Acute enhancement of insulin secretion by FFA in humans is lost with prolonged FFA elevation

Prolonged Treatment with Free Fatty Acids has Post Receptor Effect in Hepatic Insulin Resistance: Evidence that Fatty Acids, Oleate and Palmitate have Insignificant Effect on the Insulin Receptor Beta In Vivo and Ex Vivo primary Hepatocytes

In the current study, we used immunoprecipitation and immunoblotting to examine the levels and phosphorylation status of the insulin receptor-beta subunit (IR-β), as well as the down stream target in PI3K pathway, total PKB/Akt as well as their phosphorylated forms. The assessment of FFAs treatment showed no direct and significant effect on the PI3K stimulation, specifically the IR-β in primary hepatic control cells treated with insulin. Cells treated with either oleate or palmitate (360 μM) showed no statistically significant values following insulin stimulation (P > 0.05). To further investigate the effect of both FFAs and high insulin (1 μg), we examined the effects of oleate and palmitate at 360 μM concentration on IR-β as well as PKB. There was no significant difference in the total protein levels and their phosphorylated forms in cells treated with or without oleate or plamitate. Interestingly, IR-β tyrosine phosphorylation showed a similar insignificant effect in vivo and ex vivo hepatic cells treated with oleate or palmitate in comparison to their controls in the fructose fed hamsters.

Lessons learned from studying families genetically predisposed to type 2 diabetes mellitus

Early interventions to prevent type 2 diabetes mellitus (T2DM) demand a better understanding of its underlying mechanisms. Nonobese healthy subjects with a strong family history of T2DM (FH(+) subjects) hold a key to this end by allowing the study of the disease before the development of confounding factors, such as obesity or hyperglycemia. In this article, we share our experience over the past decade in studying FH(+) subjects and how lipotoxicity alters glucose metabolism in such individuals, in particular pancreatic beta-cell function. FH(+) subjects have no obvious clinical abnormalities, but when carefully studied, reveal severe hepatic/muscle/adipose tissue insulin resistance and subtle defects in beta-cell function. In most subjects, metabolic adaption allows freedom from diabetes for decades. However, the obesity epidemic is drastically changing this. Given the unique susceptibility of pancreatic beta cells to free fatty acids in FH(+) subjects, interventions that protect against obesity-induced lipotoxicity may hold the greatest promise for preventing T2DM in genetically predisposed individuals.

Effects of prolonged fasting and sustained lipolysis on insulin secretion and insulin sensitivity in normal subjects

Normal beta-cells adjust their function to compensate for any decrease in insulin sensitivity. Our aim was to explore whether a prolonged fast would allow a study of the effects of changes in circulating free fatty acid (FFA) levels on insulin secretion and insulin sensitivity and whether any potential effects could be reversed by the antilipolytic agent acipimox. Fourteen (8 female, 6 male) healthy young adults (aged 22.8-26.9 yr) without a family history of diabetes and a body mass index of 22.6 +/- 3.2 kg/m(2) were studied on three occasions in random order. Growth hormone and FFA levels were regularly measured overnight (2200-0759), and subjects underwent an intravenous glucose tolerance test in the morning (0800-1100) on each visit. Treatment A was an overnight fast, treatment B was a 24-h fast with regular administrations of a placebo, and treatment C was a 24-h fast with regular ingestions of 250 mg of acipimox. The 24-h fast increased overnight FFA levels (as measured by the area under the curve) 2.8-fold [51.3 (45.6-56.9) vs. 18.4 (14.4-22.5) *10(4) micromol/l*min, P < 0.0001], and it led to decreases in insulin sensitivity [5.7 (3.6-8.9) vs. 2.6 (1.3-4.7) *10(-4) min(-1) per mU/l, P < 0.0001] and the acute insulin response [16.3 (10.9-21.6) vs. 12.7 (8.7-16.6) *10(2) pmol/l*min, P = 0.02], and therefore a reduction in the disposition index [93.1 (64.8-121.4) vs. 35.5 (21.6-49.4) *10(2) pmol/mU, P < 0.0001]. Administration of acipimox during the 24-h fast lowered FFA levels by an average of 20% (range: -62 to +49%; P = 0.03), resulting in a mean increase in the disposition index of 31% (P = 0.03). In conclusion, the 24-h fast was accompanied by substantial increases in fasting FFA levels and induced reductions in the acute glucose-simulated insulin response and insulin sensitivity. The use of acipimox during the prolonged fast increased the disposition index, suggesting a partial reversal of the effects of fasting on the acute insulin response and insulin sensitivity.

Acute administration of GPR40 receptor agonist potentiates glucose-stimulated insulin secretion in vivo in the rat

Recently, several in vitro studies have shown that GPR40 receptor activation by free fatty acids (FFAs) results in glucose-dependent insulin secretion. However, whether GPR40 receptor activation results in glucose-dependent insulin secretion in vivo in rats is not known. Therefore, we evaluated the effect of synthetic GPR40 receptor agonist (compound 1) on glucose tolerance test (GTT) in fed, fasted, and insulin-resistant rats. In oral GTT, intraperitoneal GTT, and intravenous GTT, GPR40 receptor agonist improved glucose tolerance, which was associated with increase in plasma insulin level. Interestingly, in GTTs, the rise in insulin levels in agonist-treated group was directly proportional to the rate of rise and peak levels of glucose in control group. Although glibenclamide, a widely used insulin secretagogue, improved glucose tolerance in all GTTs, it did not display insulin release in intraperitoneal GTT or intravenous GTT. In the absence of glucose load, GPR40 receptor agonist did not significantly change the plasma insulin concentration, but did decrease the plasma glucose concentration. Fasted rats exhibited impaired glucose-stimulated insulin secretion (GSIS) as compared with fed rats. Compound 1 potentiated GSIS in fasted state but failed to do so in fed state. Suspecting differential pharmacokinetics, a detailed pharmacokinetic evaluation was performed, which revealed the low plasma concentration of compound 1 in fed state. Consequently, we examined the absorption profile of compound 1 at higher doses in fed state; and at a dose at which its absorption was comparable with that in fasted state, we observed significant potentiation of GSIS. Chronic high-fructose (60%) diet feeding resulted in impaired glucose tolerance, which was improved by GPR40 receptor agonist. Therefore, our results demonstrate for the first time that acute GPR40 receptor activation leads to potentiation of GSIS in vivo and improves glucose tolerance even in insulin-resistant condition in rats. Taken together, these results suggest that GPR40 receptor agonists could be potential therapeutic alternatives to sulfonylureas.

Glucolipotoxicity of the pancreatic beta-cell: myth or reality?

The glucolipotoxicity hypothesis postulates that chronically elevated levels of glucose and fatty acids adversely affect pancreatic beta-cell function and thereby contribute to the deterioration of insulin secretion in Type 2 diabetes. Whereas ample experimental evidence in in vitro systems supports the glucolipotoxicity hypothesis, the contribution of this phenomenon to beta-cell dysfunction in human Type 2 diabetes has been questioned. The reasons for this controversy include: differences between in vitro systems and in vivo situations; time-dependent effects of fatty acids on insulin secretion (acutely stimulatory and chronically inhibitory); and the ill-defined use of the suffix '-toxicity'. In vitro, prolonged exposure of insulin-secreting cells or isolated islets to concomitantly elevated levels of fatty acids and glucose impairs insulin secretion, inhibits insulin gene expression and, under certain circumstances, induces beta-cell death by apoptosis. Recent studies in our laboratory have shown that cyclical and alternate infusions of glucose and Intralipid in rats impair insulin gene expression, providing evidence that inhibition of the insulin gene under glucolipotoxic conditions is an early defect that might indeed contribute to beta-cell failure in Type 2 diabetes, although this hypothesis remains to be tested in humans.

