Lipotoxicity of beta-cells in obesity and in other causes of fatty acid spillover

Differences in insulin sensitivity, pancreatic beta cell function and circulating adiponectin across glucose tolerance status in Thai obese and non-obese women

Although adiponectin levels are associated with obesity and insulin insensitivity, the role of adiponectin in the progression to diabetes in non-obese subjects is unclear. Therefore, 289 women aged 50-80 years without previous history of diabetes or impaired glucose tolerance (IGT) were studied. They were classified as normal glucose tolerance (NGT), IGT or diabetes based on WHO criteria. Insulin sensitivity (S) and beta cell function (B) indices were calculated using homeostasis model assessment (HOMA). In obese women with BMI > or = 25 kg/m(2) (n = 161), there were declines in HOMA-%S (P < 0.001), HOMA-%B (P < 0.05) and circulating adiponectin (P < 0.001) across glucose tolerance status. In non-obese women with BMI < 25 kg/m(2) (n = 128), there was no significant change in HOMA-%S in women with IGT and diabetes as compared to women with NGT. However, HOMA-%B (P < 0.05) and serum adiponectin levels (P < 0.001) were significantly decreased across glucose tolerance. Serum adiponectin levels were correlated to HOMA-%S in both obese and non-obese women while negative correlations between circulating adiponectin and HOMA-%B were demonstrated only in obese women. We have demonstrated in the present study the predominant role of beta cell dysfunction as compared to that of insulin resistance in the deterioration of glucose tolerance in non-obese women. Circulating adiponectin appears to be inversely related to beta cell dysfunction in addition to insulin resistance only in obese women.

Differences in adipocyte long chain fatty acid uptake in Osborne-Mendel and S5B/Pl rats in response to high-fat diets

OBJECTIVE

To determine whether strain differences in adipocyte uptake of long chain fatty acids (LCFAs) contribute to differences in weight gain by Osborne-Mendel (OM) and S5B/Pl rats (S) fed a high-fat diet (HFD).

SUBJECTS

Ninety-four adult (12-14-week old) male OM and S rats.

MEASUREMENTS

Body weight; epididymal fat pad weight; adipocyte size, number, LCFA uptake kinetics; and plasma insulin and leptin during administration of HFD or chow diets (CDs).

RESULTS

In both strains, rate of weight gain (RWG) was greater on an HFD than a CD; RWG on an HFD was greater, overall, in OM than S. A significant RWG increase occurred on days 1 and 2 in both strains. It was normalized in S by days 6-9 but persisted at least till day 14 in OM. RWGs were significantly correlated (P<0.001) with the V(max) for saturable adipocyte LCFA uptake (V(max)). In S, an increase in V(max) on day 1 returned to baseline by day 7 and was correlated with both plasma insulin and leptin levels throughout. In OM, a greater increase in V(max) was evident by day 2, and persisted for at least 14 days, during which both insulin and leptin levels remained elevated. Growth in epididymal fat pads on the HFD correlated with body weight, reflecting hypertrophy in OM and both hypertrophy and hyperplasia in S.

CONCLUSIONS

(a) Changes in V(max) contribute significantly to changes in RWG on HFDs. (b) There are important strain differences in circulating insulin and leptin responses to an HFD. (c) Both insulin and leptin responses to an HFD are closely correlated with V(max) of adipocyte fatty acid uptake in S animals, but suggest early onset of insulin resistance in OM. Thus, differences in hormonal regulation of adipocyte LCFA uptake may underlie the different responses of OM and S to HFD.

Protection of pancreatic beta-cells: is it feasible?

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

Experimental and computational studies investigating trehalose protection of HepG2 cells from palmitate-induced toxicity

Understanding the mechanism of saturated fatty acid-induced hepatocyte toxicity may provide insight into cures for diseases such as obesity-associated cirrhosis. Trehalose, a nonreducing disaccharide shown to protect proteins and cellular membranes from inactivation or denaturation caused by different stress conditions, also protects hepatocytes from palmitate-induced toxicity. Our results suggest that trehalose serves as a free radical scavenger and alleviates damage from hydrogen peroxide secreted by the compromised cells. We also observe that trehalose protects HepG2 cells by interacting with the plasma membrane to counteract the changes in membrane fluidity induced by palmitate. The experimental results are supported by molecular dynamics simulations of model cell membranes that closely reflect the experimental conditions. Simulations were performed to understand the specific interactions between lipid bilayers, palmitate, and trehalose. The simulations results reveal the early stages of how palmitate induces biophysical changes to the cellular membrane and the role of trehalose in protecting the membrane structure.

Disruption of leptin receptor expression in the pancreas directly affects beta cell growth and function in mice

Obesity is characterized by hyperinsulinemia, hyperleptinemia, and an increase in islet volume. While the mechanisms that hasten the onset of diabetes in obese individuals are not known, it is possible that the adipose-derived hormone leptin plays a role. In addition to its central actions, leptin exerts biological effects by acting in peripheral tissues including the endocrine pancreas. To explore the impact of disrupting leptin signaling in the pancreas on beta cell growth and/or function, we created pancreas-specific leptin receptor (ObR) KOs using mice expressing Cre recombinase under the control of the pancreatic and duodenal homeobox 1 (Pdx1) promoter. The KOs exhibited improved glucose tolerance due to enhanced early-phase insulin secretion, and a greater beta cell mass secondary to increased beta cell size and enhanced expression and phosphorylation of p70S6K. Similar effects on p70S6K were observed in MIN6 beta cells with knockdown of the ObR gene, suggesting crosstalk between leptin and insulin signaling pathways. Surprisingly, challenging the KOs with a high-fat diet led to attenuated acute insulin secretory response to glucose, poor compensatory islet growth, and glucose intolerance. Together, these data provide direct genetic evidence, from a unique mouse model lacking ObRs only in the pancreas, for a critical role for leptin signaling in islet biology and suggest that altered leptin action in islets is one factor that contributes to obesity-associated diabetes.

Inhibitory effect of (-)-epigallocatechin-3-gallate on lipid accumulation of 3T3-L1 cells

OBJECTIVE

The objective of this study was to investigate the molecular mechanisms underlying the attenuating effect of (-)-epigallocatechin-3-gallate (EGCG) on proliferation and lipid accumulation of 3T3-L1 cells, with a focus on the duration of EGCG treatment.

RESEARCH METHODS AND PROCEDURES

Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assay and diamidino-2-phenylindole staining. The anti-adipogenic effect of EGCG on 3T3-L1 cells was analyzed by glycerol-3-phosphate dehydrogenase activity and Oil red O staining. Western blot analysis was used to detect adenosine monophosphate-activated protein kinase (AMPK) activation and phosphorylation of its substrate, acetyl-CoA carboxylase (ACC), and expression of insulin (INS) receptor, INS receptor substrate-1 (IRS-1), and adipocyte marker proteins.

RESULTS

Exposure to EGCG during the early period of adipogenesis (7 days) was sufficient to prevent lipid accumulation. During this period, EGCG greatly decreased expression of the adipocyte marker proteins peroxisome proliferator-activated receptor gamma2 (PPARgamma2) and liver X receptor (LXR)-alpha. Furthermore, EGCG significantly induced generation of reactive oxygen species (ROS), which led to AMPK activation, and these effects were eliminated by N-acetylcysteine (NAC) treatment. Also, EGCG increased the tyrosine phosphorylation of INS receptor and INS-1 with increasing incubation time. In contrast, EGCG treatment did not alter glycerol release in the presence or absence of 2',5'-dideoxyadenosine (DDA), indicating that EGCG had no effect on lipolysis.

DISCUSSION

Our data demonstrate that EGCG decreased cell viability and inhibited differentiation of 3T3-L1 cells in a manner dependent on the duration of treatment. Also, we showed that inhibition of adipocyte differentiation by EGCG was associated with decreased glycerol-3-phosphate dehydrogenase (GPDH) activity accompanied by a strong inhibition of PPARgamma2-induced transcriptional activity. Furthermore, the inhibition of adipocyte differentiation by EGCG involved generation of ROS and activation of AMPK.

