goals.

RESULTS

No significant difference was observed in the glucose lowering effect of glyburide between subjects with the R230C and R230R alleles. However, the dose of sulfonylurea was significantly higher in the R230C participants compared with the R230R subjects (3.3±2.1 vs 6.3±5 mg/day, p<0.001). A higher percentage of R230C participants required at least 5mg of glyburide per day to achieve treatment goals. The glyburide dose was determined by the presence of the risk allele, among other factors.

CONCLUSIONS

Patients with type 2 diabetes who have the R230C allele of ABCA1 needed a higher dose of glyburide in order to achieve the same glucose lowering effect as that in persons with the wild type variant.

High density lipoprotein as a source of cholesterol for adrenal steroidogenesis: a study in individuals with low plasma HDL-C

Few studies have addressed the delivery of lipoprotein-derived cholesterol to the adrenals for steroid production in humans. While there is evidence against a role for low-density lipoprotein (LDL), it is unresolved whether high density lipoprotein (HDL) contributes to adrenal steroidogenesis. To study this, steroid hormone profiles in urine were assessed in male subjects suffering from functional mutations in ATP binding cassette transporter A1 (ABCA1) (n = 24), lecithin:cholesterol acyltransferase (LCAT) (n = 40), as well as in 11 subjects with low HDL cholesterol (HDL-C) without ABCA1/LCAT mutations. HDL-C levels were 39% lower in the ABCA1, LCAT, and low HDL-C groups compared with controls (all P < 0.001). In all groups with low HDL-C levels, urinary excretion of 17-ketogenic steroids was reduced by 33%, 27%, and 32% compared with controls (all P < 0.04). In seven carriers of either type of mutation, adrenocorticotropic hormone (ACTH) stimulation did not reveal differences from normolipidemic controls. In conclusion, this study shows that basal but not stimulated corticosteroid metabolism is attenuated in subjects with low HDL-C, irrespective of its molecular origin. These findings lend support to a role for HDL as a cholesterol donor for basal adrenal steroidogenesis in humans.

MicroRNAs in metabolic disease

Alterations in the metabolic control of lipid and glucose homeostasis predispose an individual to develop cardiometabolic diseases, such as type 2-diabetes mellitus and atherosclerosis. Work over the last years has suggested that microRNAs (miRNAs) play an important role in regulating these physiological processes. The contribution of miRNAs in regulating metabolism is exemplified by miR-33, an intronic miRNA encoded in the Srebp genes. miR-33 controls cellular cholesterol export and fatty acid degradation, whereas its host genes stimulate cholesterol and fatty acid synthesis. Other miRNAs, such as miR-122, also play a critical role in regulating lipid homeostasis by controlling cholesterol synthesis and lipoprotein secretion in the liver. This review article summarizes the recent findings in the field, highlighting the contribution of miRNAs in regulating lipid and glucose metabolism. We will also discuss how the modulation of specific miRNAs may be a promising strategy to treat metabolic diseases.

Coordinate changes in histone modifications, mRNA levels, and metabolite profiles in clonal INS-1 832/13 β-cells accompany functional adaptations to lipotoxicity

Lipotoxicity is a presumed pathogenetic process whereby elevated circulating and stored lipids in type 2 diabetes cause pancreatic β-cell failure. To resolve the underlying molecular mechanisms, we exposed clonal INS-1 832/13 β-cells to palmitate for 48 h. We observed elevated basal insulin secretion but impaired glucose-stimulated insulin secretion in palmitate-exposed cells. Glucose utilization was unchanged, palmitate oxidation was increased, and oxygen consumption was impaired. Halting exposure of the clonal INS-1 832/13 β-cells to palmitate largely recovered all of the lipid-induced functional changes. Metabolite profiling revealed profound but reversible increases in cellular lipids. Glucose-induced increases in tricarboxylic acid cycle intermediates were attenuated by exposure to palmitate. Analysis of gene expression by microarray showed increased expression of 982 genes and decreased expression of 1032 genes after exposure to palmitate. Increases were seen in pathways for steroid biosynthesis, cell cycle, fatty acid metabolism, DNA replication, and biosynthesis of unsaturated fatty acids; decreases occurred in the aminoacyl-tRNA synthesis pathway. The activity of histone-modifying enzymes and histone modifications of differentially expressed genes were reversibly altered upon exposure to palmitate. Thus, Insig1, Lss, Peci, Idi1, Hmgcs1, and Casr were subject to epigenetic regulation. Our analyses demonstrate that coordinate changes in histone modifications, mRNA levels, and metabolite profiles accompanied functional adaptations of clonal β-cells to lipotoxicity. It is highly likely that these changes are pathogenetic, accounting for loss of glucose responsiveness and perturbed insulin secretion.

Telmisartan improves insulin resistance of skeletal muscle through peroxisome proliferator-activated receptor-δ activation

The mechanisms of the improvement of glucose homeostasis through angiotensin receptor blockers are not fully elucidated in hypertensive patients. We investigated the effects of telmisartan on insulin signaling and glucose uptake in cultured myotubes and skeletal muscle from wild-type and muscle-specific peroxisome proliferator-activated receptor (PPAR) δ knockout (MCK-PPARδ(-/-)) mice. Telmisartan increased PPARδ expression and activated PPARδ transcriptional activity in cultured C2C12 myotubes. In palmitate-induced insulin-resistant C2C12 myotubes, telmisartan enhanced insulin-stimulated Akt and Akt substrate of 160 kDa (AS160) phosphorylation as well as Glut4 translocation to the plasma membrane. These effects were inhibited by antagonizing PPARδ or phosphatidylinositol-3 kinase, but not by PPARγ and PPARα inhibition. Palmitate reducing the insulin-stimulated glucose uptake in C2C12 myotubes could be restored by telmisartan. In vivo experiments showed that telmisartan treatment reversed high-fat diet-induced insulin resistance and glucose intolerance in wild-type mice but not in MCK-PPARδ(-/-) mice. The protein levels of PPARδ, phospho-Akt, phospho-AS160, and Glut4 translocation to the plasma membrane in the skeletal muscle on insulin stimulation were reduced by high-fat diet and were restored by telmisartan administration in wild-type mice. These effects were absent in MCK-PPARδ(-/-) mice. These findings implicate PPARδ as a potential therapeutic target in the treatment of hypertensive subjects with insulin resistance.