Berberine acutely inhibits insulin secretion from beta-cells through 3',5'-cyclic adenosine 5'-monophosphate signaling pathway

Berberine, a hypoglycemic agent, has recently been shown to activate AMP-activated protein kinase (AMPK) contributing to its beneficial metabolic effects in peripheral tissues. However, whether berberine exerts a regulatory effect on beta-cells via AMPK or other signaling pathways and counteracts glucolipotoxicity remains uncertain. In the present study, the impact of berberine on beta-cell function was investigated in vivo and in vitro. In high-fat-fed rats, berberine treatment for 6 wk significantly decreased plasma glucose and insulin levels before and after an oral glucose challenge along with the reduction of body weight and improvement of blood lipid profile. In accordance with the in vivo results, berberine acutely decreased glucose-stimulated insulin secretion (GSIS) and palmitate-potentiated insulin secretion in MIN6 cells and rat islets. However, pretreated with berberine for 24 h augmented the response of MIN6 cells and rat islets to glucose and attenuated the glucolipotoxicity. Berberine acutely increased AMPK activity in MIN6 cells. However, compound C, an AMPK inhibitor, completely reversed troglitazone-suppressed GSIS, not berberine-suppressed GSIS. Otherwise, berberine decreased cAMP-raising agent-potentiated insulin secretion in MIN6 cells and rat islets. These results suggest that the activation of AMPK is required for troglitazone-suppressed GSIS, whereas cAMP signaling pathway contributes, at least in part, to the regulatory effect of berberine on insulin secretion.

Insulin secretion in metabolically obese, but normal weight, and in metabolically healthy but obese individuals

Metabolically obese but normal-weight (MONW) individuals present metabolic disturbances typical of obese individuals. Additionally, metabolically healthy but obese (MHO) individuals have been identified who are relatively insulin sensitive and have a favorable cardiovascular risk profile. We compared insulin secretion patterns of MONW and MHO with those of two age-matched groups comprising nonobese individuals or obese insulin-resistant subjects, respectively. To this end, 110 nonobese subjects and 87 obese subjects were stratified into quartile based on their insulin-stimulated glucose disposal (M(FFM)). Insulin secretion was estimated by acute insulin response (AIR) during an intravenous glucose-tolerance test (IVGTT), and the disposition index was calculated as AIR x M(FFM). We found that, as defined, M(FFM) was lower in MONW, who exhibited higher triglycerides, free-fatty acid (FFA), and 2-h postchallenge glucose levels compared to normal nonobese group. Insulin secretion was higher in MONW than in normal nonobese subjects, but disposition index was lower in MONW. Disposition index did not differ between MONW and insulin-resistant obese. M(FFM) was higher in MHO who exhibited lower waist circumference, blood pressure (BP), triglycerides, FFA, insulin levels, and higher high-density lipoprotein (HDL) cholesterol compared to insulin-resistant obese. Insulin secretion did not differ between insulin-resistant obese and MHO, but disposition index was lower in the former group. In conclusion, MONW and insulin-resistant obese showed decreased compensatory insulin secretion compared to normal nonobese and MHO subjects, respectively. Because these subjects also exhibited a worse metabolic risk profile, these findings may account for their increased risk for type 2 diabetes.

Nutrigenomics, beta-cell function and type 2 diabetes

INTRODUCTION

The present investigation was designed to investigate the accuracy and precision of lactate measurement obtained with contemporary biosensors (Chiron Diagnostics, Nova Biomedical) and standard enzymatic photometric procedures (Sigma Diagnostics, Abbott Laboratories, Analyticon).

MATERIALS AND METHODS

Measurements were performed in vitro before and after the stepwise addition of 1 molar sodium lactate solution to samples of fresh frozen plasma to systematically achieve lactate concentrations of up to 20 mmol/l.

RESULTS

Precision of the methods investigated varied between 1% and 7%, accuracy ranged between 2% and -33% with the variability being lowest in the Sigma photometric procedure (6%) and more than 13% in both biosensor methods.

CONCLUSION

Biosensors for lactate measurement provide adequate accuracy in mean with the limitation of highly variable results. A true lactate value of 6 mmol/l was found to be presented between 4.4 and 7.6 mmol/l or even with higher difference. Biosensors and standard enzymatic photometric procedures are only limited comparable because the differences between paired determinations presented to be several mmol. The advantage of biosensors is the complete lack of preanalytical sample preparation which appeared to be the major limitation of standard photometry methods.

Glucolipotoxicity: fuel excess and beta-cell dysfunction

Glucotoxicity, lipotoxicity, and glucolipotoxicity are secondary phenomena that are proposed to play a role in all forms of type 2 diabetes. The underlying concept is that once the primary pathogenesis of diabetes is established, probably involving both genetic and environmental forces, hyperglycemia and very commonly hyperlipidemia ensue and thereafter exert additional damaging or toxic effects on the beta-cell. In addition to their contribution to the deterioration of beta-cell function after the onset of the disease, elevations of plasma fatty acid levels that often accompany insulin resistance may, as glucose levels begin to rise outside of the normal range, also play a pathogenic role in the early stages of the disease. Because hyperglycemia is a prerequisite for lipotoxicity to occur, the term glucolipotoxicity, rather than lipotoxicity, is more appropriate to describe deleterious effects of lipids on beta-cell function. In vitro and in vivo evidence supporting the concept of glucotoxicity is presented first, as well as a description of the underlying mechanisms with an emphasis on the role of oxidative stress. Second, we discuss the functional manifestations of glucolipotoxicity on insulin secretion, insulin gene expression, and beta-cell death, and the role of glucose in the mechanisms of glucolipotoxicity. Finally, we attempt to define the role of these phenomena in the natural history of beta-cell compensation, decompensation, and failure during the course of type 2 diabetes.

Impaired plasma nonesterified fatty acid tolerance is an early defect in the natural history of type 2 diabetes

CONTEXT

Abnormal plasma nonesterified fatty acid (NEFA) metabolism may play a role in the development of type 2 diabetes.

OBJECTIVES

Our objectives were to demonstrate whether there is a defect in insulin-mediated suppression of plasma NEFA appearance (RaNEFA) and oxidation (OxNEFA) during enhanced intravascular triacylglycerol lipolysis early in the natural history of type 2 diabetes, and if so, to determine whether other mechanisms than reduced insulin-mediated suppression of intracellular lipolysis are involved.

DESIGN

These are cross-sectional studies.

SETTING

The studies were performed at an academic clinical research center.

PARTICIPANTS

Nine healthy subjects with both parents with type 2 diabetes (FH+) and nine healthy subjects with no first-degree relatives with type 2 diabetes (FH-) with similar anthropometric features were included in the studies.

INTERVENTIONS

Pancreatic clamps and iv infusion of stable isotopic tracers ([1,1,2,3,3-(2)H5]-glycerol and [U-(13)C]-palmitate or [1,2-(13)C]-acetate) were performed while intravascular triacylglycerol lipolysis was simultaneously clamped by iv infusion of heparin plus Intralipid at low (fasting) and high insulin levels. Oral nicotinic acid (NA) was used to inhibit intracellular lipolysis.

MAIN OUTCOME MEASURES

RaNEFA and OxNEFA were determined.

RESULTS

During heparin plus Intralipid infusion at high plasma insulin levels, and despite similar intravascular lipolytic rates, FH+ had higher RaNEFA and OxNEFA than FH- (RaNEFA: 17.4+/-6.3 vs. 9.2+/-4.2; OxNEFA: 4.5+/-1.8 vs. 2.3+/-1.5 micromol/kg lean body mass/min), independent of NA intake, gender, age, and body composition. In the presence of NA, insulin-mediated suppression of RaNEFA was still observed in FH-, but not in FH+.

CONCLUSIONS

Increased RaNEFA and OxNEFA during intravascular lipolysis at high insulin levels occur early in the natural history of type 2 diabetes.

Oral taurine but not N-acetylcysteine ameliorates NEFA-induced impairment in insulin sensitivity and beta cell function in obese and overweight, non-diabetic men

AIMS/HYPOTHESIS

Antioxidants have been shown to ameliorate lipid-induced impairment of insulin action and beta cell function, both in vitro and in animal studies. The aim of the present study was to examine the effects of two orally administered antioxidants, N-acetylcysteine (NAC) and taurine (TAU), on lipotoxicity in humans.

METHODS

Nine non-diabetic men, who were either overweight or obese, underwent three studies each, 4-6 weeks apart, in random order: (1) i.v. infusion of saline for 48 h (SAL); (2) i.v. infusion of Intralipid and heparin for 48 h to mimic chronic elevation of plasma NEFA (IH); and (3) IH infusion for 48 h with concurrent oral NAC (IH+NAC). Six men underwent similar studies except for study 3, where instead of NAC they received a 2 week pretreatment with oral TAU (IH+TAU).

RESULTS

For both the NAC and TAU studies, a 48 h IH infusion alone without antioxidant impaired insulin sensitivity (S(I), 63% and 62% of SAL in NAC and TAU studies, respectively) and beta cell function, as evidenced by a reduction in disposition index (DI, 55% and 54% of SAL in NAC and TAU studies, respectively). NAC failed to prevent the lipid-induced increase in levels of the plasma oxidative stress marker malondialdehyde and did not prevent the lipid-induced reduction in S(I) or DI, whereas TAU completely prevented the rise in malondialdehyde and decreased 4-hydroxynonenal, and significantly improved S(I) (91% of SAL) and DI (81% of SAL).

