Starvation-induced changes of palmitate metabolism and insulin secretion in isolated rat islets stimulated by glucose

Regulatory Role of Fatty Acid Metabolism on Glucose-Induced Changes in Insulin and Glucagon Secretion by Pancreatic Islet Cells

A detailed study of palmitate metabolism in pancreatic islets subject to different experimental conditions, like varying concentrations of glucose, as well as fed or starved conditions, has allowed us to explore the interaction between the two main plasma nutrients and its consequences on hormone secretion. Palmitate potentiates glucose-induced insulin secretion in a concentration-dependent manner, in a physiological range of both palmitate (0-2 mM) and glucose (6-20 mM) concentrations; at glucose concentrations lower than 6 mM, no metabolic interaction with palmitate was apparent. Starvation (48 h) increased islet palmitate oxidation two-fold, and the effect was resistant to its inhibition by glucose (6-20 mM). Consequently, labelled palmitate and glucose incorporation into complex lipids were strongly suppressed, as well as glucose-induced insulin secretion and its potentiation by palmitate. 2-bromostearate, a palmitate oxidation inhibitor, fully recovered the synthesis of complex lipids and insulin secretion. We concluded that palmitate potentiation of the insulin response to glucose is not attributable to its catabolic mitochondrial oxidation but to its anabolism to complex lipids: islet lipid biosynthesis is dependent on the uptake of plasma fatty acids and the supply of α-glycerol phosphate from glycolysis. Islet secretion of glucagon and somatostatin showed a similar dependence on palmitate anabolism as insulin. The possible mechanisms implicated in the metabolic coupling between glucose and palmitate were commented on. Moreover, possible mechanisms responsible for islet gluco- or lipotoxicity after a long-term stimulation of insulin secretion were also discussed. Our own data on the simultaneous stimulation of insulin, glucagon, and somatostatin by glucose, as well as their modification by 2-bromostearate in perifused rat islets, give support to the conclusion that increased FFA anabolism, rather than its mitochondrial oxidation, results in a potentiation of their stimulated release. Starvation, besides suppressing glucose stimulation of insulin secretion, also blocks the inhibitory effect of glucose on glucagon secretion: this suggests that glucagon inhibition might be an indirect or direct effect of insulin, but not of glucose. In summary, there seems to exist three mechanisms of glucagon secretion stimulation: 1. glucagon stimulation through the same secretion coupling mechanism as insulin, but in a different range of glucose concentrations (0 to 5 mM). 2. Direct or indirect inhibition by secreted insulin in response to glucose (5-20 mM). 3. Stimulation by increased FFA anabolism in glucose intolerance or diabetes in the context of hyperlipidemia, hyperglycemia, and hypo-insulinemia. These conclusions were discussed and compared with previous published data in the literature. Specially, we discussed the mechanism for inhibition of glucagon release by glucose, which was apparently contradictory with the secretion coupling mechanism of its stimulation.

A role and mechanism for redox sensing by SENP1 in β-cell responses to high fat feeding

Pancreatic β-cells respond to metabolic stress by upregulating insulin secretion, however the underlying mechanisms remain unclear. Here we show, in β-cells from overweight humans without diabetes and mice fed a high-fat diet for 2 days, insulin exocytosis and secretion are enhanced without increased Ca2+ influx. RNA-seq of sorted β-cells suggests altered metabolic pathways early following high fat diet, where we find increased basal oxygen consumption and proton leak, but a more reduced cytosolic redox state. Increased β-cell exocytosis after 2-day high fat diet is dependent on this reduced intracellular redox state and requires the sentrin-specific SUMO-protease-1. Mice with either pancreas- or β-cell-specific deletion of this fail to up-regulate exocytosis and become rapidly glucose intolerant after 2-day high fat diet. Mechanistically, redox-sensing by the SUMO-protease requires a thiol group at C535 which together with Zn+-binding suppresses basal protease activity and unrestrained β-cell exocytosis, and increases enzyme sensitivity to regulation by redox signals.