The novel mechanism of recombinant human ciliary neurotrophic factor on the anti-diabetes activity

In a previous study, the ciliary neurotrophic factor (CNTF) were demonstrated to lead to weight-loss partly by up-regulating the energy metabolism and the expression of uncoupling protein-1, mitochondrial transcription factor A and nuclear respiratory factor-1 in adipose tissues or muscle. To investigate the up-stream regulators of the expression, recombinant human CNTF (rhCNTF) (0.1, 0.3 and 0.9 mg/kg/day subcutaneously) were administered to KK-Ay mice for 30 days, resulting in reduction of perirenal fat mass, serum free fatty acids and islet triacylglycerol; furthermore, the values of oral glucose tolerance test were found improved. In brown adipose tissues, the gene expressions of peroxisome proliferator-activated receptor alpha (PPARalpha) and peroxisome proliferator-activated receptor coactivator-1 alpha (PGC-1alpha) were found to be up-regulated by rhCNTF. To the best of our knowledge, the changes of gene expression of PPARalpha and PGC-1alpha represent new insights into the mechanisms of anti-diabetes by rhCNTF. In addition, the activity of mitochondrial complexII was found to be increased by rhCNTF. Stimulation of PPARalpha, PGC-1alpha, uncoupling protein-1 and enhanced activity of mitochondrial complex II may be associated with the effects of anti-diabetes. The present study indicates new mechanisms of the activity and mechanisms on anti-diabetes of rhCNTF, which may be a novel anti-diabetes reagent partly acting by enhancing energy metabolism.

Inhibition of PKCepsilon improves glucose-stimulated insulin secretion and reduces insulin clearance

In type 2 diabetes, pancreatic beta cells fail to secrete sufficient insulin to overcome peripheral insulin resistance. Intracellular lipid accumulation contributes to beta cell failure through poorly defined mechanisms. Here we report a role for the lipid-regulated protein kinase C isoform PKCepsilon in beta cell dysfunction. Deletion of PKCepsilon augmented insulin secretion and prevented glucose intolerance in fat-fed mice. Importantly, a PKCepsilon-inhibitory peptide improved insulin availability and glucose tolerance in db/db mice with preexisting diabetes. Functional ablation of PKCepsilon selectively enhanced insulin release ex vivo from diabetic or lipid-pretreated islets and optimized the glucose-regulated lipid partitioning that amplifies the secretory response. Independently, PKCepsilon deletion also augmented insulin availability by reducing both whole-body insulin clearance and insulin uptake by hepatocytes. Our findings implicate PKCepsilon in the etiology of beta cell dysfunction and highlight that enhancement of insulin availability, through separate effects on liver and beta cells, provides a rationale for inhibiting PKCepsilon to treat type 2 diabetes.

Mechanisms involved in the cytotoxic and cytoprotective actions of saturated versus monounsaturated long-chain fatty acids in pancreatic beta-cells

Long-chain saturated and monounsaturated fatty acids differ in their propensity to induce beta-cell death in vitro with palmitate (C16:0) being cytotoxic, whereas palmitoleate (C16:1n-7) is cytoprotective. We now show that this cytoprotective capacity extends to a poorly metabolised C16:1n-7 derivative, methyl-palmitoleate (0.25 mM palmitate alone: 92 +/- 4% death after 18 h; palmitate plus 0.25 mM methyl-palmitoleate: 12 +/- 2%; P < 0.001). Palmitoleate and its methylated derivative also acted as mitogens in cultured beta-cells (5-bromo-2-deoxyuridine incorporation - control: 0.15 +/- 0.01 units; 0.25 mM palmitoleate: 0.22 +/- 0.01 units; P < 0.05). It has been proposed that alterations in neutral lipid synthesis (particularly triacylglycerol (TAG) formation) might mediate the differential responses to saturated and unsaturated fatty acids and we have examined this proposition. Palmitate and palmitoleate both promoted beta-cell phospholipid remodelling and increased TAG formation (control: 0.9 +/- 0.1 nmol TAG/10(6) cells; 0.25 mM palmitate: 1.55 +/- 0.07; 0.25 mM palmitoleate: 1.4 +/- 0.05; palmitate plus palmitoleate: 2.3 +/- 0.1). By contrast, methyl-palmitoleate failed to influence TAG levels (0.25 mM methyl-palmitoleate alone: 0.95 +/- 0.06 nmol TAG/10(6) cells; methyl-palmitoleate plus palmitate: 1.5 +/- 0.05) or its fatty acid composition in beta-cells exposed to palmitate. The results suggest that monounsaturated fatty acids can promote cell viability and mitogenesis by a mechanism that does not require their metabolism and is independent of alterations in TAG formation.

Ketosis-prone type 2 diabetes: effect of hyperglycemia on beta-cell function and skeletal muscle insulin signaling

OBJECTIVE

To determine the underlying mechanism for the severe and transient beta-cell dysfunction and impaired insulin action in obese African American patients with ketosis-prone diabetes.

METHODS

The effect of sustained hyperglycemia (glucotoxicity) and increased free fatty acids (lipotoxicity) on beta-cell function was assessed by changes in insulin secretion during a 20-hour glucose (200 mg/m2 per minute) and a 48-hour Intralipid (40 mL/h) infusion, respectively. Insulin-activated signaling pathways and pattern of Akt-1 and Akt-2 expression and insulin-stimulated phosphorylation were analyzed in skeletal muscle biopsy specimens. Studies were performed in an obese African American woman within 48 hours after resolution of diabetic ketoacidosis and 1 week after discontinuation of insulin treatment.

RESULTS

Dextrose infusion rapidly increased C-peptide levels from a baseline of 3.2 ng/mL to a mean of 7.1 +/- 0.5 ng/mL during the first 8 hours of infusion; thereafter, C-peptide levels progressively declined. Lipid infusion was not associated with any deleterious effect on insulin and C-peptide secretion. Initial in vitro stimulation of muscle tissue with insulin resulted in a substantial and selectively decreased Akt-2 expression and insulin-stimulated phosphorylation on the serine residue. Improved metabolic control resulted in 70% greater Akt expression at near-normoglycemic remission in comparison with the period of hyperglycemia.

CONCLUSION

Hyperglycemia, but not increased free fatty acid levels, led to progressive beta-cell dysfunction and impaired insulin secretion. Hyperglycemia was also associated with diminished skeletal muscle Akt expression and phosphorylation in an African American woman with ketosis-prone diabetes, and this defect improved notably with aggressive insulin therapy. These results indicate the importance of glucose toxicity in the pathogenesis of ketosis-prone diabetes in obese African American patients.

Loss of stearoyl-CoA desaturase-1 improves insulin sensitivity in lean mice but worsens diabetes in leptin-deficient obese mice

The lipogenic gene stearoyl-CoA desaturase (SCD)1 appears to be a promising new target for obesity-related diabetes, as mice deficient in this enzyme are resistant to diet- and leptin deficiency-induced obesity. The BTBR mouse strain replicates many features of insulin resistance found in humans with excess visceral adiposity. Using the hyperinsulinemic-euglycemic clamp technique, we determined that insulin sensitivity was improved in heart, soleus muscle, adipose tissue, and liver of BTBR SCD1-deficient mice. We next determined whether SCD1 deficiency could prevent diabetes in leptin-deficient BTBR mice. Loss of SCD1 in leptin(ob/ob) mice unexpectedly accelerated the progression to severe diabetes; 6-week fasting glucose increased approximately 70%. In response to a glucose challenge, Scd1(-/-) leptin(ob/ob) mice had insufficient insulin secretion, resulting in glucose intolerance. A morphologically distinct class of islets isolated from the Scd1(-/-) leptin(ob/ob) mice had reduced insulin content and increased triglycerides, free fatty acids, esterified cholesterol, and free cholesterol and also a much higher content of saturated fatty acids. We believe the accumulation of lipid is due to an upregulation of lipoprotein lipase (20-fold) and Cd36 (167-fold) and downregulation of lipid oxidation genes in this class of islets. Therefore, although loss of Scd1 has beneficial effects on adiposity, this benefit may come at the expense of beta-cells, resulting in an increased risk of diabetes.