CONCLUSIONS/INTERPRETATION

Oral TAU ameliorates lipid-induced functional beta cell decompensation and insulin resistance in humans, possibly by reducing oxidative stress.

Free fatty acid-induced reduction in glucose-stimulated insulin secretion: evidence for a role of oxidative stress in vitro and in vivo

OBJECTIVE

An important mechanism in the pathogenesis of type 2 diabetes in obese individuals is elevation of plasma free fatty acids (FFAs), which induce insulin resistance and chronically decrease beta-cell function and mass. Our objective was to investigate the role of oxidative stress in FFA-induced decrease in beta-cell function.

RESEARCH DESIGN AND METHODS

We used an in vivo model of 48-h intravenous oleate infusion in Wistar rats followed by hyperglycemic clamps or islet secretion studies ex vivo and in vitro models of 48-h exposure to oleate in islets and MIN6 cells.

RESULTS

Forty-eight-hour infusion of oleate decreased the insulin and C-peptide responses to a hyperglycemic clamp (P < 0.01), an effect prevented by coinfusion of the antioxidants N-acetylcysteine (NAC) and taurine. Similar to the findings in vivo, 48-h infusion of oleate decreased glucose-stimulated insulin secretion ex vivo (P < 0.01) and induced oxidative stress (P < 0.001) in isolated islets, effects prevented by coinfusion of the antioxidants NAC, taurine, or tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl). Forty-eight-hour infusion of olive oil induced oxidative stress (P < 0.001) and decreased the insulin response of isolated islets similar to oleate (P < 0.01). Islets exposed to oleate or palmitate and MIN6 cells exposed to oleate showed a decreased insulin response to high glucose and increased levels of oxidative stress (both P < 0.001), effects prevented by taurine. Real-time RT-PCR showed increased mRNA levels of antioxidant genes in MIN6 cells after oleate exposure, an effect partially prevented by taurine.

CONCLUSIONS

Our data are the first demonstration that oxidative stress plays a role in the decrease in beta-cell secretory function induced by prolonged exposure to FFAs in vitro and in vivo.

Type-2 diabetes related intermediate phenotypic traits in north Indian diabetics

Asian Indians are known to be at a higher risk of developing T2DM, but the underlying genetic factor in this population is still not well understood. T2DM is a complex genetic trait and assessment of disease related intermediate phenotypic traits is an important initial step towards any systematic genomic study. Therefore, in the present study we have assessed diabetes related intermediate phenotypic traits of insulin secretion and insulin resistance in the patients belonging to this population. The study included 157 T2DM patients of either sex ranging in age from 45-80 years and 84 non-diabetic subjects with no family history of diabetes, ranging in age from 45 to 75 years served as controls. Intermediate phenotypic traits studied were BMI, W: H ratio, fasting free fatty acid level and Insulin resistance and secretion. Diabetics were found to have significantly higher W: H ratio (p<0.001), FFA (p<0.001) and HOMA-R (p<0.001) as compared to non-diabetics. However, there was no significant difference in their BMI and HOMA-β. There was a positive correlation between FFA level and HOMA-R among diabetics, but not among controls. These findings suggest that in abdominal obesity FFA mediated insulin resistance is an important causative factor underlying T2DM in this population. Moreover, comparable HOMA-β in diabetics reflects compensatory insulin hyper secretion in these subjects. There is a need to examine relative contribution and precise nature of genetic factor in their tendency for central obesity, free fatty acidemia and insulin resistance.

Skeletal muscle subsarcolemmal mitochondrial dysfunction in high-fat fed rats exhibiting impaired glucose homeostasis

OBJECTIVE

To investigate whether changes in body energy balance induced by long-term high-fat feeding in adult rats could be associated with modifications in energetic behaviour and oxidative stress of skeletal muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondrial populations.

DESIGN

Adult rats were fed low-fat or high-fat diet for 7 weeks.

MEASUREMENTS

Body energy balance and composition analysis together with plasma insulin and glucose level determination in the whole animal. Oxidative capacity, basal and induced proton leaks as well as aconitase and superoxide dismutase activities in SS and IMF mitochondria from skeletal muscle.

RESULTS

High-fat fed rats exhibit increased body lipid content, as well as hyperinsulinemia, hyperglycaemia and higher plasma non-esterified fatty acids. In addition, SS mitochondria display lower respiratory capacity and a different behaviour of SS and IMF mitochondria is found in the prevention from oxidative damage.

CONCLUSIONS

A deleterious consequence of decreased oxidative capacity in SS mitochondria from rats fed high-fat diet would be a reduced utilization of energy substrates, especially fatty acids, which may lead to intracellular triglyceride accumulation, lipotoxicity and insulin resistance development. Our results thus reveal a possible role for SS mitochondria in the impairment of glucose homeostasis induced by high-fat diet.

Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model.

Effects on insulin secretion and insulin action of a 48-h reduction of plasma free fatty acids with acipimox in nondiabetic subjects genetically predisposed to type 2 diabetes

Elevated plasma FFA cause beta-cell lipotoxicity and impair insulin secretion in nondiabetic subjects predisposed to type 2 diabetes mellitus [T2DM; i.e., with a strong family history of T2DM (FH+)] but not in nondiabetic subjects without a family history of T2DM. To determine whether lowering plasma FFA with acipimox, an antilipolytic nicotinic acid derivative, may enhance insulin secretion, nine FH+ volunteers were admitted twice and received in random order either acipimox or placebo (double-blind) for 48 h. Plasma glucose/insulin/C-peptide concentrations were measured from 0800 to 2400. On day 3, insulin secretion rates (ISRs) were assessed during a +125 mg/dl hyperglycemic clamp. Acipimox reduced 48-h plasma FFA by 36% (P < 0.001) and increased the plasma C-peptide relative to the plasma glucose concentration or DeltaC-peptide/Deltaglucose AUC (+177%, P = 0.02), an index of improved beta-cell function. Acipimox improved insulin sensitivity (M/I) 26.1 +/- 5% (P < 0.04). First- (+19 +/- 6%, P = 0.1) and second-phase (+31 +/- 6%, P = 0.05) ISRs during the hyperglycemic clamp also improved. This was particularly evident when examined relative to the prevailing insulin resistance [1/(M/I)], as both first- and second-phase ISR markedly increased by 29 +/- 7 (P < 0.05) and 41 +/- 8% (P = 0.02). There was an inverse correlation between fasting FFA and first-phase ISR (r2 = 0.31, P < 0.02) and acute (2-4 min) glucose-induced insulin release after acipimox (r2 =0.52, P < 0.04). In this proof-of-concept study in FH+ individuals predisposed to T2DM, a 48-h reduction of plasma FFA improves day-long meal and glucose-stimulated insulin secretion. These results provide additional evidence for the important role that plasma FFA play regarding insulin secretion in FH+ subjects predisposed to T2DM.

Mechanism of insulin-stimulated clearance of plasma nonesterified fatty acids in humans

Insulin increases plasma nonesterified fatty acid (NEFA) clearance in humans, but whether this is independent of change in plasma NEFA appearance is currently unknown. Nine nondiabetic men (age: 28+/-3 yr, body mass index: 27.2+/-1.7 kg/m2) underwent euglycemic clamps to maintain low (LINS) vs. high (HINS) physiological insulin levels for 6 h. An intravenous infusion of heparin+Intralipid (HI) was performed during 4 of the 6 h of the clamps (in the last 4 h at LINS and in the first 4 h at HINS), whereas saline infusion (SAL) was administered in the remaining 2 h to modulate plasma NEFA levels independently of plasma insulin levels. Four experimental conditions were obtained in each individual: LINS with saline (LINS/SAL) and with HI infusion (LINS/HI) and HINS with saline (HINS/SAL) and with HI infusion (HINS/HI). Plasma palmitate appearance during HINS/SAL was lower than during the three other experimental conditions (P<0.05). In contrast, plasma linoleate appearance, as expected, was increased by HI independently of insulin level (P<0.02). Plasma palmitate clearance during HINS/SAL was higher than LINS/SAL and LINS/HI (P<0.008), and this increase was blunted during HINS/HI. We observed a linear decrease in plasma palmitate clearance with increasing plasma NEFA appearance independent of insulin levels. Plasma NEFA levels increased exponentially with increase in plasma NEFA appearance. We conclude that insulin stimulates plasma NEFA clearance by reducing the endogenous appearance rate of NEFA. The relationship between plasma NEFA level and appearance rate is nonlinear.