Metabolic Role of GABA in the Secretory Function of Pancreatic β-Cells: Its Hypothetical Implication in β-Cell Degradation in Type 2 Diabetes

The stimulus-secretion coupling of a glucose-induced release is generally attributed to the metabolism of the hexose in the β-cells in the glycolytic pathway and the citric acid cycle. Glucose metabolism generates an increased cytosolic concentration of ATP and of the ATP/ADP ratio that closes the ATP-dependent K⁺-channel at the plasma membrane. The resultant depolarization of the β-cells opens voltage-dependent Ca²⁺-channels at the plasma membrane that triggers the exocytosis of insulin secretory granules. The secretory response is biphasic with a first and transient peak followed by a sustained phase. The first phase is reproduced by a depolarization of the β-cells with high extracellular KCl maintaining the KATP-channels open with diazoxide (triggering phase); the sustained phase (amplifying phase) depends on the participation of metabolic signals that remain to be determined. Our group has been investigating for several years the participation of the β-cell GABA metabolism in the stimulation of insulin secretion by three different secretagogues (glucose, a mixture of L-leucine plus L-glutamine, and some branched chain alpha-ketoacids, BCKAs). They stimulate a biphasic secretion of insulin accompanied by a strong suppression of the intracellular islet content of gamma-aminobutyric acid (GABA). As the islet GABA release simultaneously decreased, it was concluded that this resulted from an increased GABA shunt metabolism. The entrance of GABA into the shunt is catalyzed by GABA transaminase (GABAT) that transfers an amino group between GABA and alpha-ketoglutarate, resulting in succinic acid semialdehyde (SSA) and L-glutamate. SSA is oxidized to succinic acid that is further oxidized in the citric acid cycle. Inhibitors of GABAT (gamma-vinyl GABA, gabaculine) or glutamic acid decarboxylating activity (GAD), allylglycine, partially suppress the secretory response as well as GABA metabolism and islet ATP content and the ATP/ADP ratio. It is concluded that the GABA shunt metabolism contributes together with the own metabolism of metabolic secretagogues to increase islet mitochondrial oxidative phosphorylation. These experimental findings emphasize that the GABA shunt metabolism is a previously unrecognized anaplerotic mitochondrial pathway feeding the citric acid cycle with a β-cell endogenous substrate. It is therefore a postulated alternative to the proposed mitochondrial cataplerotic pathway(s) responsible for the amplification phase of insulin secretion. It is concluded the new postulated alternative suggests a possible new mechanism of β-cell degradation in type 2 (perhaps also in type 1) diabetes.

Metabolism as a central regulator of β-cell chromatin state

Pancreatic β-cells are critical mediators of glucose homeostasis in the body, and proper cellular nutrient metabolism is critical to β-cell function. Several interacting signaling networks that uniquely control β-cell metabolism produce essential substrates and co-factors for catalytic reactions, including reactions that modify chromatin. Chromatin modifications, in turn, regulate gene expression. The reactions that modify chromatin are therefore well-positioned to adjust gene expression programs according to nutrient availability. It follows that dysregulation of nutrient metabolism in β-cells may impact chromatin state and gene expression through altering the availability of these substrates and co-factors. Metabolic disorders such as type 2 diabetes (T2D) can significantly alter metabolite levels in cells. This suggests that a driver of β-cell dysfunction during T2D may be the altered availability of substrates or co-factors necessary to maintain β-cell chromatin state. Induced changes in the β-cell chromatin modifications may then lead to dysregulation of gene expression, in turn contributing to the downward cascade of events that leads to the loss of functional β-cell mass, and loss of glucose homeostasis, that occurs in T2D.

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

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

Palmitate is not an effective fuel for pancreatic islets and amplifies insulin secretion independent of calcium release from endoplasmic reticulum