Skeletal muscle insulin resistance associated with cholesterol-induced activation of macrophages is prevented by high density lipoprotein

Background

Emerging evidence suggests that high density lipoprotein (HDL) may modulate glucose metabolism through multiple mechanisms including pancreatic insulin secretion as well as insulin-independent glucose uptake into muscle. We hypothesized that HDL may also increase skeletal muscle insulin sensitivity via cholesterol removal and anti-inflammatory actions in macrophages associated with excess adiposity and ectopic lipid deposition.

Methods

Human primary and THP-1 macrophages were treated with vehicle (PBS) or acetylated low density lipoprotein (acLDL) with or without HDL for 18 hours. Treatments were then removed, and macrophages were incubated with fresh media for 4 hours. This conditioned media was then applied to primary human skeletal myotubes derived from vastus lateralis biopsies taken from patients with type 2 diabetes to examine insulin-stimulated glucose uptake.

Results

Conditioned media from acLDL-treated primary and THP-1 macrophages reduced insulin-stimulated glucose uptake in primary human skeletal myotubes compared with vehicle (primary macrophages, 168±21% of basal uptake to 104±19%; THP-1 macrophages, 142±8% of basal uptake to 108±6%; P<0.05). This was restored by co-treatment of macrophages with HDL. While acLDL increased total intracellular cholesterol content, phosphorylation of c-jun N-terminal kinase and secretion of pro- and anti-inflammatory cytokines from macrophages, none were altered by co-incubation with HDL. Insulin-stimulated Akt phosphorylation in human skeletal myotubes exposed to conditioned media was unaltered by either treatment condition.

Conclusion

Inhibition of insulin-stimulated glucose uptake in primary human skeletal myotubes by conditioned media from macrophages pre-incubated with acLDL was restored by co-treatment with HDL. However, these actions were not linked to modulation of common pro- or anti-inflammatory mediators or insulin signaling via Akt.

Long-term high density lipoprotein infusion ameliorates metabolic phenotypes of diabetic db/db mice

BACKGROUND

Lower quantity and quality of high density lipoprotein (HDL) are important characteristics of type 2 diabetes mellitus. Acute HDL infusion results in a greater fall of plasma glucose in diabetes patients. Here, we aim to investigate the influence of long-term HDL infusion on metabolic phenotypes of diabetic db/db mice.

METHODS

High density lipoprotein was introduced to db/db mice twice a week for 4 weeks. The phenotypes of the mice were monitored by analyzing metabolic parameters. Glycogen analysis was performed with amyloglucosidase. The corresponding signaling molecules were detected by western blot.

RESULTS

Long-term introduction of HDL decreased plasma glucose levels of db/db mice. Glycogen deposition was enhanced in gastrocnemius muscle, paralleling the elevated glycogen synthase kinase-3 phosphorylation. Meanwhile, increased Akt-Ser473 and adenosine monophosphate-activated protein kinase phosphorylations were detected in the muscle. Moreover, HDL reduced blood glucose and free fatty acids and improved pancreatic islet structure and function with increased C-peptide. Furthermore, decreased interleukin-6, C-reactive protein, monocyte chemoattractant protein-1, resistin, and malondialdehyde, as well as enhanced leptin levels were detected in HDL-treated mice.

CONCLUSION

Results of the present study suggest that long-term HDL infusion has positive therapeutic effects on the metabolic disturbances of db/db diabetic mice.

PPAR-γ activation increases insulin secretion through the up-regulation of the free fatty acid receptor GPR40 in pancreatic β-cells

Background

It has been reported that peroxisome proliferator-activated receptor (PPAR)-γ and their synthetic ligands have direct effects on pancreatic β-cells. We investigated whether PPAR-γ activation stimulates insulin secretion through the up-regulation of GPR40 in pancreatic β-cells.

Methods

Rat insulinoma INS-1 cells and primary rat islets were treated with rosiglitazone (RGZ) and/or adenoviral PPAR-γ overexpression. OLETF rats were treated with RGZ.

Results

PPAR-γ activation with RGZ and/or adenoviral PPAR-γ overexpression increased free fatty acid (FFA) receptor GPR40 expression, and increased insulin secretion and intracellular calcium mobilization, and was blocked by the PLC inhibitors, GPR40 RNA interference, and GLUT2 RNA interference. As a downstream signaling pathway of intracellular calcium mobilization, the phosphorylated levels of CaMKII and CREB, and the downstream IRS-2 and phospho-Akt were significantly increased. Despite of insulin receptor RNA interference, the levels of IRS-2 and phospho-Akt was still maintained with PPAR-γ activation. In addition, the β-cell specific gene expression, including Pdx-1 and FoxA2, increased in a GPR40- and GLUT2-dependent manner. The levels of GPR40, phosphorylated CaMKII and CREB, and β-cell specific genes induced by RGZ were blocked by GW9662, a PPAR-γ antagonist. Finally, PPAR-γ activation up-regulated β-cell gene expressions through FoxO1 nuclear exclusion, independent of the insulin signaling pathway. Based on immunohistochemical staining, the GLUT2, IRS-2, Pdx-1, and GPR40 were more strongly expressed in islets from RGZ-treated OLETF rats compared to control islets.