Thiazolidinediones improve beta-cell function in type 2 diabetic patients

Thiazolidinediones (TZDs) improve glycemic control and insulin sensitivity in patients with type 2 diabetes mellitus (T2DM). There is growing evidence from in vivo and in vitro studies that TZDs improve pancreatic beta-cell function. The aim of this study was to determine whether TZD-induced improvement in glycemic control is associated with improved beta-cell function. We studied 11 normal glucose-tolerant and 53 T2DM subjects [age 53+/-2 yr; BMI 29.4+/-0.8 kg/m2; fasting plasma glucose (FPG) 10.3+/-0.4 mM; Hb A1c 8.2+/-0.3%]. Diabetic patients were randomized to receive placebo or TZD for 4 mo. Subjects received 1) 2-h OGTT with determination of plasma glucose, insulin, and C-peptide concentrations and 2) two-step euglycemic insulin (40 and 160 mU.m-2.min-1) clamp with [3-(3)H]glucose. T2DM patients were then randomized to receive 4 mo of treatment with pioglitazone (45 mg/day), rosiglitazone (8 mg/day), or placebo. Pioglitazone and rosiglitazone similarly improved FPG, mean plasma glucose during OGTT, Hb A1c, and insulin-mediated total body glucose disposal (Rd) and decreased mean plasma FFA during OGTT (all P<0.01, ANOVA). The insulin secretion/insulin resistance (disposition) index [DeltaISR(AUC)/Deltaglucose(AUC)/IR] was significantly improved in all TZD-treated groups: +1.8+/-0.7 (PIO+drug-naïve diabetics), +0.7+/-0.3 (PIO+sulfonylurea-treated diabetics), and 0.7+/-0.2 (ROSI+sulfonylurea-withdrawn diabetics) vs. -0.2+/-0.3 in the two placebo groups (P<0.01, all TZDs vs. placebo, ANOVA). Improved insulin secretion correlated positively with increased body weight, fat mass, and Rd and inversely with decreased plasma glucose and FFA during the OGTT. In T2DM patients, TZD treatment leads to improved beta-cell function, which correlates strongly with improved glycemic control.

Lipid-induced beta-cell dysfunction in vivo in models of progressive beta-cell failure

We determined the effect of 48-h elevation of plasma free fatty acids (FFA) on insulin secretion during hyperglycemic clamps in control female Wistar rats (group a) and in the following female rat models of progressive beta-cell dysfunction: lean Zucker diabetic fatty (ZDF) rats, both wild-type (group b) and heterozygous for the fa mutation in the leptin receptor gene (group c); obese (fa/fa) Zucker rats (nonprediabetic; group d); obese prediabetic (fa/fa) ZDF rats (group e); and obese (fa/fa) diabetic ZDF rats (group f). FFA induced insulin resistance in all groups but increased C-peptide levels (index of absolute insulin secretion) only in obese prediabetic ZDF rats. Insulin secretion corrected for insulin sensitivity using a hyperbolic or power relationship (disposition index or compensation index, respectively, both indexes of beta-cell function) was decreased by FFA. The decrease was greater in normoglycemic heterozygous lean ZDF rats than in Wistar controls. In obese "prediabetic" ZDF rats with mild hyperglycemia, the FFA-induced decrease in beta-cell function was no greater than that in obese Zucker rats. However, in overtly diabetic obese ZDF rats, FFA further impaired beta-cell function. In conclusion, 1) the FFA-induced impairment in beta-cell function is accentuated in the presence of a single copy of a mutated leptin receptor gene, independent of hyperglycemia. 2) In prediabetic ZDF rats with mild hyperglycemia, lipotoxicity is not accentuated, as the beta-cell mounts a partial compensatory response for FFA-induced insulin resistance. 3) This compensation is lost in diabetic rats with more marked hyperglycemia and loss of glucose sensing.

Effects of acute carbohydrate supplementation during sessions of high-intensity intermittent exercise

The present study evaluated the acute effects of carbohydrate supplementation on heart rate (HR), rate of perceived exertion (RPE), metabolic and hormonal responses during and after sessions of high-intensity intermittent running exercise. Fifteen endurance runners (26 +/- 5 years, 64.5 +/- 4.9 kg) performed two sessions of intermittent exercise under carbohydrate (CHO) and placebo (PLA) ingestion. The sessions consisted of 12 x 800 m separated by intervals of 1 min 30 s at a mean velocity corresponding to the previously performed 3-km time trial. Both the CHO and PLA sessions were concluded within approximately 28 min. Blood glucose was significantly elevated in both sessions (123.9 +/- 13.2 mg dl(-1) on CHO and 147.2 +/- 16.3 mg dl(-1) on PLA) and mean blood lactate was significantly higher in the CHO (11.4 +/- 4.9 mmol l(-1)) than in the PLA condition (8.4 +/- 5.1 mmol l(-1)) (P < 0.05). The metabolic stress induced by the exercise model used was confirmed by the elevated HR (approximately 182 bpm) and RPE (approximately 18 on the 15-point Borg scale) for both conditions. No significant differences in plasma insulin, cortisol or free fatty acids were observed during exercise between the two trials. During the recovery period, free fatty acid and insulin concentrations were significantly lower in the CHO trial. Supplementation with CHO resulted in higher lactate associated with lipolytic suppression, but did not attenuate the cortisol, RPE or HR responses.

Effects of portal free fatty acid elevation on insulin clearance and hepatic glucose flux

We tested the hypothesis that, due to greater hepatic free fatty acid (FFA) load, portal delivery of FFAs, as in visceral obesity, induces hyperinsulinemia and increases endogenous glucose production to a greater extent than peripheral FFA delivery. For 5 h, 10 microeq.kg(-1).min(-1) portal oleate (n = 6), equidose peripheral oleate (n = 5), or saline (n = 6) were given intravenously to conscious dogs infused with a combination of portal and peripheral insulin to enable calculation of hepatic insulin clearance during a pancreatic euglycemic clamp. Peripheral FFAs were similar with both oleate treatments and were threefold greater than in controls. Portal FFAs were 1.5- to 2-fold greater with portal than with peripheral oleate. Peripheral insulin concentrations were greatest with portal oleate, intermediate with peripheral oleate (P < 0.001 vs. portal oleate or controls), and lowest in controls, consistent with corresponding reductions in plasma insulin clearance and hepatic insulin clearance. Although endogenous glucose production did not differ between the two routes of oleate delivery, total glucose output (endogenous glucose production plus glucose cycling) was greater with portal than with peripheral oleate (P < 0.001) despite the higher insulin levels. In conclusion, during euglycemic clamps in dogs, the main effect of short-term elevation in portal FFA is to generate peripheral hyperinsulinemia. This may, in the long term, contribute to the metabolic and cardiovascular risk of visceral obesity.

Differential effects of monounsaturated, polyunsaturated and saturated fat ingestion on glucose-stimulated insulin secretion, sensitivity and clearance in overweight and obese, non-diabetic humans

AIMS/HYPOTHESIS

Prolonged elevation of plasma specific fatty acids may exert differential effects on glucose-stimulated insulin secretion (GSIS), insulin sensitivity and clearance.

SUBJECTS AND METHODS

We examined the effect of oral ingestion, at regular intervals for 24 h, of an emulsion containing either predominantly monounsaturated (MUFA), polyunsaturated (PUFA) or saturated (SFA) fat or water (control) on GSIS, insulin sensitivity and insulin clearance in seven overweight or obese, non-diabetic humans. Four studies were conducted in each individual in random order, 4-6 weeks apart. Twenty-four hours after initiation of oral ingestion, subjects underwent a 2 h, 20 mmol/l hyperglycaemic clamp to assess GSIS, insulin sensitivity and insulin clearance.

RESULTS

Following oral ingestion of any of the three fat emulsions over 24 h, plasma NEFAs were elevated by approximately 1.5- to 2-fold over the basal level. Ingestion of any of the three fat emulsions resulted in reduction in insulin clearance, and SFA ingestion reduced insulin sensitivity. PUFA ingestion was associated with an absolute reduction in GSIS, whereas insulin secretion failed to compensate for insulin resistance in subjects who ingested SFA.