The aim of the study was to determine the acute contribution of fuel oxidation in mediating the increase in insulin secretion rate (ISR) in response to fatty acids. Measures of mitochondrial metabolism, as reflected by oxygen consumption rate (OCR) and cytochrome c reduction, calcium signaling, and ISR by rat islets were used to evaluate processes stimulated by acute exposure to palmitic acid (PA). The contribution of mitochondrial oxidation of PA was determined in the presence and absence of a blocker of mitochondrial transport of fatty acids (etomoxir) at different glucose concentrations. Subsequent to increasing glucose from 3 to 20 mM, PA caused small increases in OCR and cytosolic calcium (about 20% of the effect of glucose). In contrast, the effect of PA on ISR was almost 3 times that by glucose, suggesting that the metabolism of PA is not the dominant mechanism mediating PA's effect on ISR. This was further supported by lack of inhibition of PA-stimulated OCR and ISR when blocking entry of PA into mitochondria (with etomoxir), and PA's lack of stimulation of reduced cytochrome c in the presence of high glucose. Consistent with the lack of metabolic stimulation by PA, an inhibitor of calcium release from the endoplasmic reticulum, but not a blocker of L-type calcium channels, abolished the PA-induced elevation of cytosolic calcium. Notably, ISR was unaffected by thapsigargin showing the dissociation of endoplasmic reticulum calcium release and second phase insulin secretion. In conclusion, stimulation of ISR by PA was mediated by mechanisms largely independent of the oxidation of the fuel.

Mechanisms of β-cell functional adaptation to changes in workload

Insulin secretion must be tightly coupled to nutritional state to maintain blood glucose homeostasis. To this end, pancreatic β-cells sense and respond to changes in metabolic conditions, thereby anticipating insulin demands for a given physiological context. This is achieved in part through adjustments of nutrient metabolism, which is controlled at several levels including allosteric regulation, post-translational modifications, and altered expression of metabolic enzymes. In this review, we discuss mechanisms of β-cell metabolic and functional adaptation in the context of two physiological states that alter glucose-stimulated insulin secretion: fasting and insulin resistance. We review current knowledge of metabolic changes that occur in the β-cell during adaptation and specifically discuss transcriptional mechanisms that underlie β-cell adaptation. A more comprehensive understanding of how β-cells adapt to changes in nutrient state could identify mechanisms to be co-opted for therapeutically modulating insulin secretion in metabolic disease.

Metabolic signaling in fuel-induced insulin secretion

The pancreatic islet β cell senses circulating levels of calorigenic nutrients to secrete insulin according to the needs of the organism. Altered insulin secretion is linked to various disorders such as diabetes, hypoglycemic states, and cardiometabolic diseases. Fuel stimuli, including glucose, free fatty acids, and amino acids, promote insulin granule exocytosis primarily via their metabolism in β cells and the production of key signaling metabolites. This paper reviews our current knowledge of the pathways involved in both positive and negative metabolic signaling for insulin secretion and assesses the role of established and candidate metabolic coupling factors, keeping recent developments in focus.

Effects of acute and chronic psychological stress on isolated islets' insulin release

This study investigated the effects of acute and chronic psychological stress on glucose-stimulated insulin secretion from isolated pancreatic islets. Male Wistar rats were divided into two control and stressed groups; each further was allocated into fed and fasted groups. Stress was induced by communication box for one (acute), fifteen and thirty (chronic) days. After islet isolation, their number, size and insulin output were assessed. Plasma corticosterone level was determined. In fasted animals, acute stress increased basal and post stress plasma corticosterone level, while 30 days stress decreased it compared to day 1. In fed rats, acute stress increased only post stress plasma corticosterone concentration, however, after 15 days stress, it was decreased compared to day 1. Acute stress did not change insulin output; however, the insulin output was higher in the fed acutely stressed rats at 8.3 and 16.7 mM glucose than fasted ones. Chronic stress increased insulin output on day 15 in the fasted animals but decreased it on day 30 in the fed animals at 8.3 and 16.7 mM glucose. In the fasted control rats insulin output was lower than fed ones. In the chronic stressed rats insulin output at 8.3 and 16.7 mM glucose was higher in the fasted than fed rats. The number of islets increased in the fasted rats following 15 days stress. This study indicated that the response of the isolated islets from acute and chronically stressed rats are different and depends on the feeding status.