Influences of diabetes (db/db), obese (ob/ob) and dystrophic (dy/dy) genotype mutations on hind limb bone maturation: a morphometric, radiological and cytochemical indices analysis

The influences of single-gene missense mutations expressing diabetes (db/db), obese (ob/ob) or dystrophia (dy/dy) dysregulated metabolic syndromes on hind limb bone maturation and cytodevelopment in C57BL/KsJ mice were evaluated by radiological, macro- and cytomorphometric analysis of the resulting variances in os coxae, femur and tibia osteodevelopment indices relative to control parameters between 8 and 16 weeks of age. Associated with obesity and hyperglycaemic/hyperinsulinaemic states, both db/db and ob/ob mutants demonstrated significant suppression of hind limb maturation (length) and cytodensity indices relative to control growth parameters. By contrast, skeletal growth suppression induced by dy/dy mutation expression was associated with lean body mass and normoglycaemic/hypoinsulinaemic systemic endometabolic indices. In both db/db and ob/ob mutation syndromes, osteovascular, -interstitial and -cytolipidaemia were prominent cytochemical aberrations of the osteopaenic states relative to the dyslipidaemia/fibrodysplasia characteristic of dy/dy osteomaturation. Between 8 and 16 weeks of age, both ob/ob and db/db groups demonstrated extensive cortical interstitial (laminal) osteolipidaemia and suppressed cytodensities compared to control indices. These data demonstrate that the abnormal hyperglycaemic/hyperinsulinaemic endometabolic states associated with the expression of db/db and ob/ob genomutations promote extensive lipidaemia-induced osteopaenia, compromising hind limb osteomaturation and cytodensity indices, as compared to the hyperfibritic osteopaenia characteristic of dy/dy mutation syndromes. Recognized therapeutic modulation of the hypercytolipidaemic component of diabetes-obesity syndromes may prove to be effective towards amelioration of the deleterious influences of these expressed hyperglycaemic, dysregulated lipometabolic conditions on osteomaturation and cytodevelopment.

How effective are lifestyle changes in the prevention of type 2 diabetes mellitus?

Obesity and impaired glucose tolerance are associated with a greater risk for a number of conditions, including insulin resistance, diabetes mellitus, hypertension, dyslipidemia, coagulation abnormalities, inflammatory markers, and coronary heart disease. Lifestyle changes can delay or prevent the development of type 2 diabetes in patients with obesity and impaired glucose tolerance. The risks improve with weight loss and increased physical activity. A decrease of 7% to 10% or more from baseline weight can have a significant effect. This has now been documented in a number of randomized, controlled studies.

Proposed mechanisms of (−)-epigallocatechin-3-gallate for anti-obesity

Green tea catechins (GTCs) are polyphenolic flavonoids formerly called vitamin P. GTCs, especially (-)-epigallocatechin-3-gallate (EGCG), lower the incidence of cancers, collagen-induced arthritis, oxidative stress-induced neurodegenerative diseases, and streptozotocin-induced diabetes. Also, inhibition of adipogenesis by green tea and green tea extract has been demonstrated in cell lines, animal models, and humans. The obesity-preventive effects of green tea and its main constituent EGCG are widely supported by results from epidemiological, cell culture, animal, and clinical studies in the last decade. Studies with adipocyte cell lines and animal models have demonstrated that EGCG inhibits extracellular signal-related kinases (ERK), activates AMP-activated protein kinase (AMPK), modulates adipocyte marker proteins, and down-regulates lipogenic enzymes as well as other potential targets. Also, the catechin components of green tea have been shown to possess anti-carcinogenic properties possibly related to their anti-oxidant activity. In addition, it was shown that dietary supplementation with EGCG could potentially contribute to nutritional strategies for the prevention and treatment of type 2 diabetes mellitus. In this review, the biological activities and multiple mechanisms of EGCG in cell lines, animal models, and clinical observations are explained.

PPARalpha activation reverses adverse effects induced by high-saturated-fat feeding on pancreatic beta-cell function in late pregnancy

We examined whether the additional demand for insulin secretion imposed by dietary saturated fat-induced insulin resistance during pregnancy is accommodated at late pregnancy, already characterized by insulin resistance. We also assessed whether effects of dietary saturated fat are influenced by PPARalpha activation or substitution of 7% of dietary fatty acids (FAs) with long-chain omega-3 FA, manipulations that improve insulin action in the nonpregnant state. Glucose tolerance at day 19 of pregnancy in the rat was impaired by high-saturated-fat feeding throughout pregnancy. Despite modestly enhanced glucose-stimulated insulin secretion (GSIS) in vivo, islet perifusions revealed an increased glucose threshold and decreased glucose responsiveness of GSIS in the saturated-fat-fed pregnant group. Thus, insulin resistance evoked by dietary saturated fat is partially countered by augmented insulin secretion, but compensation is compromised by impaired islet function. Substitution of 7% of saturated FA with long-chain omega-3 FA suppressed GSIS in vivo but did not modify the effect of saturated-fat feeding to impair GSIS by perifused islets. PPARalpha activation (24 h) rescued impaired islet function that was identified using perifused islets, but GSIS in vivo was suppressed such that glucose tolerance was not improved, suggesting modification of the feedback loop between insulin action and secretion.

Glycemic control of apoptosis in the pancreatic beta cell: danger of extremes?

Excessive formation of oxygen radicals is a well-established mediator of hyperglycemic damage in diabetes to a wide range of tissues, such as neurons, retinal cells, and vascular endothelium. Increased oxygen radical formation is generally considered a toxic side effect of excessive rates of mitochondrial oxidative metabolism and electron transport in high glucose-exposed cells. Along the same line, metabolic oxidative stress is currently also regarded as crucial mediator of beta cell dysfunction and apoptosis under hyperglycemic conditions. Here the authors argue that a healthy beta cell is well equipped to deal adequately with elevated glucose metabolic rates, and demonstrate that decreased glucose catabolism leads to ROS production and apoptosis. They therefore propose that adverse metabolic conditions in poorly controlled diabetes (hyperglycemia and/or dyslipidemia) or genetic defects could decrease the viability of beta cells by interfering with normal glucose sensing and metabolism, rather than by overactivating it. This view is supported by the fragmentary data currently available on the pathways for hypergycemic and hypoglycemic beta cell death.

Diet, obesity and diabetes: a current update

The prevalence of obesity has been increasing at a rapid rate over the last few decades. Although the primary defect can be attributed to an imbalance of energy intake over energy expenditure, the regulation of energy balance is now recognized to be complex. Adipose-tissue factors play a central role in the control of energy balance and whole-body fuel homoeostasis. The regulation of adipose-tissue function, in particular its secretion of adipokines, is impaired by increases in adipose mass associated with obesity, and with the development of insulin resistance and Type 2 diabetes. This review analyses adipose-regulated energy input and expenditure, together with the impact of dietary macronutrient composition on energy balance in relation to susceptibility to the development of obesity and Type 2 diabetes, and how these metabolic conditions may be exacerbated by the consequences of abnormal adipose function. By gaining a greater understanding of how energy balance is controlled in normal, and in obese and diabetic states, a more practical approach can be employed to prevent and better treat obesity and metabolic disorders.

Effects of pioglitazone on beta-cell function in metabolic syndrome patients with impaired glucose tolerance

OBJECTIVE

To determine the potential effects of pioglitazone on beta-cell function in metabolic syndrome patients with impaired glucose tolerance and probe into the possible mechanisms.

RESEARCH DESIGN AND METHODS

Twenty-two subjects were treated with pioglitazone 30 mg/day for 4 months. At baseline and after treatment, each subject underwent an IVGTT. The acute insulin response (AIRg), the glucose disappearance rates (coefficients K) and the ratio of Deltainsulin/Deltaglucose (DeltaI/DeltaG) were calculated according to IVGTT results. Hyperglycemic clamp study was conducted to determine the second-phase insulin response, insulin sensitivity index (ISI) and glucose infusion rate (GIR). Euglycemic-hyperinsulinemic clamp study was made to measure the glucose disposal rate (GDR). Plasma glucose, free fatty acids (FFAs), serum insulin and proinsulin levels were measured.

RESULTS

AIRg unchanged (P = 0.25) after treatment, whereas the values of coefficients K (P < 0.01) and DeltaI/DeltaG increased (P < 0.05). The second-phase insulin response and GIR were both demonstrated marked increments (P < 0.01 and P < 0.01, respectively). Pioglitazone therapy also resulted in improvement of ISI value (P < 0.05). And the increment of GDR during the euglycemic-hyperinsulinemic clamp was also significant (P < 0.01). Furthermore, a decrease in fasting proinsulin level was observed (P < 0.001). And plasma glucose, FFAs and serum insulin levels all declined. The increase of DeltaI1/DeltaG1 was positively correlated with the improvement of GDR (r = 0.536, P = 0.089). And a positive relationship was observed between the change in the second-phase insulin response and change in K value (r = 0.682, P = 0.021).