CONCLUSIONS/INTERPRETATION

Oral ingestion of fats with differing degrees of saturation resulted in different effects on insulin secretion and action. PUFA ingestion resulted in an absolute reduction in insulin secretion and SFA ingestion induced insulin resistance. Failure of insulin secretion to compensate for insulin resistance implies impaired beta cell function in the SFA study.

The Effect of Insulin on the Intracellular Distribution of 14(R,S)-[18F]Fluoro-6-thia-heptadecanoic Acid in Rats

PURPOSE

The aim of this study was to determine the effect of hyperinsulinemia on myocardial and hepatic distribution and metabolism of 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA).

PROCEDURES

Mitochondrial retention and intracellular lipid incorporation of [18F]FTHA were compared to that of [14C]-2-bromopalmitate or [14C]palmitate during hyperinsulinemic clamp vs. saline infusion in male Wistar rats.

RESULTS

Mitochondrial 18F activity was increased in the heart (1.7 +/- 0.4 vs. 0.5 +/- 0.1% ID/g, P < 0.05), whereas it was reduced in the liver (1.1 +/- 0.3 vs. 1.8 +/- 0.4% ID/g, P < 0.05) during insulin vs. saline infusion, respectively. Mitochondrial [14C]-2-bromopalmitate activity was affected by insulin in a similar way in both tissues. The fractional esterification of [18F]FTHA into triglycerides was impaired compared to [14C]palmitate in both tissues, and [18F]FTHA was insensitive to the shift of esterification of fatty acids into complex lipids in response to insulin.

CONCLUSIONS

[18F]FTHA is sensitive to insulin-induced modifications of free fatty acid oxidative metabolism in rats but is insensitive to changes in nonoxidative fatty acid metabolism.

On the suppression of plasma nonesterified fatty acids by insulin during enhanced intravascular lipolysis in humans

During the fasting state, insulin reduces nonesterified fatty acid (NEFA) appearance in the systemic circulation mostly by suppressing intracellular lipolysis in the adipose tissue. In the postprandial state, insulin may also control NEFA appearance through enhanced trapping into the adipose tissue of NEFA derived from intravascular triglyceride lipolysis. To determine the contribution of suppression of intracellular lipolysis in the modulation of plasma NEFA metabolism by insulin during enhanced intravascular triglyceride lipolysis, 10 healthy nonobese subjects underwent pancreatic clamps at fasting vs. high physiological insulin level with intravenous infusion of heparin plus Intralipid. Nicotinic acid was administered orally during the last 2 h of each 4-h clamp to inhibit intracellular lipolysis and assess insulin's effect on plasma NEFA metabolism independently of its effect on intracellular lipolysis. Stable isotope tracers of palmitate, acetate, and glycerol were used to assess plasma NEFA metabolism and total triglyceride lipolysis in each participant. The glycerol appearance rate was similar during fasting vs. high insulin level, but plasma NEFA levels were significantly lowered by insulin. Nicotinic acid significantly blunted the insulin-mediated suppression of plasma palmitate appearance and oxidation rates by approximately 60 and approximately 70%, respectively. In contrast, nicotinic acid did not affect the marked stimulation of palmitate clearance by insulin. Thus most of the insulin-mediated reduction of plasma NEFA appearance and oxidation can be explained by suppression of intracellular lipolysis during enhanced intravascular triglyceride lipolysis in healthy humans. Our results also suggest that insulin may affect plasma NEFA clearance independently of the suppression of intracellular lipolysis.

Acute in vivo elevation of intravascular triacylglycerol lipolysis impairs peripheral T cell activation in humans

BACKGROUND

Previous studies have shown suppressive effects of polyunsaturated fatty acids (PUFAs) on T cell proliferation, but the precise mechanism for this effect has not been fully investigated in vivo in humans.

OBJECTIVE

The objective was to determine whether this effect is the result of altered T cell membrane properties and impaired CD3- and CD28-mediated signaling in vivo in humans.

DESIGN

Peripheral T cells were isolated from healthy subjects before and 2 h after an intravenous infusion of heparin plus a PUFA-rich lipid emulsion during a euglycemic hyperinsulinemic clamp to induce a 2.5-fold elevation in plasma linoleic acid concentration without significant change in plasma total free fatty acid concentrations.

RESULTS

Intravenous infusion of heparin plus the lipid emulsion reduced peripheral T cell membrane fluidity and altered lipid raft organization, both of which were associated with reduced T cell proliferation after stimulation with CD3 plus CD28. Tyrosine phosphorylation of linker of activated T cells and activation of protein kinase B in T cells were also impaired without a reduction in T cell receptor expression. In addition, acute PUFA elevation was associated with a reduction in T cell membrane cholesterol exchange with the cellular milieu ex vivo.

CONCLUSIONS

A selective increase in plasma linoleic acid concentration and in intravascular lipolysis has a suppressive effect on peripheral T cell CD28-dependent activation, and this effect is associated with changes in plasma membrane properties. Our results have important implications for nutritional therapy in patients at high risk of septic complications and may also be of relevance to postprandial lipid metabolism disorders such as insulin resistance and type 2 diabetes.

Saturated and cis/trans unsaturated acyl CoA esters differentially regulate wild-type and polymorphic beta-cell ATP-sensitive K+ channels

Metabolic regulation of pancreatic beta-cell ATP-sensitive K+ channel (K(ATP) channel) function plays a key role in the process of glucose-stimulated insulin secretion (GSIS). Modulation of K(ATP) channel activity by long-chain acyl CoAs represents an important endogenous regulatory mechanism. Elevated acyl CoA levels have been reported in obese and type 2 diabetic individuals and may contribute to reduced beta-cell excitability and impaired GSIS. Recent studies suggest that the composition of dietary fat may influence the effects of high-fat feeding on impaired GSIS. Therefore, we examined the effects of side-chain length and the degree of saturation of various acyl CoAs on K(ATP) channel activity. Macroscopic currents from either wild-type or polymorphic (Kir6.2[E23K/I337V]) recombinant beta-cell K(ATP) channels were measured in inside-out patches by exposing the inner surface of the membrane to acyl CoAs at physiological nanomolar concentrations. Acyl CoAs increased both wild-type and polymorphic K(ATP) channel activity with the following rank order of efficacy: C18:0, C18:1trans approximately C18:1cis, C20:4 = C16:0, C16:1, and C18:2. A significant correlation exists between activation and acyl CoA hydrophobicity, suggesting that both side-chain length and degree of saturation are critical determinants of K(ATP) channel activation. Our observations reveal a plausible mechanism behind the disparate effects of acyl CoA saturation on K(ATP) channel activation and suggest that dietary fat composition may determine the severity of impaired GSIS via differential activation of beta-cell K(ATP) channels.

Free fatty acids and insulin secretion in humans

Acute increases in plasma levels of long-chain fatty acids raise plasma insulin levels by stimulating insulin secretion or by decreasing insulin clearance. In normal subjects, long-term elevations of fatty acids also stimulate insulin secretion. In fact, they increase insulin precisely to the degree needed to compensate for the fatty acid-induced insulin resistance. In contrast, in individuals who are genetically predisposed to develop type 2 diabetes (prediabetic subjects), the free fatty acid (FFA) stimulation of insulin secretion is not sufficient to fully compensate for the FFA-induced insulin resistance. Therefore, obesity, if associated with elevated fatty acid levels, may lead to hyperglycemia in prediabetic but not in normal individuals.

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Additive effects of glucagon-like peptide 1 and pioglitazone in patients with type 2 diabetes

OBJECTIVE

To evaluate the effect of combination therapy with pioglitazone and glucagon-like peptide (GLP)-1 in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Eight patients with type 2 diabetes (BMI 32.7 +/- 1.3 kg/m(2) and fasting plasma glucose 13.5 +/- 1.2 mmol/l) underwent four different treatment regimens in random order: saline therapy, monotherapy with continuous subcutaneous infusion of GLP-1 (4.8 pmol x kg(-1) x min(-1)), monotherapy with pioglitazone (30-mg tablet of Actos), and combination therapy with GLP-1 and pioglitazone. The observation period was 48 h. End points were plasma levels of glucose, insulin, glucagon, free fatty acids (FFAs), and sensation of appetite.