Overcoming the spatial barriers of the stimulus secretion cascade in pancreatic β-cells

The ability of the pancreatic β-cells to adapt the rate of insulin release in accordance to changes in circulating glucose levels is essential for glucose homeostasis. Two spatial barriers imposed by the plasma membrane and inner mitochondrial membrane need to be overcome in order to achieve stringent coupling between the different steps in the stimulus-secretion cascade.   The first spatial barrier is overcome by the presence of a glucose transporter (GLUT) in the plasma membrane, whereas a low affinity hexokinase IV (glucokinase, GK) in the cytosol conveys glucose availability into a metabolic flux that triggers and accelerates insulin release. The mitochondrial inner membrane comprises a second spatial barrier that compartmentalizes glucose metabolism into glycolysis (cytosol) and tricarboxylate (TCA) cycle (mitochondrial matrix). The exchange of metabolites between cytosol and mitochondrial matrix is mediated via a set of mitochondrial carriers, including the aspartate-glutamate carrier (aralar1), α- ketoglutarate carrier (OGC), ATP/ADP carrier (AAC), glutamate carrier (GC1), dicarboxylate carrier (DIC) and citrate/isocitrate carrier (CIC). The scope of this review is to provide an overview of the role these carriers play in stimulus-secretion coupling and discuss the importance of these findings in the context of the exquisite glucose responsive state of the pancreatic β-cell.

Insulin improves β-cell function in glucose-intolerant rat models induced by feeding a high-fat diet

Insulin therapy has been shown to contribute to extended glycemia remission in newly diagnosed patients with type 2 diabetes mellitus. This study investigated the effects of insulin treatment on pancreatic lipid content, and β-cell apoptosis and proliferation in glucose-intolerant rats to explore the protective role of insulin on β-cell function. A rat glucose-intolerant model was induced by streptozotocin and a high-fat diet. Plasma and pancreatic triglycerides, free fatty acids, and insulin were measured; and pancreatic β-cell cell apoptosis and proliferation were detected by a propidium iodide cell death assay and immunofluorescence for proliferating cell nuclear antigen. Relative β-cell area was determined by immunohistochemistry for insulin, whereas insulin production in pancreas was assessed by reverse transcriptase polymerase chain reaction. Islet β-cell secreting function was assessed by the index ΔI30/ΔG30. Glucose-intolerant rats had higher pancreatic lipid content, more islet β-cell apoptosis, lower β-cell proliferation, and reduced β-cell area in pancreas when compared with controls. Insulin therapy reduced blood glucose, inhibited pancreatic lipid accumulation and islet β-cell apoptosis, and increased β-cell proliferation and β-cell area in glucose-intolerant rats. Furthermore, impaired insulin secretion and insulin production in glucose-intolerant rats were improved by insulin therapy. Insulin can preserve β-cell function by protecting islets from glucotoxicity and lipotoxicity. It can also ameliorate β-cell area by enhancing β-cell proliferation and reducing β-cell apoptosis.

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

The excess accumulation of lipids in islets is thought to contribute to the development of diabetes in obesity by impairing beta-cell function. However, lipids also serve a nutrient function in islets, and fatty acids acutely increase insulin secretion. A better understanding of lipid metabolism in islets will shed light on complex effects of lipids on beta-cells. Adipose differentiation-related protein (ADFP) is localized on the surface of lipid droplets in a wide range of cells and plays an important role in intracellular lipid metabolism. We found that ADFP was highly expressed in murine beta-cells. Moreover, islet ADFP was increased in mice on a high-fat diet (3.5-fold of control) and after fasting (2.5-fold of control), revealing dynamic changes in ADFP in response to metabolic cues. ADFP expression was also increased by addition of fatty acids in human islets. The downregulation of ADFP in MIN6 cells by antisense oligonucleotide (ASO) suppressed the accumulation of triglycerides upon fatty acid loading (56% of control) along with a reduction in the mRNA levels of lipogenic genes such as diacylglycerol O-acyltransferase-2 and fatty acid synthase. Fatty acid uptake, oxidation, and lipolysis were also reduced by downregulation of ADFP. Moreover, the reduction of ADFP impaired the ability of palmitate to increase insulin secretion. These findings demonstrate that ADFP is important in regulation of lipid metabolism and insulin secretion in beta-cells.

Deletion of GPR40 impairs glucose-induced insulin secretion in vivo in mice without affecting intracellular fuel metabolism in islets

OBJECTIVE

The G-protein-coupled receptor GPR40 mediates fatty acid potentiation of glucose-stimulated insulin secretion, but its contribution to insulin secretion in vivo and mechanisms of action remain uncertain. This study was aimed to ascertain whether GPR40 controls insulin secretion in vivo and modulates intracellular fuel metabolism in islets.