Conclusion

These observations suggest that PPAR-γ activation with RGZ and/or adenoviral overexpression increased intracellular calcium mobilization, insulin secretion, and β-cell gene expression through GPR40 and GLUT2 gene up-regulation.

Cholesterol reduction ameliorates glucose-induced calcium handling and insulin secretion in islets from low-density lipoprotein receptor knockout mice

AIMS/HYPOTHESIS

Changes in cellular cholesterol level may contribute to beta cell dysfunction. Islets from low density lipoprotein receptor knockout (LDLR(-/-)) mice have higher cholesterol content and secrete less insulin than wild-type (WT) mice. Here, we investigated the association between cholesterol content, insulin secretion and Ca(2+) handling in these islets.

METHODS

Isolated islets from both LDLR(-/-) and WT mice were used for measurements of insulin secretion (radioimmunoassay), cholesterol content (fluorimetric assay), cytosolic Ca(2+) level (fura-2AM) and SNARE protein expression (VAMP-2, SNAP-25 and syntaxin-1A). Cholesterol was depleted by incubating the islets with increasing concentrations (0-10mmol/l) of methyl-beta-cyclodextrin (MβCD).

RESULTS

The first and second phases of glucose-stimulated insulin secretion (GSIS) were lower in LDLR(-/-) than in WT islets, paralleled by an impairment of Ca(2+) handling in the former. SNAP-25 and VAMP-2, but not syntaxin-1A, were reduced in LDLR(-/-) compared with WT islets. Removal of excess cholesterol from LDLR(-/-) islets normalized glucose- and tolbutamide-induced insulin release. Glucose-stimulated Ca(2+) handling was also normalized in cholesterol-depleted LDLR(-/-) islets. Cholesterol removal from WT islets by 0.1 and 1.0mmol/l MβCD impaired both GSIS and Ca(2+) handling. In addition, at 10mmol/l MβCD WT islet showed a loss of membrane integrity and higher DNA fragmentation.

CONCLUSION

Abnormally high (LDLR(-/-) islets) or low cholesterol content (WT islets treated with MβCD) alters both GSIS and Ca(2+) handling. Normalization of cholesterol improves Ca(2+) handling and insulin secretion in LDLR(-/-) islets.

Novel biological functions of high-density lipoprotein cholesterol

In addition to its role in reverse cholesterol transport, high-density lipoprotein (HDL) cholesterol has direct action on numerous cell types that influence cardiovascular and metabolic health. Cellular responses to HDL entail its capacity to invoke cholesterol efflux that causes signal initiation via scavenger receptor class B, type I, and plasma membrane receptor activation by HDL cargo molecules. In endothelial cells and their progenitors, HDL attenuates apoptosis and stimulates proliferation and migration. HDL also has diverse anti-inflammatory actions in both endothelial cells and leukocytes. In vascular smooth muscles, HDL tempers proinflammatory, promigratory, and degradative processes, and through actions on endothelium and platelets HDL is antithrombotic. There are additional actions of HDL of potential cardiovascular consequence that are indirect, including the capacities to promote pancreatic β-cell insulin secretion, to protect pancreatic β cells from apoptosis, and to enhance glucose uptake by skeletal muscle myocytes. Furthermore, HDL decreases white adipose tissue mass, increases energy expenditure, and promotes the production of adipose-derived cytokine adiponectin that has its own vascular-protective properties. Many of these numerous actions of HDL have been observed not only in cell culture and animal models but also in human studies, and assessments of these functions are now being applied to patient populations to better-elucidate which actions of HDL may contribute to its cardioprotective potential and how they can be quantified and targeted. Further work on the many mechanisms of HDL action promises to reveal new prophylactic and therapeutic strategies to optimize both cardiovascular and metabolic health.

Novel biological functions of high-density lipoprotein cholesterol

In addition to its role in reverse cholesterol transport, high-density lipoprotein (HDL) cholesterol has direct action on numerous cell types that influence cardiovascular and metabolic health. Cellular responses to HDL entail its capacity to invoke cholesterol efflux that causes signal initiation via scavenger receptor class B, type I, and plasma membrane receptor activation by HDL cargo molecules. In endothelial cells and their progenitors, HDL attenuates apoptosis and stimulates proliferation and migration. HDL also has diverse anti-inflammatory actions in both endothelial cells and leukocytes. In vascular smooth muscles, HDL tempers proinflammatory, promigratory, and degradative processes, and through actions on endothelium and platelets HDL is antithrombotic. There are additional actions of HDL of potential cardiovascular consequence that are indirect, including the capacities to promote pancreatic β-cell insulin secretion, to protect pancreatic β cells from apoptosis, and to enhance glucose uptake by skeletal muscle myocytes. Furthermore, HDL decreases white adipose tissue mass, increases energy expenditure, and promotes the production of adipose-derived cytokine adiponectin that has its own vascular-protective properties. Many of these numerous actions of HDL have been observed not only in cell culture and animal models but also in human studies, and assessments of these functions are now being applied to patient populations to better-elucidate which actions of HDL may contribute to its cardioprotective potential and how they can be quantified and targeted. Further work on the many mechanisms of HDL action promises to reveal new prophylactic and therapeutic strategies to optimize both cardiovascular and metabolic health.

ABC transporter genes and risk of type 2 diabetes: a study of 40,000 individuals from the general population

OBJECTIVE

Alterations of pancreatic β-cell cholesterol content may contribute to β-cell dysfunction. Two important determinants of intracellular cholesterol content are the ATP-binding cassette (ABC) transporters A1 (ABCA1) and -G1 (ABCG1). Whether genetic variation in ABCA1 and ABCG1 predicts risk of type 2 diabetes in the general population is unknown.