RESULTS

Fasting plasma glucose decreased from 13.5 +/- 1.2 mmol/l (saline) to 11.7 +/- 1.2 (GLP-1) and 11.5 +/- 1.2 (pioglitazone) and further decreased to 9.9 +/- 1.0 (combination) (P < 0.001). Eight-hour mean plasma glucose levels were reduced from 13.7 +/- 1.1 mmol/l (saline) to 10.6 +/- 1.0 (GLP-1) and 12.0 +/- 1.2 (pioglitazone) and were further reduced to 9.5 +/- 0.8 (combination) (P < 0.0001). Insulin levels increased during monotherapy with GLP-1 compared with monotherapy with pioglitazone (P < 0.01). Glucagon levels were reduced in GLP-1 and combination therapy compared with saline and monotherapy with pioglitazone (P < 0.01). FFAs during breakfast (area under the curve, 0-3 h) were reduced in combination therapy compared with saline (P = 0.03). Sensation of appetite was reduced during monotherapy with GLP-1 and combination therapy (P < 0.05).

CONCLUSIONS

GLP-1 and pioglitazone show an additive glucose-lowering effect. A combination of the two agents may, therefore, be a valuable therapeutic approach for the treatment of type 2 diabetes.

Evaluation of insulin sensitivity and beta-cell function indexes obtained from minimal model analysis of a meal tolerance test

Modeling analysis of glucose, insulin, and C-peptide following a meal has been proposed as a means to estimate insulin sensitivity (S(i)) and beta-cell function from a single test. We compared the model-derived meal indexes with analogous indexes obtained from an intravenous glucose tolerance test (IVGTT) and hyperglycemic clamp (HGC) in 17 nondiabetic subjects (14 men, 3 women, aged 50 +/- 2 years [mean +/- SE], BMI 25.0 +/- 0.7 kg/m(2)). S(i) estimated from the meal was correlated with S(i) estimated from the IVGTT and the HGC (r = 0.59 and 0.76, respectively; P < 0.01 for both) but was approximately 2.3 and 1.4 times higher (P < 0.05 for both). The meal-derived estimate of the beta-cell's response to a steady-state change in glucose (static secretion index) was correlated with the HGC second-phase insulin response (r = 0.69; P = 0.002), but the estimated rate-of-change component (dynamic secretion index) was not correlated with first-phase insulin release from either the HGC or IVGTT. Indexes of beta-cell function obtained from the meal were significantly higher than those obtained from the HGC. In conclusion, insulin sensitivity and beta-cell indexes derived from a meal are not analogous to those from the clamp or IVGTT. Further work is needed before these indexes can be routinely used in clinical and epidemiological studies.

Rosiglitazone prevents the impairment of human islet function induced by fatty acids: evidence for a role of PPARgamma2 in the modulation of insulin secretion

Peroxisome proliferator-activated receptors (PPARs) are a subgroup of the superfamily of nuclear receptors, with three distinct main types: alpha, beta and gamma (subdivided into gamma(1) and gamma(2)). Recently, the presence of PPARgamma has been reported in human islets. Whether other PPAR types can be found in human islets, how islet PPARgamma mRNA expression is regulated by the metabolic milieu, their role in insulin secretion, and the effects of a PPARgamma agonist are not known. In this study, human pancreatic islets were prepared by collagenase digestion and density gradient purification from nonobese adult donors. The presence of PPAR mRNAs was assessed by RT-PCR, and the effect was evaluated of exposure for up to 24 h to either 22.2 mmol/l glucose and/or 0.25, 0.5, or 1.0 mmol/l long-chain fatty acid mixture (oleate to palmitate, 2:1). PPARbeta and, to a greater extent, total PPARgamma and PPARgamma(2) mRNAs were expressed in human islets, whereas PPARalpha mRNA was not detected. Compared with human adipose tissue, PPARgamma mRNA was expressed at lower levels in the islets, and PPARbeta at similar levels. The expression of PPARgamma(2) mRNA was not affected by exposure to 22.2 mmol/l glucose, whereas it decreased markedly and time-dependently after exposure to progressively higher free fatty acids (FFA). This latter effect was not affected by the concomitant presence of high glucose. Exposure to FFA caused inhibition of insulin mRNA expression, glucose-stimulated insulin release, and reduction of islet insulin content. The PPARgamma agonists rosiglitazone and 15-deoxy-Delta-(12,14)prostaglandin J(2) prevented the cytostatic effect of FFA as well as the FFA-induced changes of PPAR and insulin mRNA expression. In conclusion, this study shows that PPARgamma mRNA is expressed in human pancreatic islets, with predominance of PPARgamma(2); exposure to FFA downregulates PPARgamma(2) and insulin mRNA expression and inhibits glucose-stimulated insulin secretion; exposure to PPARgamma agonists can prevent these effects.

Prolonged increase of plasma non-esterified fatty acids fully abolishes the stimulatory effect of 24 hours of moderate hyperglycaemia on insulin sensitivity and pancreatic beta-cell function in obese men

AIMS/HYPOTHESIS

A prolonged increase of plasma NEFA impairs acute glucose-stimulated insulin secretion (GSIS) in vitro and in vivo. Our study therefore examined the combined effect of increased plasma NEFA and glucose on GSIS in humans.

METHODS

We examined GSIS on four occasions in eight obese men during a 10 mmol/l hyperglycaemic clamp and after a 24-h infusion of (i) normal saline, (ii) intralipid and heparin to raise plasma NEFA about two-fold above basal, (iii) 20% dextrose to raise plasma glucose to about 7.5 mmol/l and (iv) intralipid and heparin combined with 20% dextrose to raise plasma NEFA and glucose.

RESULTS

In study (iii) insulin sensitivity was about 20% greater than in study (i) and the disposition index was about 50% higher. Insulin sensitivity tended to be lower in study (ii) whereas the disposition index was lower than in study (i), confirming previous observations. The combination of increased plasma NEFA and glucose (study iv) reduced insulin sensitivity in comparison with study (i) and completely abolished the increase in insulin sensitivity and disposition index seen in study (iii), but did not reduce the latter to a lower value than that in the saline control study (study i).

CONCLUSIONS/INTERPRETATION

We showed that a prolonged increase of plasma NEFA completely abolishes the stimulatory effect of moderate hyperglycaemia on insulin sensitivity and beta-cell function in obese humans. This suggests that previous observations, showing that a prolonged increase of plasma NEFA impairs pancreatic beta-cell function, also apply to the hyperglycaemic state.

A sustained increase in plasma free fatty acids impairs insulin secretion in nondiabetic subjects genetically predisposed to develop type 2 diabetes

Acute elevations in free fatty acids (FFAs) stimulate insulin secretion, but prolonged lipid exposure impairs beta-cell function in both in vitro studies and in vivo animal studies. In humans data are limited to short-term (< or =48 h) lipid infusion studies and have led to conflicting results. We examined insulin secretion and action during a 4-day lipid infusion in healthy normal glucose tolerant subjects with (FH+ group, n = 13) and without (control subjects, n = 8) a family history of type 2 diabetes. Volunteers were admitted twice to the clinical research center and received, in random order, a lipid or saline infusion. On days 1 and 2, insulin and C-peptide concentration were measured as part of a metabolic profile after standardized mixed meals. Insulin secretion in response to glucose was assessed with a +125 mg/dl hyperglycemic clamp on day 3. On day 4, glucose turnover was measured with a euglycemic insulin clamp with [3-3H]glucose. Day-long plasma FFA concentrations with lipid infusion were increased within the physiological range, to levels seen in type 2 diabetes (approximately 500-800 micromol/l). Lipid infusion had strikingly opposite effects on insulin secretion in the two groups. After mixed meals, day-long plasma C-peptide levels increased with lipid infusion in control subjects but decreased in the FH+ group (+28 vs. -30%, respectively, P < 0.01). During the hyperglycemic clamp, lipid infusion enhanced the insulin secretion rate (ISR) in control subjects but decreased it in the FH+ group (first phase: +75 vs. -60%, P < 0.001; second phase: +25 vs. -35%, P < 0.04). When the ISR was adjusted for insulin resistance (ISRRd = ISR / [1/Rd], where Rd is the rate of insulin-stimulated glucose disposal), the inadequate beta-cell response in the FH+ group was even more evident. Although ISRRd was not different between the two groups before lipid infusion, in the FH+ group, lipid infusion reduced first- and second-phase ISR(Rd) to 25 and 42% of that in control subjects, respectively (both P < 0.001 vs. control subjects). Lipid infusion in the FH+ group (but not in control subjects) also caused severe hepatic insulin resistance with an increase in basal endogenous glucose production (EGP), despite an elevation in fasting insulin levels, and impaired suppression of EGP to insulin. In summary, in individuals who are genetically predisposed to type 2 diabetes, a sustained physiological increase in plasma FFA impairs insulin secretion in response to mixed meals and to intravenous glucose, suggesting that in subjects at high risk of developing type 2 diabetes, beta-cell lipotoxicity may play an important role in the progression from normal glucose tolerance to overt hyperglycemia.