CONCLUSIONS

Short-term pioglitazone therapy improved beta-cell dysfunction, the mechanism might involve the attenuation of insulin resistance.

Life and death decisions of the pancreatic β-cell: the role of fatty acids

Both stimulatory and detrimental effects of NEFAs (non-esterified fatty acids) on pancreatic β-cells have been recognized. Acute exposure of the pancreatic β-cell to high glucose concentrations and/or saturated NEFAs results in a substantial increase in insulin release, whereas chronic exposure results in desensitization and suppression of secretion, followed by induction of apoptosis. Some unsaturated NEFAs also promote insulin release acutely, but they are less toxic to β-cells during chronic exposure and can even exert positive protective effects. Therefore changes in the levels of NEFAs are likely to be important for the regulation of β-cell function and viability under physiological conditions. In addition, the switching between endogenous fatty acid synthesis or oxidation in the β-cell, together with alterations in neutral lipid accumulation, may have critical implications for β-cell function and integrity. Long-chain acyl-CoA (formed from either endogenously synthesized or exogenous fatty acids) controls several aspects of β-cell function, including activation of specific isoenzymes of PKC (protein kinase C), modulation of ion channels, protein acylation, ceramide formation and/or NO-mediated apoptosis, and transcription factor activity. In this review, we describe the effects of exogenous and endogenous fatty acids on β-cell metabolism and gene and protein expression, and have explored the outcomes with respect to insulin secretion and β-cell integrity.

Appetite suppression through delayed fat digestion

High-fat diets are often associated with greater caloric intake and weight gain. Since satiety during fat intake is induced by fat in the intestine we investigated the efficiency of a lipid compound that retards fat digestion to regulate fat intake. We found this compound to reduce high-fat food intake, body weight and blood lipids in Sprague-Dawley rats, without causing steatorrhea. The absence of steatorrhea is explained by an increased pancreatic lipase/colipase secretion, compensating the impaired lipolysis by the added compound. The animals also had an elevated CCK secretion. The satiety for fat may be the consequence of elevated CCK and procolipase/enterostatin levels. We conclude that compounds can be found that delay intestinal fat digestion and control high-fat food intake through the release of satiety signals, without causing steatorrhea. The absence of steatorrhea makes such compounds advantageous over lipase inhibitors in the treatment of obesity.

Limited role for SREBP-1c in defective glucose-induced insulin secretion from Zucker diabetic fatty rat islets: a functional and gene profiling analysis

Accumulation of intracellular lipid may contribute to defective insulin secretion in type 2 diabetes. Although Zucker diabetic fatty (ZDF; fa/fa) rat islets are fat-laden and overexpress the lipogenic master gene, sterol regulatory element binding protein 1c (SREBP-1c), the contribution of SREBP-1c to the secretory defects observed in this model remains unclear. Here we compare the gene expression profile of lean control (fa/+) and ZDF rat islets in the absence or presence of dominant-negative SREBP-1c (SREBP-1c DN). ZDF islets displayed elevated basal insulin secretion at 3 mmol/l glucose but a severely depressed response to 17 mmol/l glucose. While SREBP-1c DN reduced basal insulin secretion from ZDF islets, glucose-stimulated insulin secretion was not improved. Of 57 genes differentially regulated in ZDF islets and implicated in glucose metabolism, vesicle trafficking, ion fluxes, and/or exocytosis, 21 were upregulated and 5 were suppressed by SREBP-1c DN. Genes underrepresented in ZDF islets were either unaffected (Glut-2, Kir6.2, Rab3), stimulated (voltage-dependent Ca(2+) channel subunit alpha1D, CPT2, SUR2, rab9, syt13), or inhibited (syntaxin 7, secretogranin-2) by SREBP-1c inhibition. Correspondingly, SREBP-1c DN largely corrected decreases in the expression of the transcription factors Pdx-1 and MafA but did not affect the abnormalities in Pax6, Arx, hepatic nuclear factor-1alpha (HNF1alpha), HNF3beta/Forkhead box-a2 (Foxa2), inducible cyclic AMP early repressor (ICER), or transcription factor 7-like 2 (TCF7L2) expression observed in ZDF islets. We conclude that upregulation of SREBP-1c and mild increases in triglyceride content do not explain defective glucose-stimulated insulin secretion from ZDF rats. However, overexpression of SREBP-1c may contribute to enhanced basal insulin secretion in this model.

Down-regulation of PPARgamma2-induced adipogenesis by PEGylated conjugated linoleic acid as the pro-drug: Attenuation of lipid accumulation and reduction of apoptosis

This study is designed to evaluate whether the PEGylated conjugated linoleic acid (PCLA) as the pro-drug can have favorable stability, bioavailability, and anti-adipogenic activity in 3T3-L1 cells for anti-obesity when compared with conjugated linoleic acid (CLA) itself. The CLA was simply coupled to poly(ethylene glycol) (PEG) at the melting state without solvents or catalysts through ester linkages between the carboxylic group of CLA and the hydroxyl group of PEG. To confirm of PCLA as the pro-drug, CLA release from PCLA was investigated by using high-performance liquid chromatographic (HPLC), showing that CLA release from PCLA was almost 90% in a nearly continuous fashion over the next 75h. Apoptosis was promoted by both CLA- and PCLA-treatments with increasing concentrations. However, the level of cell apoptosis induced by PCLA was lower than that induced by CLA owing to the biocompatible and hydrophilic properties of PEG. Moreover, the PCLA decreased glycerol-3-phosphate dehydrogenase (GPDH) activity in 3T3-L1 cells by acting upon major adipocyte marker proteins such as PPARgamma2, C/EBPalpha, and aP2 modulators. Furthermore, either CLA or PCLA stimulated basal, but not isoproterenol-sensitive, lipolysis in our cell model, suggesting that both CLA and PCLA may stimulate lipolysis via hormone sensitive lipase (HSL)-independent mechanisms. These results suggest that the PCLA may prove to be a stable pro-drug to control the deposition of fat in the human body, and that the anti-adipogenic effect of the PCLA on 3T3-L1 cells will offer a challenging approach for anti-obesity.

Membrane topology and identification of key functional amino acid residues of murine acyl-CoA:diacylglycerol acyltransferase-2

Triacylglycerols are the predominant molecules of energy storage in eukaryotes. However, excessive accumulation of triacylglycerols in adipose tissue leads to obesity and, in nonadipose tissues, is associated with tissue dysfunction. Hence, it is of great importance to have a better understanding of the molecular mechanisms of triacylglycerol synthesis. The final step in triacylglycerol synthesis is catalyzed by the acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2. Although recent studies have shed light on metabolic functions of these enzymes, little is known about the molecular aspects of their structures or functions. Here we report the topology for murine DGAT2 and the identification of key amino acids that likely contribute to enzymatic function. Our data indicate that DGAT2 is an integral membrane protein with both the N and C termini oriented toward the cytosol. A long hydrophobic region spanning amino acids 66-115 likely comprises two transmembrane domains or, alternatively, a single domain that is embedded in the membrane bilayer. The bulk of the protein lies distal to the transmembrane domains. This region shares the highest degree of homology with other enzymes of the DGAT2 family and contains a sequence HPHG that is conserved in all family members. Mutagenesis of this sequence in DGAT2 demonstrated that it is required for full enzymatic function. Additionally, a neutral lipid-binding domain that is located in the putative first transmembrane domain was also required for full enzymatic function. Our findings provide the first insights into the topography and molecular aspects of DGAT2 and related enzymes.

Islet beta cell failure in type 2 diabetes

The major focus of this Review is on the mechanisms of islet beta cell failure in the pathogenesis of obesity-associated type 2 diabetes (T2D). As this demise occurs within the context of beta cell compensation for insulin resistance, consideration is also given to the mechanisms involved in the compensation process, including mechanisms for expansion of beta cell mass and for enhanced beta cell performance. The importance of genetic, intrauterine, and environmental factors in the determination of "susceptible" islets and overall risk for T2D is reviewed. The likely mechanisms of beta cell failure are discussed within the two broad categories: those with initiation and those with progression roles.