RESEARCH DESIGN AND METHODS

Insulin secretion and sensitivity were assessed in GPR40 knockout mice and their wild-type littermates by hyperglycemic clamps and hyperinsulinemic euglycemic clamps, respectively. Transcriptomic analysis, metabolic studies, and lipid profiling were used to ascertain whether GPR40 modulates intracellular fuel metabolism in islets.

RESULTS

Both glucose- and arginine-stimulated insulin secretion in vivo were decreased by approximately 60% in GPR40 knockout fasted and fed mice, without changes in insulin sensitivity. Neither gene expression profiles nor intracellular metabolism of glucose and palmitate in isolated islets were affected by GPR40 deletion. Lipid profiling of isolated islets revealed that the increase in triglyceride and decrease in lyso-phosphatidylethanolamine species in response to palmitate in vitro was similar in wild-type and knockout islets. In contrast, the increase in intracellular inositol phosphate levels observed in wild-type islets in response to fatty acids in vitro was absent in knockout islets.

CONCLUSIONS

These results indicate that deletion of GPR40 impairs insulin secretion in vivo not only in response to fatty acids but also to glucose and arginine, without altering intracellular fuel metabolism in islets, via a mechanism that may involve the generation of inositol phosphates downstream of GPR40 activation.

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.

The fatty acid receptor GPR40 plays a role in insulin secretion in vivo after high-fat feeding

OBJECTIVE

The G-protein-coupled receptor GPR40 is expressed in pancreatic beta-cells and is activated by long-chain fatty acids. Gene deletion studies have shown that GPR40 mediates, at least in part, fatty acid-amplification of glucose-induced insulin secretion (GSIS) but is not implicated in GSIS itself. However, the role of GPR40 in the long-term effects of fatty acids on insulin secretion remains controversial. This study aimed to test the hypothesis that GPR40 plays a role in insulin secretion after high-fat feeding. RESEARCH DESIGN AND METHOD GPR40 knockout (KO) mice on a C57BL/6 background and their wild-type (WT) littermates were fed a high-fat diet (HFD) for 11 weeks. Glucose tolerance, insulin tolerance, and insulin secretion in response to glucose and Intralipid were assessed during the course of the diet period.

RESULTS

GPR40 KO mice had fasting hyperglycemia. They became as obese, glucose intolerant, and insulin resistant as their WT littermates given HFD and developed a similar degree of liver steatosis. Their fasting blood glucose levels increased earlier than those of control mice during the course of the HFD. The remarkable increase in insulin secretory responses to intravenous glucose and Intralipid seen in WT mice after HFD was of much lower magnitude in GPR40 KO mice.

CONCLUSIONS

GPR40 plays a role not only in fatty acid modulation of insulin secretion, but also in GSIS after high-fat feeding. These observations raise doubts on the validity of a therapeutic approach based on GPR40 antagonism for the treatment of type 2 diabetes.

Chronic prehepatic portal hypertension in the rat: is it a type of metabolic inflammatory syndrome?

BACKGROUND

A progressive development of hepatic steatosis with an increase in the lipid hepatocyte content and the formation of megamitochondria have been demonstrated in rats with prehepatic portal hypertension. The aim of this study is to verify the existence of liver and serum lipid metabolism impairments in rats with long-term (2 years) portal hypertension.

METHODS

Male Wistar rats: Control (n = 10) and with prehepatic portal hypertension by triple partial portal vein ligation (n = 9) were used. Liver content of Triglycerides (TG), phospholipids (PL) and cholesterol and serum cholesterol, lipoproteins (HDL and LDL), TG, glucose and Lipid Binding Protein (LBP) were assayed with specific colorimetric commercial kits. Serum levels of insulin and somatostatin were assayed by RIA.