Fatty acid metabolism and insulin secretion in pancreatic beta cells

Increases in glucose or fatty acids affect metabolism via changes in long-chain acyl-CoA formation and chronically elevated fatty acids increase total cellular CoA. Understanding the response of pancreatic beta cells to increased amounts of fuel and the role that altered insulin secretion plays in the development and maintenance of obesity and Type 2 diabetes is important. Data indicate that the activated form of fatty acids acts as an effector molecule in stimulus-secretion coupling. Glucose increases cytosolic long-chain acyl-CoA because it increases the "switch" compound malonyl-CoA that blocks mitochondrial beta-oxidation, thus implementing a shift from fatty acid to glucose oxidation. We present arguments in support of the following: (i) A source of fatty acid either exogenous or endogenous (derived by lipolysis of triglyceride) is necessary to support normal insulin secretion; (ii) a rapid increase of fatty acids potentiates glucose-stimulated secretion by increasing fatty acyl-CoA or complex lipid concentrations that act distally by modulating key enzymes such as protein kinase C or the exocytotic machinery; (iii) a chronic increase of fatty acids enhances basal secretion by the same mechanism, but promotes obesity and a diminished response to stimulatory glucose; (iv) agents which raise cAMP act as incretins, at least in part, by stimulating lipolysis via beta-cell hormone-sensitive lipase activation. Furthermore, increased triglyceride stores can give higher rates of lipolysis and thus influence both basal and stimulated insulin secretion. These points highlight the important roles of NEFA, LC-CoA, and their esterified derivatives in affecting insulin secretion in both normal and pathological states.

Free fatty acid-mediated impairment of glucose-stimulated insulin secretion in nondiabetic Oji-Cree individuals from the Sandy Lake community of Ontario, Canada: a population at very high risk for developing type 2 diabetes

The Oji-Cree population of the Sandy Lake region of Ontario, Canada, has the third highest prevalence of type 2 diabetes in the world. Changes in their diet and physical activity over the past half-century, particularly the marked increase in consumption of dietary fats, are felt to be important factors accounting for this epidemic. The aim of the present study was to examine the beta-cell response to a 48-h approximately twofold elevation of plasma free fatty acids (FFAs) (induced by Intralipid and heparin infusion) in members of the Sandy Lake Oji-Cree population (n = 12) and to compare the response to that in healthy age-matched nondiabetic Caucasian subjects (n = 16). The insulin secretion rate, insulin sensitivity index (S(I)), and disposition index (D(I)) (an index of insulin secretion that takes into account the ambient S(I)) were assessed in response to a 4-h graded intravenous glucose infusion followed by a 20 mmol/l 2-h hyperglycemic clamp. Total insulin secretory response to the graded glucose infusion did not change after a 48-h FFA elevation versus saline control in Caucasians and increased by approximately 30% in Oji-Cree individuals (P = 0.04 for difference between the two groups). Infusion of heparin-Intralipid reduced S(I) by approximately 40% in both groups (P = 0.002). Although D(I) was markedly reduced by heparin-Intralipid infusion in Caucasians (by approximately 40%), it was reduced by only 15% in Oji-Cree individuals (P = 0.03 for difference of response between the two groups). However, S(I) and D(I) in the Oji-Cree individuals were already much lower than in Caucasians at baseline, in keeping with the very high risk of type 2 diabetes in this population. It is concluded that Oji-Cree individuals from a community at very high risk for developing type 2 diabetes are not more susceptible to the FFA-induced desensitization of glucose-stimulated insulin secretion than healthy non-Natives and, in fact, appear to be less susceptible. Whether this reflects an inherent resistance to lipotoxicity or an already-present lipotoxic effect in this population will require further study.

Elevated plasma nonesterified fatty acids are associated with deterioration of acute insulin response in IGT but not NGT

High concentrations of nonesterified fatty acids (NEFA) are a risk factor for developing type 2 diabetes in Pima Indians. In vitro and in vivo, chronic elevation of NEFA decreases glucose-stimulated insulin secretion. We hypothesized that high fasting plasma NEFA would increase the risk of type 2 diabetes by inducing a worsening of glucose-stimulated insulin secretion in Pima Indians. To test this hypothesis, fasting plasma NEFA concentrations, body composition, insulin action (M), acute insulin response (AIR, 25-g IVGTT), and glucose tolerance (75-g OGTT) were measured in 151 Pima Indians [107 normal glucose tolerant (NGT), 44 impaired glucose tolerant (IGT)] at the initial visit. These subjects, participants in ongoing studies of the pathogenesis of obesity and type 2 diabetes, had follow-up measurements of body composition, glucose tolerance, M, and AIR. In NGT individuals, cross-sectionally, high fasting plasma NEFA concentrations at the initial visit were negatively associated with AIR after adjustment for age, sex, percent body fat, and M (P = 0.03). Longitudinally, high fasting plasma NEFA concentrations at the initial visit were not associated with change in AIR. In individuals with IGT, cross-sectionally, high fasting plasma NEFA concentrations at the initial visit were not associated with AIR. Longitudinally, high fasting plasma NEFA concentrations at the initial visit were associated with a decrease in AIR before (P < 0.0001) and after adjustment for sex, age at follow-up, time of follow-up, change in percent body fat and insulin sensitivity, and AIR at the initial visit (P = 0.0006). In conclusion, findings in people with NGT indicate that fasting plasma NEFA concentrations are not a primary etiologic factor for beta-cell failure. However, in subjects who have progressed to a state of IGT, chronically elevated NEFA seem to have a deleterious effect on insulin-secretory capacity.

The relative contributions of insulin resistance and beta-cell dysfunction to the pathophysiology of Type 2 diabetes

The relative contributions of insulin resistance and beta-cell dysfunction to the pathophysiology of Type 2 diabetes have been debated extensively. The concept that a feedback loop governs the interaction of the insulin-sensitive tissues and the beta cell as well as the elucidation of the hyperbolic relationship between insulin sensitivity and insulin secretion explains why insulin-resistant subjects exhibit markedly increased insulin responses while those who are insulin-sensitive have low responses. Consideration of this hyperbolic relationship has helped identify the critical role of beta-cell dysfunction in the development of Type 2 diabetes and the demonstration of reduced beta-cell function in high risk subjects. Furthermore, assessments in a number of ethnic groups emphasise that beta-cell function is a major determinant of oral glucose tolerance in subjects with normal and reduced glucose tolerance and that in all populations the progression from normal to impaired glucose tolerance and subsequently to Type 2 diabetes is associated with declining insulin sensitivity and beta-cell function. The genetic and molecular basis for these reductions in insulin sensitivity and beta-cell function are not fully understood but it does seem that body-fat distribution and especially intra-abdominal fat are major determinants of insulin resistance while reductions in beta-cell mass contribute to beta-cell dysfunction. Based on our greater understanding of the relative roles of insulin resistance and beta-cell dysfunction in Type 2 diabetes, we can anticipate advances in the identification of genes contributing to the development of the disease as well as approaches to the treatment and prevention of Type 2 diabetes.

Novel experimental protocol to increase specific plasma nonesterified fatty acids in humans

This study reports a novel protocol to increase plasma monounsaturated, polyunsaturated, and saturated nonesterified fatty acids (NEFA) in eight healthy volunteers (age 29-54 yr, body mass index 23-26 kg/m(2)). This was achieved by feeding small boluses of fat at different time points (35 g at 0 min and 8 g at 30, 60, 90, 120, 150, 180, and 210 min) in combination with a continuous low-dose heparin infusion. Olive oil, safflower oil, or palm stearin were used to increase monounsaturated, polyunsaturated, or saturated NEFAs, respectively. Plasma NEFA concentrations were increased for 2 h, when fat and heparin were given (olive oil: 745 +/- 35 micromol/l; safflower oil: 609 +/- 37 micromol/l, and palm stearin: 773 +/- 38 micromol/l) compared with the control test (no fat and no heparin: 445 +/- 41 micromol/l). During the heparin infusion, 18:1 n-9 was the most abundant fatty acid for the olive oil test compared with 18:2 n-6 for the safflower oil test and 16:0 for the palm stearin test (P < 0.01). The method described here successfully increases several types of plasma NEFA concentrations and could be used to investigate differential effects of elevated individual NEFAs on metabolic processes.