Metformin influences cardiomyocyte cell death by pathways that are dependent and independent of caspase-3

AIMS/HYPOTHESIS

Metformin has been shown to increase fatty acid oxidation, an effect mediated by AMP activated protein kinase (AMPK). We hypothesised that metformin could prevent both caspase-3 activation and apoptosis when induced by palmitic acid.

MATERIALS AND METHODS

Cardiomyocytes were incubated with 1 mmol/l palmitic acid, in the absence or presence of metformin (1-5 mmol/l). Following 1 to 16 h, cell damage was evaluated by measuring lactate dehydrogenase released into the incubation medium, and Hoechst staining. To investigate the mechanism of metformin's effect on cardiomyocytes, substrate utilisation and phosphorylation of AMPK and acetyl-CoA carboxylase were measured. Intracellular mediators of apoptosis were also evaluated.

RESULTS

Incubation of myocytes with palmitic acid for 16 h increased apoptosis, an effect that was partly blunted by 1 and 2 mmol/l metformin. This beneficial effect of metformin was associated with increased AMPK phosphorylation, palmitic acid oxidation and suppression of high-fat-induced increases in (1) long chain base biosynthesis protein 1 levels, (2) ceramide levels, and (3) caspase-3 activity. Unexpectedly, 5 mmol/l metformin dramatically increased apoptosis in myocytes incubated with high fat. This effect was associated with a robust increase in glycolysis, lactate accumulation, and a significant drop of pH in the myocyte incubation medium.

CONCLUSIONS/INTERPRETATION

Our study demonstrates that metformin reduces high-fat-induced cardiac cell death, probably through inhibition of ceramide synthesis. However, at high concentrations, metformin causes proton and lactate accumulation, leading to cell damage that is independent of caspase-3.

Hypercytolipidemia-induced cellular lipoapoptosis: Cytostructural and endometabolic basis of progressive organo-involution following expression of diabetes (db/db) and obese (ob/ob) mutation syndromes

Onset expression of Type 2 (NIDDM) diabetes and obesity metabolic syndromes (DOS) are characterized by premature, progressive cytoatrophy and organo-involution induced by dysregulated cellular gluco- and lipo-metabolic cascades. The consequential systemic, interstitial and intracellular hyperlipidemia disrupts normal cytointegrity and metabolic responsivity to the established hypercaloric pericellular environments. The sequential cytostructural, metabolic and endocrine disturbances associated with the development of progressive DOS-associated hypercytolipidemia compromises cellular metabolic response cascades and promotes cytochemical disturbances which culminate with nuclear lipoapoptosis and cytoatrophy. The dramatic alterations in interstitial glucose and lipid (free fatty acids/triglycerides) concentrations are recognized to influence interstitial and cytoplasmic microchemical environments, which markedly alter cellular nutrient diffusion and active trans-membrane flux rates. The progressive exacerbation of interstitial and cytoplasmic lipid imbibition has been demonstrated to be associated with DNA fragmentation by lipo-infiltration into the chromatin matrix, inducing structural disruption and physical dissolution, indexed as nuclear lipoapoptosis. Therapeutic reduction of the severity of hypercytolipidemia-induced structural and cytochemical compromise promotes the restoration of homeostatic metabolic support for normalized cytostructural indices and supportive cellular gluco- and lipo-metabolic cascades. The re-establishment of a homeostatic interstitial microenvironment moderates the severity of cytolipidemic compromise within affected cell types, reduces nuclear lipo-infiltration and DNA lipo-dissolution, resulting in the preservation of cytostructural integrity. Through the therapeutic restoration of extra- and intra-cellular microchemical environments in genetically dysregulated metabolic syndrome models, the coincident cytochemical, endocrine and metabolic disturbances associated with progressive hypercytolipidemia, resulting from the expressed systemic hypercaloric environmental and hepato-pancreatic endometabolic disturbances which characterize Type 2 (NIDDM) diabetes-obesity and metabolic (X) syndromes, may be ameliorated.

Regulation of lipid metabolism by soy protein and its implication in diseases mediated by lipid disorders

Soybeans have a high-quality protein that has been consumed for approximately 5000 years in Oriental countries. The awareness that soy products are healthy has increased their consumption in Western countries. Substantial data from epidemiological surveys and nutritional interventions in humans and animals indicate that soy protein reduces serum total and low-density lipoprotein (LDL) cholesterol and triglycerides as well as hepatic cholesterol and triglycerides. This review examines the evidence on the possible mechanisms for which soy protein has beneficial effects in diabetes, obesity and some forms of chronic renal disease. Consumption of soy protein due to low methionine content reduces serum homocysteine concentration, decreasing the risk of acquiring a cardiovascular disease. On the other hand, soy protein reduces the insulin/glucagon ratio, which in turn down-regulates the expression of the hepatic transcription factor sterol regulatory element binding protein (SREBP)-1. The reduction of this factor decreases the expression of several lipogenic enzymes, decreasing in this way serum and hepatic triglycerides as well as LDL cholesterol and very LDL triglycerides in diabetes and obesity, reducing lipotoxicity in the liver. Soy protein intake also reduces hepatic lipotoxicity by maintaining the number of functional adipocytes, preventing the transfer of fatty acids to extra adipose tissues. Furthermore, soy protein isoflavones stimulate the transcription factor SREBP-2, increasing serum cholesterol clearance. The reduction of serum cholesterol and triglyceride concentrations by soy protein intake produces beneficial effects in the kidney preventing the inflammatory response, increasing the renal flow by releasing endothelial nitric oxide (NO) synthase from the caveolae, facilitating the synthesis of NO. Thus, soy protein consumption may reduce the clinical and biochemical abnormalities in diseases mediated by lipid disorders.

Leptin-induced suppression of cardiomyocyte contraction is amplified by ceramide

Uncorrected obesity is often accompanied by ventricular contractile dysfunction, elevation of the lipotoxic mediator ceramide and the obesity gene product leptin. Both ceramide and leptin participate in the regulation of cardiac function and are speculated to play roles in obesity-related cardiac dysfunctions. The purpose of this study was to examine the effect of ceramide on leptin-elicited cardiac contractile response. Adult rat left ventricular myocytes were incubated for 24 h with low (5 nM) or high (50 nM) concentration of leptin in the absence or presence of the active ceramide analog C2-dihydroceramide (25 microM). Contractile and intracellular Ca2+ properties were evaluated using an IonOptix MyoCam system including peak shortening (PS), maximal velocity of shortening/relengthening (+/-dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR90), intracellular Ca2+ rise (Delta[Ca2+]) and intracellular Ca2+ decay. While ceramide did not elicit any effect on cell mechanics and intracellular Ca2+ transients, it sensitized leptin-induced effects on myocyte shortening and intracellular Ca2+ transients. In the absence of ceramide, 5 nM leptin had no effect on cell mechanics while 50 nM depressed PS, +/-dL/dt, Delta[Ca2+] and prolonged TR90. With ceramide co-incubation, 5 nM leptin depressed PS, +/-dL/dt, Delta[Ca2+] and prolonged TR90 whereas 50 nM leptin-elicited effects on PS, +/-dL/dt, Delta[Ca2+] and TR90 were significantly potentiated in addition to slowing intracellular Ca2+ decay. In summary, our data demonstrated that ceramide sensitizes cardiac depressive effects of leptin and may contribute to hyperleptinemia-related cardiac contractile dysfunction.

Insulin resistance--a common link between type 2 diabetes and cardiovascular disease

Evidence suggests that diabetes and cardiovascular disease (CVD) may share an underlying cause(s), a theory known as the 'common soil' hypothesis. Insulin resistance is central both to the progression from normal glucose tolerance to type 2 diabetes and to a constellation of cardiovascular risk factors known as the metabolic syndrome. These risk factors include visceral obesity and dyslipidaemia characterized by low levels of high-density lipoprotein cholesterol, hypertriglyceridaemia and raised small dense low-density lipoprotein particle levels. Changes in adipose tissue mass and metabolism may link insulin resistance and visceral obesity, a condition that is common in type 2 diabetes. Furthermore, weight reduction, increased physical activity, metformin and acarbose have been shown to reduce the development of type 2 diabetes in genetically predisposed subjects and may decrease the high cardiovascular risk of patients with diabetes. Some fatty acid derivatives can affect energy metabolism by activating peroxisome proliferator-activated receptors (PPARs), nuclear receptors that play a key role in energy homeostasis. These receptors represent an ideal therapeutic target for reducing cardiovascular risk, because they are involved in the regulation of both insulin action and lipid metabolism. In addition to lifestyle changes, PPARgamma agonists such as thiazolidinediones are frequently beneficial and have been shown to ameliorate insulin resistance, while activation of PPARalpha (e.g. by fibrates) can lead to improvements in free fatty acid oxidation and lipid profile, and a reduction in cardiovascular events. The development of agents with both PPARalpha and PPARgamma activity promises added benefits with amelioration of insulin resistance, delayed progression to and of type 2 diabetes and a reduction of CVD.