RESULTS

The liver content of TG (6.30 +/- 1.95 vs. 4.17 +/- 0.59 microg/ml; p < 0.01) and cholesterol (1.48 +/- 0.15 vs. 1.10 +/- 0.13 microg/ml; p < 0.001) increased in rats with portal hypertension. The serum levels of cholesterol (97.00+26.02 vs. 114.78 +/- 37.72 mg/dl), TG (153.41 +/- 80.39 vs. 324.39 +/- 134.9 mg/dl; p < 0.01), HDL (20.45 +/- 5.14 vs. 55.15 +/- 17.47 mg/dl; p < 0.001) and somatostatin (1.32 +/- 0.31 vs. 1.59 +0.37 mg/dl) decreased, whereas LDL (37.83 +/- 15.39 vs. 16.77 +/- 6.81 mg/dl; p < 0.001) and LBP (308.47 +/- 194.53 vs. 60.27 +/- 42.96 ng/ml; p < 0.001) increased.

CONCLUSION

Portal hypertension in the rat presents changes in the lipid and carbohydrate metabolisms similar to those produced in chronic inflammatory conditions and sepsis in humans. These underlying alterations could be involved in the development of hepatic steatosis and, therefore, in those described in the metabolic syndrome in humans.

Hepatic lipid metabolism changes in short- and long-term prehepatic portal hypertensive rats

AIM: To verify the impairment of the hepatic lipid metabolism in prehepatic portal hypertension.

METHODS: The concentrations of free fatty acids, diacylglycerol, triglycerides, and phospholipids were assayed by using D-[U-¹⁴C] glucose incorporation in the different lipid fractions and thin-layer chromatography and cholesterol was measured by spectrophotometry, in liver samples of Wistar rats with partial portal vein ligation at short- (1 mo) and long-term (1 year) (i.e. portal hypertensive rats) and the control rats.

RESULTS: In the portal hypertensive rats, liver phospholipid synthesis significantly decreased (7.42 ± 0.50 vs 4.70 ± 0.44 nCi/g protein; P < 0.01) and was associated with an increased synthesis of free fatty acids (2.08 ± 0.14 vs 3.36 ± 0.33 nCi/g protein; P < 0.05), diacylglycerol (1.93 ± 0.2 vs 2.26 ± 0.28 nCi/g protein), triglycerides (2.40 ± 0.30 vs 4.49 ± 0.15 nCi/g protein) and cholesterol (24.28 ± 2.12 vs 57.66 ± 3.26 mg/g protein; P < 0.01).

CONCLUSION: Prehepatic portal hypertension in rats impairs the liver lipid metabolism. This impairment consists in an increase in lipid deposits (triglycerides, diacylglycerol and cholesterol) in the liver, accompanied by a decrease in phospholipid synthesis.

Chronic effects of different non-esterified fatty acids on pancreatic islets of rats

AIMS

The aim of this study was to examine the chronic effects of different non-esterified fatty acids (NEFA) on insulin secretion by pancreatic islets of normal Wistar rats in vitro.

METHODS

Pancreatic islets were isolated from normal Wistar rats, and were incubated with 0.2, 0.4, or 0.8 mmol/L palmitate (C16:0), stearate (C18:0), oleate (C18:1), or linoleate (C18:2) for 24 h, then the insulin secretion and pyruvate dehydrogenase (PDH) activity were examined.

RESULTS

Neither islet insulin content nor islet DNA content differed among islets incubated with each kind of NEFA. Compared with control, linoleate significantly inhibited glucose-stimulated insulin secretion (GSIS) and PDH activity at each concentration (p < 0.05), while others inhibited GSIS and PDH activity significantly only at 0.4 and 0.8 mmol/L (p < 0.05). There was no significant difference in GSIS and PDH activity among islets pretreated by palmitate, stearate, and oleate at the same concentration (p > 0.05). However, linoleate decreased GSIS more than others at the same concentration (p < 0.05), while linoleate (0.4 or 0.8 mmol/L) inhibited PDH activity more than others at the same concentration (p < 0.05).

CONCLUSIONS

Elevation of palmitate, stearate, oleate or linoleate decreases the beta-cell secretory response to glucose, through inhibiting PDH activity. Linoleate exerts more negative effect on GSIS than other NEFA.