RESEARCH DESIGN AND METHODS

We tested whether genetic variation in the promoter and coding regions of ABCA1 and ABCG1 predicted risk of type 2 diabetes in the general population. Twenty-seven variants, identified by previous resequencing of both genes, were genotyped in the Copenhagen City Heart Study (CCHS) (n = 10,185). Two loss-of-function mutations (ABCA1 N1800H and ABCG1 g.-376C>T) (n = 322) and a common variant (ABCG1 g.-530A>G) were further genotyped in the Copenhagen General Population Study (CGPS) (n = 30,415).

RESULTS

Only one of the variants examined, ABCG1 g.-530A>G, predicted a decreased risk of type 2 diabetes in the CCHS (P for trend = 0.05). Furthermore, when validated in the CGPS or in the CCHS and CGPS combined (n = 40,600), neither the two loss-of-function mutations (ABCA1 N1800H, ABCG1 g.-376C>T) nor ABCG1 g.-530A>G were associated with type 2 diabetes (P values >0.57 and >0.30, respectively).

CONCLUSIONS

Genetic variations in ABCA1 and ABCG1 were not associated with increased risk of type 2 diabetes in the general population. These data were obtained in general population samples harboring the largest number of heterozygotes for loss-of-function mutations in ABCA1 and ABCG1.

Mapping body fat distribution: a key step towards the identification of the vulnerable patient?

Although excess body fat is a significant health hazard, estimation of body fat content with the body mass index may not adequately reflect the amount of atherogenic adipose tissue (AT), i.e. visceral and ectopic fat. As opposed to subcutaneous AT that supposedly acts as a metabolic sink buffering excess dietary energy, visceral or intra-abdominal AT depots respond to several external stimuli that trigger lipolysis and secretion of free fatty acids (FFAs). Reaching the liver, FFAs accumulate in the liver and, over time, promote a chronic condition known as non-alcoholic fatty liver disease (NAFLD). The liver of the typical NAFLD patient secretes large amounts of very-low-density lipoproteins, the lipid content of which may accumulate in additional organs (skeletal muscle, heart, and pancreas). Here, we review the evidence emerging from functional and population studies that point towards an important role of ectopic fat accumulation in the pathophysiology of type 2 diabetes and cardiovascular disease. We conclude that although patients with impaired glycemic control or type 2 diabetes are at increased cardiovascular disease (CVD) risk, estimating cardiovascular risk goes wellbeyond the assessment of glycemic control and traditional CVD risk factors, and the estimation of visceral/ectopic fat deposition via readily available imaging techniquesshould be considered.

Dyslipidemia and diabetes: reciprocal impact of impaired lipid metabolism and Beta-cell dysfunction on micro- and macrovascular complications

Patients with diabetes frequently exhibit the combined occurrence of hyperglycemia and dyslipidemia. Published data on their coexistence are often controversial. Some studies provide evidence for suboptimal lifestyle and exogenous hyperinsulinism at "mild insulin resistance" in adult diabetic patients as main pathogenic factors. In contrast, other studies confirm that visceral adiposity and insulin resistance are the basic features of dyslipidemia in type 2 diabetes (T2D). The consequence is an excess of free fatty acids, which causes hepatic gluconeogenesis to increase, metabolism in muscles to shift from glucose to lipid, beta-cell lipotoxicity, and an appearance of the classical "lipid triad", without real hypercholesterolemia. Recently, it has been proposed that cholesterol homeostasis is important for an adequate insulin secretory performance of beta-cells. The accumulation of cholesterol in beta-cells, caused by defective high-density lipoprotein (HDL) cholesterol with reduced cholesterol efflux, induces hyperglycemia, impaired insulin secretion, and beta-cell apoptosis. Data from animal models and humans, including humans with Tangier disease, who are characterized by very low HDL cholesterol levels, are frequently associated with hyperglycemia and T2D. Thus, there is a reciprocal influence of dyslipidemia on beta-cell function and inversely of beta-cell dysfunction on lipid metabolism and micro- and macrovascular complications. It remains to be clarified how these different but mutually influencing adverse effects act in together to define measures for a more effective prevention and treatment of micro- and macrovascular complications in diabetes patients. While the control of circulating low-density lipoprotein (LDL) cholesterol and the level of HDL cholesterol are determinant targets for the reduction of cardiovascular risk, based on recent data, these targets should also be considered for the prevention of beta-cell dysfunction and the development of type 2 diabetes. In this review, we analyze consolidated data and recent advances on the relationship between lipid metabolism and diabetes mellitus, with particular attention to the reciprocal effects of the two features of the disease and the development of vascular complications.

ATP-binding cassette transporter G1 intrinsically regulates invariant NKT cell development

ATP-binding cassette transporter G1 (ABCG1) plays a role in the intracellular transport of cholesterol. Invariant NKT (iNKT) cells are a subpopulation of T lymphocytes that recognize glycolipid Ags. In this study, we demonstrate that ABCG1 regulates iNKT cell development and functions in a cell-intrinsic manner. Abcg1(-/-) mice displayed reduced frequencies of iNKT cells in thymus and periphery. Thymic iNKT cells deficient in ABCG1 had reduced membrane lipid raft content, and showed impaired proliferation and defective maturation during the early stages of development. Moreover, we found that Abcg1(-/-) mice possess a higher frequency of Vβ7(+) iNKT cells, suggesting alterations in iNKT cell thymic selection. Furthermore, in response to CD3ε/CD28 stimulation, Abcg1(-/-) thymic iNKT cells showed reduced production of IL-4 but increased production of IFN-γ. Our results demonstrate that changes in intracellular cholesterol homeostasis by ABCG1 profoundly impact iNKT cell development and function.