The Genetic Basis of Type 2 Diabetes

Type 2 Diabetes results from a complex physiologic process that includes the pancreatic beta cells, peripheral glucose uptake in muscle, the secretion of multiple cytokines and hormone-like molecules from adipocytes, hepatic glucose production, and likely the central nervous system. Consistent with the complex web of physiologic defects, the emerging picture of the genetics will involve a large number of risk susceptibility genes, each individually with relatively small effect (odds ratios below 1.2 in most cases). The challenge for the future will include cataloging and confirming the genetic risk factors, and understanding how these risk factors interact with each other and with the known environmental and lifestyle risk factors that increase the propensity to type 2 diabetes.

Unique ultrastructure of exorbital lacrimal glands in male NOD and BALB/c mice

Lacrimal glands of male NOD and BALB/c mice have very small, pleomorphic acinar lumens. Acini contain isolated zones of highly complex cell surface interdigitations at the basal surface, sometimes occurring between acinar and myoepithelial cells. In NOD mice, cytological abnormalities, including mitochondrial deterioration, pleomorphic and heterogeneous cytoplasmic vacuoles, and lipid accumulation are evident within acinar cells at 1 month. Accumulation of lipid is further increased as the animal ages, accompanied by lymphocytic infiltration and destruction of acini. These results demonstrate alterations from normal cytology as early as 1 month in NOD mice, well before detection of clinical signs of Sjögren syndrome.

Influences of obese (ob/ob) and diabetes (db/db) genotype mutations on lumber vertebral radiological and morphometric indices: skeletal deformation associated with dysregulated systemic glucometabolism

Background

Both diabetes and obesity syndromes are recognized to promote lumbar vertebral instability, premature osteodegeneration, exacerbate progressive osteoporosis and increase the propensity towards vertebral degeneration, instability and deformation in humans.

Methods

The influences of single-gene missense mutations, expressing either diabetes (db/db) or obese (ob/ob) metabolic syndromes on vertebral maturation and development in C57BL/KsJ mice were evaluated by radiological and macro-morphometric analysis of the resulting variances in osteodevelopment indices relative to control parameters between 8 and 16 weeks of age (syndrome onset @ 4 weeks), and the influences of low-dose 17-B-estradiol therapy on vertebral growth expression evaluated.

Results

Associated with the indicative genotypic obesity and hyper-glycemic/-insulinemic states, both db/db and ob/ob mutants demonstrated a significant (P ≤ 0.05) elongation of total lumbar vertebrae column (VC) regional length, and individual lumbar vertebrae (LV1-5) lengths, relative to control VC and LV parameters. In contrast, LV1-5 width indices were suppressed in db/db and ob/ob mutants relative to control LV growth rates. Between 8 and 16 weeks of age, the suppressed LV1-5 width indices were sustained in both genotype mutant groups relative to control osteomaturation rates. The severity of LV1-5 width osteosuppression correlated with the severe systemic hyperglycemic and hypertriglyceridemic conditions sustained in ob/ob and db/db mutants. Low-dose 17-B-estradiol therapy (E2-HRx: 1.0 ug/ 0.1 ml oil s.c/3.5 days), initiated at 4 weeks of age (i.e., initial onset phase of db/db and ob/ob expressions) re-established control LV 1–5 width indices without influencing VC or LV lengths in db/db groups.

Conclusion

These data demonstrate that the abnormal systemic endometabolic states associated with the expression of db/db and ob/ob genomutation syndromes suppress LV 1–5 width osteomaturation rates, but enhanced development related VC and LV length expression, relative to control indices in a progressive manner similar to recognized human metabolic syndrome conditions. Therapeutic E2 modulation of the hyperglycemic component of diabetes-obesity syndrome protected the regional LV from the mutation-induced osteopenic width-growth suppression. These data suggest that these genotype mutation models may prove valuable for the evaluation of therapeutic methodologies suitable for the treatment of human diabetes- or obesity-influenced, LV degeneration-linked human conditions, which demonstrate amelioration from conventional replacement therapies following diagnosis of systemic syndrome-induced LV osteomaturation-associated deformations.

Insulin secretion after dietary supplementation with conjugated linoleic acids and n-3 polyunsaturated fatty acids in normal and insulin-resistant mice

Conjugated linoleic acids (CLAs) and n-3 polyunsaturated fatty acids (PUFAs) improve insulin sensitivity in insulin-resistant rodents. However, the effects of these fatty acids on insulin secretion are not known but are of importance to completely understand their influence on glucose homeostasis. We therefore examined islet function after dietary supplementation consisting of 1% CLAs in combination with 1% n-3 enriched PUFAs for 12 wk to mice on a normal diet and to insulin-resistant mice fed a high-fat diet (58% fat). In the mice fed a normal diet, CLA/PUFA supplementation resulted in insulin resistance associated with low plasma adiponectin levels and low body fat content. Intravenous and oral glucose tolerance tests revealed a marked increase in insulin secretion, which nevertheless was insufficient to counteract the insulin resistance, resulting in glucose intolerance. In freshly isolated islets from mice fed the normal diet, both basal and glucose-stimulated insulin secretion were adaptively augmented by CLA/PUFA, and at a high glucose concentration this was accompanied by elevated glucose oxidation. In contrast, in high-fat-fed mice, CLA/PUFA did not significantly affect insulin secretion, insulin resistance, or glucose tolerance. It is concluded that dietary supplementation of CLA/PUFA in mice fed the normal diet augments insulin secretion, partly because of increased islet glucose oxidation, but that this augmentation is insufficient to counterbalance the induction of insulin resistance, resulting in glucose intolerance. Furthermore, the high-fat diet partly prevents the deleterious effects of CLA/PUFA, but this dietary supplementation was not able to counteract high-fat-diet-induced insulin resistance.

Genetic and genomic studies of the BTBR ob/ob mouse model of type 2 diabetes

The BTBR mouse strain harbors alleles promoting insulin resistance. When made genetically obese (ob/ob), these mice develop severe type 2 diabetes (fasting glucose >400 mg/dL). By contrast, C57BL/6 ob/ob mice are able to compensate for the obesity-induced insulin resistance by increasing pancreatic insulin secretion and thus maintain only slightly elevated plasma glucose levels (<250 mg/dL). Islet insulin secretory responses to glucose are undiminished in the remaining islets of BTBR ob/ob mice. A genome-wide linkage analysis identified 3 major loci influencing plasma glucose and/or insulin levels in an F2ob/ob sample derived from the 2 strains. A locus on chromosome 2 affects insulin sensitivity and is independent of obesity. Loci on chromosomes 16 and 19 affect fasting glucose and insulin levels and likely affect beta-cell mass or function. Analysis of mRNA expression patterns revealed a reduction in lipogenic gene expression in adipose tissue associated with obesity. Conversely, hepatic lipogenic gene expression increases in obese mice, but to a much greater extent in the diabetes-resistant C57BL/6 strain. We propose that hepatic lipogenic capacity affects susceptibility to obesity-induced diabetes.