Pancreatic islet adaptation to fasting is dependent on peroxisome proliferator-activated receptor alpha transcriptional up-regulation of fatty acid oxidation

The cellular response to fasting and starvation in tissues such as heart, skeletal muscle, and liver requires peroxisome proliferator-activated receptor-alpha (PPARalpha)-dependent up-regulation of energy metabolism toward fatty acid oxidation (FAO). PPARalpha null (PPARalphaKO) mice develop hyperinsulinemic hypoglycemia in the fasting state, and we previously showed that PPARalpha expression is increased in islets at low glucose. On this basis, we hypothesized that enhanced PPARalpha expression and FAO, via depletion of lipid-signaling molecule(s) for insulin exocytosis, are also involved in the normal adaptive response of the islet to fasting. Fasted PPARalphaKO mice compared with wild-type mice had supranormal ip glucose tolerance due to increased plasma insulin levels. Isolated islets from the PPARalpha null mice had a 44% reduction in FAO, normal glucose use and oxidation, and enhanced glucose-induced insulin secretion. In normal rats, fasting for 24 h increased islet PPARalpha, carnitine palmitoyltransferase 1, and uncoupling protein-2 mRNA expression by 60%, 62%, and 82%, respectively. The data are consistent with the view that PPARalpha, via transcriptionally up-regulating islet FAO, can reduce insulin secretion, and that this mechanism is involved in the normal physiological response of the pancreatic islet to fasting such that hypoglycemia is avoided.

Anaplerotic input is sufficient to induce time-dependent potentiation of insulin release in rat pancreatic islets

Nutrients that induce biphasic insulin release, such as glucose and leucine, provide acetyl-CoA and anaplerotic input in the beta-cell. The first phase of release requires increased ATP production leading to increased intracellular Ca(2+) concentration ([Ca(2+)](i)). The second phase requires increased [Ca(2+)](i) and anaplerosis. There is strong evidence to indicate that the second phase is due to augmentation of Ca(2+)-stimulated release via the K(ATP) channel-independent pathway. To test whether the phenomenon of time-dependent potentiation (TDP) has similar properties to the ATP-sensitive K(+) channel-independent pathway, we monitored the ability of different agents that provide acetyl-CoA and anaplerotic input or both of these inputs to induce TDP. The results show that anaplerotic input is sufficient to induce TDP. Interestingly, among the agents tested, the nonsecretagogue glutamine, the nonhydrolyzable analog of leucine aminobicyclo[2.2.1]heptane-2-carboxylic acid, and succinic acid methyl ester all induced TDP, and all significantly increased alpha-ketoglutarate levels in the islets. In conclusion, anaplerosis that enhances the supply and utilization of alpha-ketoglutarate in the tricarboxylic acid cycle appears to play an essential role in the generation of TDP.

Palmitate increases L-type Ca2+ currents and the size of the readily releasable granule pool in mouse pancreatic beta-cells

We have investigated the in vitro effects of the saturated free fatty acid palmitate on mouse pancreatic beta-cells by a combination of electrophysiological recordings, intracellular Ca(2+) ([Ca(2+)](i)) microfluorimetry and insulin release measurements. Addition of palmitate (1 mm, bound to fatty acid-free albumin) to intact islets exposed to 15 mm glucose increased the [Ca(2+)](i) by approximately 30% and insulin secretion 2-fold. Palmitate remained capable of increasing [Ca(2+)](i) and insulin release in the presence of tolbutamide and in islets depolarized by high K(+) in combination with diazoxide, indicating that the stimulation occurs independently of closure of ATP-regulated K(+) channels (K(ATP) channels). Palmitate (0.5 mm) augmented exocytosis (measured as an increase in cell capacitance) in single beta-cells and increased the size of the readily releasable pool (RRP) of granules 2-fold. Whole-cell peak Ca(2+) currents rose by approximately 25% following addition of 0.5 mm palmitate, an effect that was abolished in the presence of 10 microm isradipine indicating that the free fatty acid specifically acts on L-type Ca(2+) channels. The actions of palmitate on exocytosis and Ca(2+) currents were not mimicked by intracellular application of palmitoyl-CoA. We conclude that palmitate increases insulin secretion by a K(ATP) channel-independent mechanism exerted at the level of exocytosis and that involves both augmentation of L-type Ca(2+) currents and an increased size of the RRP.