Acute lowering of circulating fatty acids improves insulin secretion in a subset of type 2 diabetes subjects

We tested the effects of acute perturbations of elevated fatty acids (FA) on insulin secretion in type 2 diabetes. Twenty-one type 2 diabetes subjects with hypertriglyceridemia (triacylglycerol >2.2 mmol/l) and 10 age-matched nondiabetic subjects participated. Glucose-stimulated insulin secretion was monitored during hyperglycemic clamps for 120 min. An infusion of Intralipid and heparin was added during minutes 60-120. In one of two tests, the subjects ingested 250 mg of Acipimox 60 min before the hyperglycemic clamp. A third test (also with Acipimox) was performed in 17 of the diabetic subjects after 3 days of a low-fat diet. Acipimox lowered FA levels and enhanced insulin sensitivity in nondiabetic and diabetic subjects alike. Acipimox administration failed to affect insulin secretion rates in nondiabetic subjects and in the group of diabetic subjects as a whole. However, in the diabetic subjects, Acipimox increased integrated insulin secretion rates during minutes 60-120 in the 50% having the lowest levels of hemoglobin A(1c) (379 +/- 34 vs. 326 +/- 30 pmol x kg(-1) x min(-1) without Acipimox, P < 0.05). A 3-day dietary intervention diminished energy from fat from 39 to 23% without affecting FA levels and without improving the insulin response during clamps. Elevated FA levels may tonically inhibit stimulated insulin secretion in a subset of type 2 diabetic subjects.

Free fatty acid-induced hepatic insulin resistance: a potential role for protein kinase C-delta

The mechanisms of the impairment in hepatic glucose metabolism induced by free fatty acids (FFAs) and the importance of FFA oxidation in these mechanisms remain unclear. FFA-induced peripheral insulin resistance has been linked to membrane translocation of novel protein kinase C (PKC) isoforms, but the role of PKC in hepatic insulin resistance has not been assessed. To investigate the biochemical pathways that are induced by FFA in the liver and their relation to glucose metabolism in vivo, we determined endogenous glucose production (EGP), the hepatic content of citrate (product of acetyl-CoA derived from FFA oxidation and oxaloacetate), and hepatic PKC isoform translocation after 2 and 7 h Intralipid + heparin (IH) or SAL in rats. Experiments were performed in the basal state and during hyperinsulinemic clamps (insulin infusion rate, 5 mU. kg(-1). min(-1)). IH increased EGP in the basal state (P < 0.001) and during hyperinsulinemia (P < 0.001) at 2 and 7 h. Also, 7-h infusion of IH induced resistance to the suppressive effect of insulin on EGP (P < 0.05). Glycerol infusion (resulting in plasma glycerol levels similar to IH infusion) did not have any effect on EGP. IH increased hepatic citrate content by twofold, independent of the insulin levels and the duration of IH infusion. IH induced hepatic PKC-delta translocation from the cytosolic to membrane fraction in all groups. PKC-delta translocation was greater at 7 compared with 2 h (P < 0.05). In conclusion, 1) increased FFA oxidation may contribute to the FFA-induced increase in EGP in the basal state and during hyperinsulinemia but is not associated with FFA-induced hepatic insulin resistance, and 2) the progressive insulin resistance induced by FFA in the liver is associated with a progressive increase in hepatic PKC-delta translocation.

Long-term high-fat feeding leads to severe insulin resistance but not diabetes in Wistar rats

Although lipid excess can impair beta-cell function in vitro, short-term high-fat feeding in normal rats produces insulin resistance but not hyperglycemia. This study examines the effect of long-term (10-mo) high polyunsaturated fat feeding on glucose tolerance in Wistar rats. The high fat-fed compared with the chow-fed group was 30% heavier and 60% fatter, with approximately doubled fasting hyperinsulinemia (P < 0.001) but only marginal fasting hyperglycemia (7.5 +/- 0.1 vs. 7.2 +/- 0.1 mmol/l, P < 0.01). Insulin sensitivity was approximately 67% lower in the high-fat group (P < 0.01). The acute insulin response to intravenous arginine was approximately double in the insulin-resistant high-fat group (P < 0.001), but that to intravenous glucose was similar in the two groups. After the intravenous glucose bolus, plasma glucose decline was slower in the high fat-fed group, confirming mild glucose intolerance. Therefore, despite severe insulin resistance, there was only a mildly elevated fasting glucose level and a relative deficiency in glucose-stimulated insulin secretion; this suggests that a genetic or congenital susceptibility to beta-cell impairment is required for overt hyperglycemia to develop in the presence of severe insulin resistance.

The composition of dietary fat directly influences glucose-stimulated insulin secretion in rats

Acute elevations of plasma free fatty acid (FFA) levels augment glucose-stimulated insulin secretion (GSIS). Prolonged elevations of FFA levels reportedly impair GSIS, but no one has previously compared GSIS after prolonged exposure to saturated or unsaturated fat. Rats received a low-fat diet (Low-Fat) or one enriched with either saturated (Lard) or unsaturated fat (Soy) for 4 weeks. Insulin responses during hyperglycemic clamps were augmented by saturated but not unsaturated fat (580 +/- 25, 325 +/- 30, and 380 +/- 50 pmol x l(-1) x min(-1) in Lard, Soy, and Low-Fat groups, respectively). Despite hyperinsulinemia, the amount of glucose infused was lower in the Lard compared with the Low-Fat group. Separate studies measured GSIS from the perfused pancreas. Without fatty acids in the perfusate, insulin output in the Lard group (135 +/- 22 ng/30 min) matched that of Low-Fat rats (115 +/- 13 ng/30 min), but exceeded that of Soy rats (80 +/- 7 ng/30 min). When FFAs in the perfusate mimicked the quantity and composition of plasma FFAs in intact animals, in vivo insulin secretory patterns were restored. Because the GSIS of rats consuming Lard diets consistently exceeded that of the Soy group, we also assessed responses after 48-h infusions of lard or soy oil. Again, lard oil exhibited greater insulinotropic potency. These data indicate that prolonged exposure to saturated fat enhances GSIS (but this does not entirely compensate for insulin resistance), whereas unsaturated fat, given in the diet or by infusion, impairs GSIS. Inferences regarding the impact of fatty acids on GSIS that are based on models using unsaturated fat may not reflect the effects of saturated fat.

The effect of systemic versus portal insulin delivery in pancreas transplantation on insulin action and VLDL metabolism

Combined kidney-pancreas transplantation (KPT) with anastomosis of the pancreatic vein to the systemic circulation (KPT-S) or to the portal circulation (KPT-P) provides a human model in which the chronic effects of portal versus systemic insulin delivery on glucose and VLDL metabolism can be examined. Despite similar plasma glucose and C-peptide levels, KPT-S (n = 9) had an approximate twofold elevation of fasting and intravenous glucose-stimulated plasma insulin levels compared with both KPT-P (n = 7) and healthy control subjects (n = 15). The plasma free fatty acid (FFA) levels were elevated in both transplant groups versus control subjects, but the plasma insulin elevation necessary to lower plasma FFA by 50% was approximately two times higher in KPT-S versus KPT-P and control subjects. Endogenous glucose production was similar in KPT-S and KPT-P, despite approximately 35% higher hepatic insulin levels in the latter, and was suppressed to a greater extent during a euglycemic-hyperinsulinemic clamp in KPT-S versus KPT-P. Total-body glucose utilization during the euglycemic-hyperinsulinemic clamp was approximately 40% lower in KPT-S versus KPT-P, indicating peripheral tissue but not hepatic insulin resistance in KPT-S versus KPT-P. Both transplant groups had an approximate twofold elevation of triglyceride (TG)-rich lipoprotein apolipoprotein B (apoB) and lipids versus control subjects. Elevation of VLDL-apoB and VLDL-TG in both transplant groups was entirely explained by an approximately 50% reduction in clearance of VLDL compared with healthy control subjects. In the presence of increased FFA load but in the absence of hepatic overinsulinization and marked hepatic insulin resistance, there was no elevation of VLDL secretion in KPT-S versus KPT-P and control subjects. These findings suggest that chronic systemic hyperinsulinemia and peripheral tissue insulin resistance with the consequent elevation of plasma FFA flux are insufficient per se to cause VLDL overproduction and that additional factors, such as hepatic hyperinsulinemia and/or gross insulin resistance, may be an essential prerequisite in the pathogenesis of VLDL overproduction in the common form of the insulin resistance syndrome.