New Insights into Fatty Acid Modulation of Pancreatic β‐Cell Function

Insulin resistance states as found in type 2 diabetes and obesity are frequently associated with hyperlipidemia. Both stimulatory and detrimental effects of free fatty acids (FFA) on pancreatic beta cells have long been recognized. Acute exposure of the pancreatic beta cell to both high glucose concentrations and saturated FFA results in a substantial increase of insulin release, whereas a chronic exposure results in desensitization and suppression of secretion. Reduction of plasma FFA levels in fasted rats or humans severely impairs glucose-induced insulin release but palmitate can augment insulin release in the presence of nonstimulatory concentrations of glucose. These results imply that changes in physiological plasma levels of FFA are important for regulation of beta-cell function. Although it is widely accepted that fatty acid (FA) metabolism (notably FA synthesis and/or formation of LC-acyl-CoA) is necessary for stimulation of insulin secretion, the key regulatory molecular mechanisms controlling the interplay between glucose and fatty acid metabolism and thus insulin secretion are not well understood but are now described in detail in this review. Indeed the correct control of switching between FA synthesis or oxidation may have critical implications for beta-cell function and integrity both in vivo and in vitro. LC-acyl-CoA (formed from either endogenously synthesized or exogenous FA) controls several aspects of beta-cell function including activation of certain types of PKC, modulation of ion channels, protein acylation, ceramide- and/or NO-mediated apoptosis, and binding to and activating nuclear transcriptional factors. The present review also describes the possible effects of FAs on insulin signaling. We have previously reported that acute exposure of islets to palmitate up-regulates some key components of the intracellular insulin signaling pathway in pancreatic islets. Another aspect considered in this review is the potential source of fatty acids for pancreatic islets in addition to supply in the blood. Lipids can be transferred from leukocytes (macrophages) to pancreatic islets in coculture. This latter process may provide an additional source of FAs that may play a significant role in the regulation of insulin secretion.

Alpha-lipoic acid prevents endothelial dysfunction in obese rats via activation of AMP-activated protein kinase

OBJECTIVE

Lipid accumulation in vascular endothelial cells may play an important role in the pathogenesis of atherosclerosis in obese subjects. We showed previously that alpha-lipoic acid (ALA) activates AMP-activated protein kinase (AMPK) and reduces lipid accumulation in skeletal muscle of obese rats. Here, we investigated whether ALA improves endothelial dysfunction in obese rats by activating AMPK in endothelial cells.

METHODS AND RESULTS

Endothelium-dependent vascular relaxation was impaired, and the number of apoptotic endothelial cells was higher in the aorta of obese rats compared with control rats. In addition, triglyceride and lipid peroxide levels were higher, and NO synthesis was lower. Administration of ALA improved all of these abnormalities. AMPK activity was lower in aortic endothelium of obese rats, and ALA normalized it. Incubation of human aortic endothelial cells with ALA activated AMPK and protected cells from linoleic acid-induced apoptosis. Dominant-negative AMPK inhibited the antiapoptotic effects of ALA.

CONCLUSIONS

Reduced AMPK activation may play an important role in the genesis of endothelial dysfunction in obese rats. ALA improves vascular dysfunction by normalizing lipid metabolism and activating AMPK in endothelial cells.

Gonadal steroid modulation of the diabetes (db/db) mutation-induced hyperlipometabolic, hypogonadal syndrome: Restoration of female reproductive tract cytochemical and structural indices

The gonadal steroids, 17-B-estradiol (E2) and progesterone (P), are recognized to stimulate cellular gluco- and lipo-metabolic compensatory cascades which counteract the deleterious influences of the diabetes (db/db) mutation (i.e., leptin membrane receptor defect) which promotes a progressive, hypercytolipidemia-induced premature involution of the female reproductive tract (FRT). The current studies define the therapeutic efficacy of E2 (1 microg/3.5 days) and P (1 mg/3.5 days) treatments (HRx) on utero-ovarian structural and cytochemical (gluco-/lipo-metabolic) maintenance, and the prevention of premature nuclear apoptosis and cytostructural disruption, following the expression of progressive db/db-induced hypercytolipidemia. Control (normal: +/+ and +/?) and diabetes (db/db) genotype groups of 8-week-old (i.e., overt phase of the db/db-hypogonadal syndrome) C57BL/KsJ mice were prepared for high resolution (HRLM) cytochemical and transmission electron (TEM) microscopic analysis of cytolipidemia and nuclear apoptosis (TUNEL-labeled 3'-DNA fragmentation) indices from uterine and ovarian secondary (early antral) follicular tissue samples. Compared to HRx controls, the db/db mutation induced a dramatic increase in cytolipid vacuole volume and density within all ovarian follicular granulosa cells (GC) and uterine endometrial epithelial (UEE) layers. The co-localization of nuclear apoptotic 3'-DNA fragments within identified hyperlipidemic granulosa cells was coincident with the cytochemical and ultrastructural identification of lipid penetration through the nuclear envelope in db/db mutants. P-HRx moderated the severity of db/db-induced GC and UEE hypercytolipidemia, reducing the cytodensity of lipid vacuole accumulations and maintaining cytoplasmic organelle structure, organization, and nuclear membrane integrity. In contrast, E2-HRx resulted in a dramatic reduction in db/db cytolipidemia in both ovarian GC and UEE tissue compartments. Following E2-HRx, UEE cells demonstrated non-pycnotic nuclear profiles, reduced nuclear apoptosis TUNEL-labeling, increased cytoplasmic organelle density profiles and a pronounced cytoplasmic cisternal expansion indicative of active cellular nutrient/metabolite trafficking. Ovarian follicular GC populations demonstrated minimal cytolipidemia, a restored cytoarchitecture with prominent organelle compartments and reduced TUNEL-indexed nuclear lipoapoptosis. These results are the first cytochemical and ultrastructural indications that P- and E2-HRx compensate for the genetic db/db mutation-induced metabolic disturbances, which promote utero-ovarian hypercytolipidemia and the coincident nuclear lipoapoptosis culminating in the expressed diabetes hypogonadal syndrome. The capability of P-HRx to moderate the severity of utero-ovarian involution in db/db mutants, and of E2-HRx to restore and maintain viable GC and UEE cyto-chemical and -structural indices under normoglycemic conditions, suggests that chronic, low-dose cyclic P- and E2-HRx stimulate cellular gluco- and lipo-metabolic cascades which compensate for the lack of leptin signaling in these single-gene, obese-Type II diabetic mutants. The compensatory endometabolic maintenance of utero-ovarian cellular and nuclear architecture suggests that the gluco- and lipo-metabolic disregulation may be therapeutically prevented or reversed, restoring reproductive tract cytointegrity and function, reducing the manifestation of hypogonadal reproductive sterility and db/db compromise of the female reproductive tract.

Glucose-induced lipogenesis in pancreatic beta-cells is dependent on SREBP-1

High concentrations of glucose induce de novo fatty acid synthesis in pancreatic beta-cells and chronic exposure of elevated glucose and fatty acids synergize to induce accumulation of triglycerides, a phenomenon termed glucolipotoxicity. Here we investigate the role of sterol-regulatory element binding proteins in glucose-induced lipogenesis in the pancreatic beta-cell line INS-1E. We show that glucose induces SREBP-1c expression and SREBP-1 activity independent of insulin secretion and signaling. Using adenoviral expression of SREBP-1c and a SREBP-mutant we show that lipogenic gene expression, de novo fatty acid synthesis and lipid accumulation are induced primarily through sterol-regulatory elements (SREs) and not E-Boxes. Adenoviral expression of a dominant negative SREBP compromises glucose induction of some lipogenic genes and significantly reduces glucose-induction of de novo fatty acid synthesis. Thus, we demonstrate for the first time that SREBP activity is necessary for full glucose induction of de novo fatty acid synthesis in pancreatic beta-cells.

ER stress and SREBP-1 activation are implicated in beta-cell glucolipotoxicity

The reduction in insulin secretory capacity and beta-cell mass observed in type 2 diabetes is thought to be caused by glucolipotoxicity secondary to hyperglycemia and hyperlipidemia. Our aim in this study was to elucidate the underlying molecular mechanisms. We found a strong correlation between chronic high-glucose treatment and SREBP-1c activation in INS-1 cells and rat islets. Both high-glucose treatment and SREBP-1c activation in INS-1 cells resulted in lipid accumulation, impaired glucose-stimulated insulin secretion, apoptosis, and strikingly similar gene expression patterns, including upregulation of lipogenic and pro-apoptotic genes and downregulation of IRS2, Bclxl and Pdx1. These lipotoxic effects of high glucose were largely prevented by induction of a dominant-negative mutant of SREBP-1c, suggesting SREBP-1c is a major factor responsible for beta cell glucolipotoxicity. Moreover, overexpression of another lipogenic transcription factor, ChREBP, in INS-1 cells did not cause lipotoxicity. Intriguingly, chronic high glucose treatment in INS-1 cells led to pronounced induction of the ER stress marker genes, BIP and Chop10. Treatment of rat islets with both chronic high glucose and two ER stress inducers, thapsigargin and tunicamycin, enhanced SREBP-1 binding to the human IRS2 promoter. These results suggest that SREBP-1 activation caused by ER stress is implicated in beta-cell glucolipotoxicity.