Rosiglitazone promotes PPARγ-dependent and -independent alterations in gene expression in mouse islets

The glitazone class of insulin-sensitizing agents act, in part, by the activation of peroxisome proliferator-activated receptor (PPAR)-γ in adipocytes. However, it is unclear whether the expression of PPARγ in the islets is essential for their potential β-cell-sparing properties. To investigate the in vivo effects of rosiglitazone on β-cell biology, we used an inducible, pancreatic and duodenal homeobox-1 enhancer element-driven, Cre recombinase to knockout PPARγ expression specifically in adult β-cells (PPARgKO). Subjecting the PPARgKO mice to a chow diet led to virtually undetectable changes in glucose or insulin sensitivity, which was paralleled by minimal changes in islet gene expression. Similarly, challenging the mutant mice with a high-fat diet and treatment with rosiglitazone did not alter insulin sensitivity, glucose-stimulated insulin secretion, islet size, or proliferation in the knockout mice despite PPARγ-dependent and -independent changes in islet gene expression. These data suggest that PPARγ expression in the β-cells is unlikely to be directly essential for normal β-cell function or the insulin-sensitizing actions of rosiglitazone.

Metabolic stress, IAPP and islet amyloid

Amyloid forms within pancreatic islets in type 2 diabetes from aggregates of the β-cell peptide islet amyloid polypeptide (IAPP). These aggregates are toxic to β-cells, inducing β-cell death and dysfunction, as well as inciting islet inflammation. The β-cell is subject to a number of other stressors, including insulin resistance and hyperglycaemia, that may contribute to amyloid formation by increasing IAPP production by the β-cell. β-Cell dysfunction, evident as impaired glucose-stimulated insulin secretion and defective prohormone processing and exacerbated by metabolic stress, is also a likely prerequisite for islet amyloid formation to occur in type 2 diabetes. Islet transplants in patients with type 1 diabetes face similar stressors, and are subject to rapid amyloid formation and impaired proinsulin processing associated with progressive loss of β-cell function and mass. Declining β-cell mass is predicted to increase metabolic demand on remaining β-cells, promoting a feed-forward cycle of β-cell decline.

Cholesterol accumulation increases insulin granule size and impairs membrane trafficking

The formation of mature secretory granules is essential for proper storage and regulated release of hormones and neuropeptides. In pancreatic β cells, cholesterol accumulation causes defects in insulin secretion and may participate in the pathogenesis of type 2 diabetes. Using a novel cholesterol analog, we show for the first time that insulin granules are the major sites of intracellular cholesterol accumulation in live β cells. This is distinct from other, non-secretory cell types, in which cholesterol is concentrated in the recycling endosomes and the trans-Golgi network. Excess cholesterol was delivered specifically to insulin granules, which caused granule enlargement and retention of syntaxin 6 and VAMP4 in granule membranes, with concurrent depletion of these proteins from the trans-Golgi network. Clathrin also accumulated in the granules of cholesterol-overloaded cells, consistent with a possible defect in the last stage of granule maturation, during which clathrin-coated vesicles bud from the immature granules. Excess cholesterol also reduced the docking and fusion of insulin granules at the plasma membrane. Together, the data support a model in which cholesterol accumulation in insulin secretory granules impairs the ability of these vesicles to respond to stimuli, and thus reduces insulin secretion.

Myeloid cell-specific ABCA1 deletion protects mice from bacterial infection

RATIONALE

ATP-binding cassette transporter A1 (ABCA1) plays a critical role in eliminating excess free cholesterol from tissues by effluxing cellular free cholesterol and phospholipids to lipid-poor apolipoprotein AI. Macrophage ABCA1 also dampens proinflammatory myeloid differentiation primary-response protein 88-dependent toll-like receptor signaling by reducing cellular membrane free cholesterol and lipid raft content, indicating a role of ABCA1 in innate immunity. However, whether ABCA1 expression has a role in regulating macrophage function in vivo is unknown.

OBJECTIVE

We investigated whether macrophage ABCA1 expression impacts host defense function, including microbial killing and chemotaxis.

METHODS AND RESULTS

Myeloid cell-specific ABCA1 knockout (MSKO) vs wild-type mice were infected with Listeria monocytogenes (Lm) for 36 hours or 72 hours before euthanasia. Lm-induced monocytosis was similar for wild-type and MSKO mice; however, MSKO mice were more resistant to Lm infection, with significantly less body weight loss, less Lm burden in liver and spleen, and less hepatic damage 3 days postinfection. In addition, Lm-infected MSKO mouse livers had: (1) greater monocyte chemoattractant protein-1 and macrophage inflammatory protein-2 expression; (2) more monocyte/macrophage infiltration; (3) less neutral lipid accumulation; and (4) diminished expression of lipogenic genes. MSKO macrophages showed enhanced chemotaxis toward chemokines in vitro and increased migration from peritoneum in response to lipopolysaccharide in vivo. Lm infection of wild-type macrophages markedly reduced expression of ABCA1 protein, as well as other cholesterol export proteins (such as ATP-binding cassette transporter G1 and apolipoprotein E).

CONCLUSIONS

Myeloid-specific ABCA1 deletion favors host response to and clearance of Lm. Macrophage Lm infection reduces expression of cholesterol export proteins, suggesting that diminished cholesterol efflux enhances innate immune function of macrophages.

Relationship between blood lipid profiles and pancreatic islet β cell function in Chinese men and women with normal glucose tolerance: a cross-sectional study

Background

Dyslipidemia is present in people with diabetes as well as subjects with normal glucose tolerance (NGT). The purpose of this study was to investigate the relationship between lipid profiles and β cell function in Chinese individuals with NGT but without history of diabetes or prediabetes.

Methods

A total of 893 men and 1454 women aged 18–76 years living in Sichuan, China, who were not being treated with lipid-lowering drugs were enrolled in this study. Insulin sensitivity (IR) was evaluated using the homeostasis model assessment –IR (HOMA-IR), β-cell function was calculated by the following equation: ΔI30/ΔG30/ HOMA-IR (ΔI30/ΔG30: the ratio of incremental glucose and insulin 30 min after glucose intake). Multivariate linear regression analyses were performed to estimate the relationship between blood lipid and β cell function as standardized coefficients (β).