Alpha-lipoic acid increases insulin sensitivity by activating AMPK in skeletal muscle

Triglyceride accumulation in skeletal muscle contributes to insulin resistance in obesity. We recently showed that alpha-lipoic acid (ALA) reduces body weight and prevents the development of diabetes in diabetes-prone obese rats by reducing triglyceride accumulation in non-adipose tissues. AMP-activated protein kinase (AMPK) is a major regulator of cellular energy metabolism. We examined whether ALA lowers triglyceride accumulation in skeletal muscle by activating AMPK. Alpha2-AMPK activity was decreased in obese rats compared to control rats. Administration of ALA to obese rats increased insulin-stimulated glucose disposal in whole body and in skeletal muscle. ALA also increased fatty acid oxidation and activated AMPK in skeletal muscle. Adenovirus-mediated administration of dominant negative AMPK into skeletal muscle prevented the ALA-induced increases in fatty acid oxidation and insulin-stimulated glucose uptake. These results suggest that ALA-induced improvement of insulin sensitivity is mediated by activation of AMPK and reduced triglyceride accumulation in skeletal muscle.

Cytochemical analysis of pancreatic islet lipoapoptosis: hyperlipidemia-induced cytoinvolution following expression of the diabetes (db/db) mutation

The diabetes (db/db) genotype mutation induces a hyperglycemic-hyperinsulinemic endometabolic state in C57BL/KsJ mice, manifesting a type II NIDDM diabetes-obesity syndrome (DOS) associated with intrinsic leptin receptor expression defects. The severity of the DOS-induced premature pancreatic dysfunction and cytoatrophic involution has been linked to the severity of hypercytolipidemia which develops in pancreatic islets following systemic lipoidosis. The current studies define the cytochemical changes associated with pancreatic islet and acinar vesicular degranulation (deproteinization), cytoinvolution and B-cell dysfunction relative to the onset of cellular (nuclear DNA fragmentation) apoptosis in 20- to 26-week-old chronic db/db mutants relative to control (+/?) indices. The db/db mutation induced dramatic increases in body weights, blood glucose as well as serum and tissue triglyceride concentrations relative to +/? parameters. In contrast, pancreatic tissue weights and insulin concentrations were significantly decreased in db/db groups in association with premature islet cytoatrophy relative to +/? indices. Concurrent elevations in db/db tissue triglyceride concentrations and islet cytolipid depositions accompanied the progressive pancreatic cytoatrophic alterations. Diminished B-cell vesicular (insulin) granulation was pronounced in atrophic pancreatic islets, which were also characterized by hyperplasic acinar cellular intrusion and subsequent proteolytic B-cell dissolution coincident with 3'-DNA fragmentation-indexed (TUNEL-labeled) nuclear apoptosis. The chronic expression of the db/db mutation exacerbated these pancreatic islet B-cell atrophy indices, characterized by insulin vesicular degranulation, suppressed systemic insulin concentrations, invasive hypercytolipidemia, progressive cellular atrophy and hyperplasic acinar proteolytic dissolution, culminating in islet volume/mass reduction and chronic db/db-related pancreatic involution. The results of these studies indicate that pancreatic islet B-cell apoptosis is coincident with the progressive hypercytolipidemia component of the type II DOS promoted by the db/db genotypic mutation. These data suggest that the severity of progressive pancreatic lipoapoptosis disrupts regulatory cellular metabolic cascades, resulting in nuclear fragmentation, organelle dissolution and the subsequent promotion of a nonhomeostatic cytochemical milieu which ultimately renders islet B-cell populations susceptible to acinar proteolytic dissolution and progressive pancreatic involution.

Palmitate-induced apoptosis in cultured bovine retinal pericytes: roles of NAD(P)H oxidase, oxidant stress, and ceramide

Apoptosis of pericytes (PCs) is an early event in diabetic retinopathy. It is generally thought to be a consequence of sustained hyperglycemia. In keeping with this, long-term (>7 days) incubation of cultured PCs in a high-glucose media has been shown to increase apoptosis. We examine here whether the saturated free fatty acid palmitate, the concentration of which is often elevated in diabetes, has similar effects on cultured PCs. Incubation with 0.4 mmol/l palmitate for 24 h induced both oxidant stress and apoptosis, as evidenced by a sixfold increase in DCF fluorescence and a twofold increase in caspase-3 activation, respectively. NAD(P)H oxidase appeared to be involved in these responses, since overexpression of dominant-negative subunits of NAD(P)H oxidase, such as phox47(DN), diminished oxidant stress, and phox67(DN) and N-17 RAC1(DN) prevented the increase in caspase-3 activity. Likewise, overexpression of vRAC, a constitutively active RAC1, increased caspase-3 activity to the same extent as palmitate alone. The effects of vRAC and palmitate were not additive. In parallel with the increases in oxidative stress, the redox-sensitive transcription factor nuclear factor-kappaB (NF-kappaB) was activated in cells incubated with 0.4 mmol/l palmitate. Furthermore, inhibition of NF-kappaB activation by various means inhibited caspase-3 activation. Finally, incubation with palmitate increased the cellular content of ceramide, a molecule linked to apoptosis and increases in oxidative stress and NF-kappaB activation in other cells. In keeping with such a role, in PCs both coincubation with fumonisin B1 (a ceramide synthase inhibitor) and overexpression of ceramidase I reversed the proapoptotic effect of palmitate. On the other hand, they increased rather than decreased DCF fluorescence. In conclusion, the results suggest that palmitate-induced apoptosis in PCs is associated with activation of NAD(P)H oxidase and NF-kappaB and an increase in ceramide. The precise interactions between these molecules in causing apoptosis and the importance of oxidant stress as a contributory factor remain to be determined.

Ovarian follicular lipoapoptosis: structural, cytochemical and metabolic basis of reproductive tract atrophy following expression of the hypogonadal diabetes (db/db) syndrome

The diabetes (db/db) mutation (i.e., leptin membrane receptor defect) promotes a progressive, hypercytolipidemia within ovarian follicular granulosa, thecal and interstitial layers of C57BL/KsJ mice which manifests an infertile, acyclic hypogonadal syndrome. The current studies focus on the structural, cytochemical and gluco-/lipo-metabolic changes which induce cellular lipoapoptosis and the resulting cytostructural disruption of db/db follicular populations, relative to littermate control indices, following the expression of progressive ovarian hypercytolipidemia. Control (normal: +/+ and +/?) and diabetes (db/db) genotype groups were prepared for high resolution light microscopic (HRLM) analysis of cytolipidemia and nuclear apoptosis (TUNEL-labeled 3'-DNA fragmentation) indices and compared to the transmission electron (TEM) microscopic analysis of ovarian follicular samples collected from 8-16-week-old groups. Compared to controls, the db/db mutation induced a dramatic increase in cytolipid vacuole volume and density within all ovarian follicular layers. TEM analysis revealed that the lipid vacuoles initially aggregated along the inner membrane compartments of affected thecal and granulosa cells in response to the interstitial and vaso-lipidemic-hyperglycemic conditions which characterized the ovarian microenvironment of db/db follicles. Progressive cytoplasmic movement of lipid pools into the perinuclear compartment of affected granulosa cells induced nuclear isolation from cytoplasmic organelles that were displaced towards peripheral intracellular compartments. Cytochemical analysis of lipid vacuole accumulations indicated attraction towards, and incorporation within, the nuclear envelope of hyperlipidemic cells. Co-localization of nuclear apoptotic 3'-DNA fragments within identified hyperlipidemic granulosa cells was coincident with the cytochemical and ultrastructural identification of lipid penetration through the nuclear envelope in db/db mutants. These results are the first cytochemical evidence that the lipometabolic disturbances in db/db mutants, which promote hypercytolipidemia-induced premature ovarian involution, are coincident with lipoapoptosis-induced nuclear dissolution within follicular granulosa layers. The lipidemia-induced alterations in cellular and nuclear architecture suggests that the disturbances in glucose and lipid metabolic cascade activities in diabetes (db/db) mutants disrupts follicular cytointegrity, culminating in nuclear disregulation (as indicated by lipoapoptosis) which results in premature reproductive tract organo-involution and manifest reproductive sterility.