Results

β cell function decreased in men and women with increasing age. We found inverse relationships between β cell function and total cholesterol (TC) in men and women (β = −0.157 and −0.113, respectively, both p < 0.001), low-density lipoprotein–cholesterol (LDL-C; β = −0.130 and −0.068, respectively, both p < 0.001), TC/high-density lipoprotein–cholesterol (HDL-C) ratio (β = −0.084, p < 0.01 and −0.096, p < 0.001), and triglycerides (TG) (women only; β = −0.053, p < 0.05). However, β cell function was not associated with HDL-C in men or women (β = −0.034 and 0.000, respectively, both p > 0.05) or the TG/HDL-C ratio (β = −0.035 and −0.013, respectively, both p > 0.05). β cell function was significantly worse in males than in females in all age groups, except in subjects aged > 70 years.

Conclusions

Dyslipidemia is associated with dysfunction of pancreatic β cells in subjects with NGT and this is particularly evident in people with elevated TC and LDL-C levels, especially males.

Trial Registration Number

#TR-CCH-Chi CTR-CCH-00000361

Balsamic Vinegar Improves High Fat-Induced Beta Cell Dysfunction via Beta Cell ABCA1

BACKGROUND

The aim of this study was to investigate the effects of balsamic vinegar on β-cell dysfunction.

METHODS

In this study, 28-week-old Otsuka Long-Evans Tokushima Fatty (OLETF) rats were fed a normal chow diet or a high-fat diet (HFD) and were provided with tap water or dilute balsamic vinegar for 4 weeks. Oral glucose tolerance tests and histopathological analyses were performed thereafter.

RESULTS

In rats fed both the both chow diet and the HFD, the rats given balsamic vinegar showed increased insulin staining in islets compared with tap water administered rats. Balsamic vinegar administration also increased β-cell ATP-binding cassette transporter subfamily A member 1 (ABCA1) expression in islets and decreased cholesterol levels.

CONCLUSION

These findings provide the first evidence for an anti-diabetic effect of balsamic vinegar through improvement of β-cell function via increasing β-cell ABCA1 expression.

Associations of ATP-binding cassette transporter A1 and G1 with insulin secretion in human insulinomas

OBJECTIVES

Adenosine triphosphate-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) are 2 important cholesterol transporters in human pancreatic β-cells. The aim of this study was to investigate their alteration in insulinomas and their potential associations with abnormal insulin secretion in these patients.

METHODS

Six patients with insulinoma and 6 healthy controls were recruited. Lipid profiles and glucose metabolism were measured. Insulin content, ABCA1, and ABCG1 in insulinomas and the adjacent islets of the 6 patients with insulinoma were detected by immunohistochemistry or immunofluorescence.

RESULTS

Plasma total cholesterol, high-density lipoprotein, low-density lipoprotein, and triglyceride were comparable between the controls and the patients with insulinoma. Fasting glucose was less than 2.8 mmol/L, and insulin release index was greater than 0.3 in each patient. Serum insulin fell extremely, and blood glucose reached the reference range within an hour after the cutting of the tumors in 2 patients with insulinoma. Adenosine triphosphate-binding cassette transporter G1 increased in insulinomas compared with the adjacent islets. However, ABCA1 was detected neither in the adjacent islets nor in insulinomas. Adenosine triphosphate-binding cassette transporter G1 expression in insulinomas was significantly associated with fasting insulin level and insulin release index.

CONCLUSIONS

Increased ABCG1 may contribute to insulin hypersecretion in insulinomas. In contrast, the undetectable ABCA1 in insulinomas may reflect a negative feedback in insulin secretion in these patients.

Hepatic ABCA1 and VLDL triglyceride production

Elevated plasma triglyceride (TG) and reduced high density lipoprotein (HDL) concentrations are prominent features of metabolic syndrome (MS) and type 2 diabetes (T2D). Individuals with Tangier disease also have elevated plasma TG concentrations and a near absence of HDL, resulting from mutations in ATP binding cassette transporter A1 (ABCA1), which facilitates the efflux of cellular phospholipid and free cholesterol to assemble with apolipoprotein A-I (apoA-I), forming nascent HDL particles. In this review, we summarize studies focused on the regulation of hepatic very low density lipoprotein (VLDL) TG production, with particular attention on recent evidence connecting hepatic ABCA1 expression to VLDL, LDL, and HDL metabolism. Silencing ABCA1 in McArdle rat hepatoma cells results in diminished assembly of large (>10nm) nascent HDL particles, diminished PI3 kinase activation, and increased secretion of large, TG-enriched VLDL1 particles. Hepatocyte-specific ABCA1 knockout (HSKO) mice have a similar plasma lipid phenotype as Tangier disease subjects, with a two-fold elevation of plasma VLDL TG, 50% lower LDL, and 80% reduction in HDL concentrations. This lipid phenotype arises from increased hepatic secretion of VLDL1 particles, increased hepatic uptake of plasma LDL by the LDL receptor, elimination of nascent HDL particle assembly by the liver, and hypercatabolism of apoA-I by the kidney. These studies highlight a novel role for hepatic ABCA1 in the metabolism of all three major classes of plasma lipoproteins and provide a metabolic link between elevated TG and reduced HDL levels that are a common feature of Tangier disease, MS, and T2D. This article is part of a Special Issue entitled: Triglyceride Metabolism and Disease.

Pancreatic β-cell function relates positively to HDL functionality in well-controlled type 2 diabetes mellitus

BACKGROUND

High density lipoproteins (HDLs) have been implicated in glucose homeostasis. Among subjects with normal fasting glucose (NFG), impaired fasting glucose (IFG) and Type 2 diabetes mellitus (T2DM) we tested whether pancreatic β-cell function relates to HDL functionality, as determined by HDL anti-oxidative capacity and cellular cholesterol efflux to plasma.

SUBJECTS AND METHODS

HDL anti-oxidative capacity (inhibition of LDL oxidation in vitro), cellular cholesterol efflux (the ability of plasma to stimulate cholesterol efflux out of cultured fibroblasts obtained from a single human donor), glucose and insulin were determined in fasting plasma samples from 37 subjects with NFG, 36 with IFG and 22 with T2DM (no glucose lowering drug or insulin treatment; HbA1c 6.0±1.0%). Homeostasis model assessment was used to estimate pancreatic β-cell function (HOMA-β) and insulin resistance (HOMAir).

RESULTS

HOMA-β was lowest, whereas HOMAir was highest in T2DM (P<0.01 and P<0.001 vs. NFG). HDL anti-oxidative capacity and cellular cholesterol efflux did not differ significantly according to glucose tolerance category. In univariate analysis and after controlling for HOMAir both HDL anti-oxidative capacity (P<0.05) and cellular cholesterol efflux (P<0.01) were positively correlated with HOMA-β in T2DM, but not in NFG and IFG. In age-, sex- and HOMAir-adjusted analyses, T2DM status interacted positively with HDL anti-oxidative capacity (P=0.001) and cellular cholesterol efflux (P=0.042) on HOMA-β. HbA1c interacted similarly with HDL functionality measures on HOMA-β.

CONCLUSIONS

Pancreatic β-cell function relates to pathophysiologically relevant measures of HDL function in T2DM, but not in NFG and IFG. Better HDL functionality may contribute to maintenance of β-cell function in subjects with well-controlled T2DM.

Influence of cholesterol on cellular signaling and fusion pore kinetics

Cholesterol is an important lipid component of cellular membranes. Recent studies have shown that changes in cellular cholesterol level can affect cellular functions. Here, we summarize our recent findings on the impact of cholesterol on the glucose-stimulated Ca(2+) signaling in rat pancreatic β cells and the fusion pore kinetics of large dense core granules in rat chromaffin cells. In mouse pancreatic β cells, pharmacological elevation of cellular cholesterol (but not cholesterol extraction) reduced the current density of the delayed rectifier K(+) channels, the ATP-dependent K(+) channels, and voltage-gated Ca(2+) channels. Importantly, cholesterol enrichment impaired glucose-stimulated Ca(2+) signaling in mouse pancreatic β cells via a suppression of voltage-gated Ca(2+) channels and a decrease in mitochondrial ATP production, which in turn led to a reduction in the glucose-evoked depolarization. In rat chromaffin cells, we found that the persistence of the semi-stable fusion pore was increased by cholesterol enrichment, and acute cholesterol extraction from the cytosolic side of the cell destabilized the semi-stable fusion pore. Overall, our findings highlight the importance of cholesterol in the regulation of cellular signaling and exocytosis.

miR-33a modulates ABCA1 expression, cholesterol accumulation, and insulin secretion in pancreatic islets

Changes in cellular cholesterol affect insulin secretion, and β-cell-specific deletion or loss-of-function mutations in the cholesterol efflux transporter ATP-binding cassette transporter A1 (ABCA1) result in impaired glucose tolerance and β-cell dysfunction. Upregulation of ABCA1 expression may therefore be beneficial for the maintenance of normal islet function in diabetes. Studies suggest that microRNA-33a (miR-33a) expression inversely correlates with ABCA1 expression in hepatocytes and macrophages. We examined whether miR-33a regulates ABCA1 expression in pancreatic islets, thereby affecting cholesterol accumulation and insulin secretion. Adenoviral miR-33a overexpression in human or mouse islets reduced ABCA1 expression, decreased glucose-stimulated insulin secretion, and increased cholesterol levels. The miR-33a-induced reduction in insulin secretion was rescued by cholesterol depletion by methyl-β-cyclodextrin or mevastatin. Inhibition of miR-33a expression in apolipoprotein E knockout islets and ABCA1 overexpression in β-cell-specific ABCA1 knockout islets rescued normal insulin secretion and reduced islet cholesterol. These findings confirm the critical role of β-cell ABCA1 in islet cholesterol homeostasis and β-cell function and highlight modulation of β-cell miR-33a expression as a means to influence insulin secretion.

Loss of both ABCA1 and ABCG1 results in increased disturbances in islet sterol homeostasis, inflammation, and impaired β-cell function

Cellular cholesterol homeostasis is important for normal β-cell function. Disruption of cholesterol transport by decreased function of the ATP-binding cassette (ABC) transporter ABCA1 results in impaired insulin secretion. Mice lacking β-cell ABCA1 have increased islet expression of ABCG1, another cholesterol transporter implicated in β-cell function. To determine whether ABCA1 and ABCG1 have complementary roles in β-cells, mice lacking ABCG1 and β-cell ABCA1 were generated and glucose tolerance, islet sterol levels, and β-cell function were assessed. Lack of both ABCG1 and β-cell ABCA1 resulted in increased fasting glucose levels and a greater impairment in glucose tolerance compared with either ABCG1 deletion or loss of ABCA1 in β-cells alone. In addition, glucose-stimulated insulin secretion was decreased and sterol accumulation increased in islets lacking both transporters compared with those isolated from knockout mice with each gene alone. Combined deficiency of ABCA1 and ABCG1 also resulted in significant islet inflammation as indicated by increased expression of interleukin-1β and macrophage infiltration. Thus, lack of both ABCA1 and ABCG1 induces greater defects in β-cell function than deficiency of either transporter individually. These data suggest that ABCA1 and ABCG1 each make complimentary and important contributions to β-cell function by maintaining islet cholesterol homeostasis in vivo.

A prospective study of prepregnancy dietary fat intake and risk of gestational diabetes

BACKGROUND

Fatty acids play a vital role in glucose homeostasis; however, studies on habitual dietary fat intakes and gestational diabetes mellitus (GDM) risk are limited and provide conflicting findings.

OBJECTIVE

We determined whether the total amount and the type and source of prepregnancy dietary fats are related to risk of GDM.

DESIGN

A prospective study was conducted in 13,475 women who reported a singleton pregnancy between 1991 and 2001 in the Nurses' Health Study II. In these women, 860 incident GDM cases were reported. The adjusted RR of GDM was estimated for quintiles of total fat, specific fat, and the source of fat intakes by pooled logistic regression.

RESULTS

Higher animal fat and cholesterol intakes were significantly associated with increased GDM risk. Across increasing quintiles of animal fat, RRs (95% CIs) for GDM were 1.00 (reference), 1.55 (1.20, 1.98), 1.43 (1.09, 1.88), 1.40 (1.04, 1.89), and 1.88 (1.36, 2.60) (P-trend = 0.05). Corresponding RRs (95% CIs) for dietary cholesterol were 1.00 (reference), 1.08 (0.84, 1.32), 1.02 (0.78, 1.29), 1.20 (0.93, 1.55), and 1.45 (1.11, 1.89) (P-trend = 0.04). The substitution of 5% of energy from animal fat for an equal percentage of energy from carbohydrates was associated with significantly increased risk of GDM [RR (95% CI): 1.13 (1.08, 1.18); P < 0.0001]. No significant associations were observed between dietary polyunsaturated fat, monounsaturated fat, or trans fat intakes and GDM risk.

CONCLUSION

Higher prepregnancy intakes of animal fat and cholesterol were associated with elevated GDM risk.

The emerging role of HDL in glucose metabolism

A low plasma level of HDL cholesterol is an atherosclerotic risk factor; however, emerging evidence suggests that low HDL levels might also contribute to the pathophysiology of type 2 diabetes mellitus (T2DM) through direct effects on plasma glucose. In the past decade, animal and clinical studies have uncovered a previously undescribed spectrum of HDL actions, indicating that HDL may control glucose homeostasis through mechanisms including insulin secretion, direct glucose uptake by muscle via the AMP-activated protein kinase, and possibly enhanced insulin sensitivity. These effects are mediated by multiple cell types via mechanisms including preservation of cell function through cellular lipid removal and also via direct signaling events. We suggest a paradigm shift from HDL being a bystander to being an active player in diabetic pathophysiology, which raises the possibility that HDL elevation could be a novel therapeutic avenue for T2DM. The entry of HDL-raising agents of the cholesteryl ester transfer protein (CETP) inhibitor class into late-phase clinical trials creates potential for rapid clinical translation. This Review will discuss the emerging evidence for a role of HDL-mediated glucose regulation in the pathophysiology of T2DM, and will also outline the therapeutic potential for HDL elevation for the prevention and management of T2DM.

A dietary pattern including nopal, chia seed, soy protein, and oat reduces serum triglycerides and glucose intolerance in patients with metabolic syndrome

Metabolic syndrome (MetS) is a health problem throughout the world and is associated with cardiovascular disease and diabetes. Thus, the purpose of the present work was to evaluate the effects of a dietary pattern (DP; soy protein, nopal, chia seed, and oat) on the biochemical variables of MetS, the AUC for glucose and insulin, glucose intolerance (GI), the relationship of the presence of certain polymorphisms related to MetS, and the response to the DP. In this randomized trial, the participants consumed their habitual diet but reduced by 500 kcal for 2 wk. They were then assigned to the placebo (P; n = 35) or DP (n = 32) group and consumed the reduced energy diet plus the P or DP beverage (235 kcal) minus the energy provided by these for 2 mo. All participants had decreases in body weight (BW), BMI, and waist circumference during the 2-mo treatment (P < 0.0001); however, only the DP group had decreases in serum TG, C-reactive protein (CRP), and AUC for insulin and GI after a glucose tolerance test. Interestingly, participants in the DP group with MetS and the ABCA1 R230C variant had a greater decrease in BW and an increase in serum adiponectin concentration after 2 mo of dietary treatment than those with the ABCA1 R230R variant. The results from this study suggest that lifestyle interventions involving specific DP for the treatment of MetS could be more effective if local foods and genetic variations of the population are considered.

Investigation of ABCA1 C69T polymorphism in patients with type 2 diabetes mellitus

INTRODUCTION

Non insulin dependent diabetes mellitus is the most common type of diabetes. Genetic factors, lipid profiles, hypertension are potential risk factors for diabetes mellitus. Adenosine binding cassette transporter proteins 1 (ABCA1) plays a role in cholesterol metabolism, especially high density lipoprotein (HDL-cholesterol). There are multiple mechanisms by which HDL-cholesterol can be atheroprotective, it is clear that the relative activity of ABCA1 plays a major role. We aimed to investigate association of ABCA1 C69T gene polymorphism with lipid levels in Turkish type 2 diabetic patients.

MATERIALS AND METHODS

After isolation of DNA by ethanol precipitation we determined ABCA1 gene polymorphism by using polimerase chain reaction--restriction fragment lenght polymorphism (PCR-RFLP) method in 107 type 2 diabetic patients and 50 healthy controls.

RESULTS

We have observed that the frequency of TT genotype is significantly higher in healthy controls compared to patients (14% vs. 3%; P = 0.008). Also frequency of T allele was higher in controls than in patients (34% vs. 21%; P = 0.020; OR (95% CI) = 0.52 (0.30-0.88)). There was no association of lipid levels and ABCA1 C69T polymorphism subgroups.

CONCLUSION

We have found significantly higher frequency of both T allele and genotype in control group when compared to patients that made us think that T allele may be a protective factor against diabetes mellitus. But, we could not find a relationship between genotypes and lipid concentrations in our two groups. Larger studies will help us to understand the relationship between ABCA1 C69T genotype and lipid parameters in diabetes mellitus.