Palmitate potentiation of glucose-induced insulin release: a study using 2-bromopalmitate

The mechanisms whereby fatty acids (FA) potentiate glucose-induced insulin secretion from the pancreatic beta cell are incompletely understood. In this study, the effects of palmitate on insulin secretion were investigated in isolated rat islets. Palmitate did not initiate insulin secretion at nonstimulatory glucose concentrations, but markedly stimulated insulin release at concentrations of glucose > or = 5.6 mmol/L. At concentrations of palmitate > or =0.5 mmol/L, the important determinant of the potency of the FA was its unbound concentration. At total concentrations < or = 0.5 mmol/L, both the total and unbound concentrations appeared important. Surprisingly, 2-bromopalmitate did not affect palmitate oxidation, but significantly diminished palmitate esterification into cellular lipids. Neither methyl palmitate, which is not activated into a long-chain acyl-CoA ester, nor 2-bromopalmitate affected glucose-stimulated insulin release. Further, 2-bromopalmitate partly inhibited the potentiating effect of palmitate. These results support the concept that FA potentiation of insulin release is mediated by FA-derived signals generated in the esterification pathway.

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.

Selective modification of pyruvate dehydrogenase kinase isoform expression in rat pancreatic islets elicited by starvation and activation of peroxisome proliferator-activated receptor-alpha: implications for glucose-stimulated insulin secretion

The pyruvate dehydrogenase complex (PDC) has a pivotal role in islet metabolism. The pyruvate dehydrogenase kinases (PDK1-4) regulate glucose oxidation through inhibitory phosphorylation of PDC. Starvation increases islet PDK activity (Am J Physiol Endocrinol Metab 270:E988-E994, 1996). In this study, using antibodies against PDK1, PDK2, and PDK4 (no sufficiently specific antibodies are as yet available for PDK3), we identified the PDK isoform profile of the pancreatic islet and delineated the effects of starvation (48 h) on protein expression of individual PDK isoforms. Rat islets were demonstrated to contain all three PDK isoforms, PDK1, PDK2, and PDK4. Using immunoblot analysis with antibodies raised against the individual recombinant PDK isoforms, we demonstrated increased islet protein expression of PDK4 in response to starvation (2.3-fold; P < 0.01). Protein expression of PDK1 and PDK2 was suppressed in response to starvation (by 27% [P < 0.01] and 10% [NS], respectively). We demonstrated that activation of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) by the selective agonist WY14,643 for 24 h in vivo leads to specific upregulation of islet PDK4 protein expression by 1.8-fold (P < 0.01), in the absence of change in islet PDK1 and PDK2 protein expression but in conjunction with a 2.2-fold increase (P < 0.01) in islet PPAR-alpha protein expression. Thus, although no changes in islet PPAR-alpha expression were observed after the starvation protocol, activation of PPAR-alpha in vivo may be a potential mechanism underlying upregulation of islet PDK4 protein expression in starvation. We evaluated the effects of antecedent changes in PDK profile and/or PPAR-alpha activation induced by starvation or PPAR-alpha activation in vivo on glucose-stimulated insulin secretion (GSIS) in isolated islets. GSIS at 20 mmol/l glucose was modestly impaired on incubation with exogenous triglyceride (1 mmol/l triolein) ( approximately 20% inhibition; P < 0.05) in islets from fed rats. Starvation (48 h) impaired GSIS in the absence of triolein (by 57%; P < 0.001), but GSIS after the further addition of triolein did not differ significantly between islets from fed or starved rats. GSIS by islets prepared from WY14,643-treated fed rats did not differ significantly from that seen with islets from control fed rats, and the response to triolein addition resembled that of islets prepared from fed rather than starved rats. PPAR-alpha activation in vivo led to increased insulin secretion at low glucose concentrations. Our results are discussed in relation to the potential impact of changes in islet PDK profile on the insulin secretory response to lipid and of PPAR-alpha activation in the cause of fasting hyperinsulinemia.

Enzymes in pancreatic islets that use NADP(H) as a cofactor including evidence for a plasma membrane aldehyde reductase

Recent evidence of a pyruvate malate shuttle capable of transporting a large amount of NADPH equivalents out of mitochondria in pancreatic islets suggests that cytosolic NADP(H) plays a role in beta cell metabolism. To obtain clues about these processes the activities of several NADPH-utilizing enzymes were estimated in pancreatic islets. Low levels of pyrroquinolone quinone (PQQ) and low levels of enzyme activity that reduce PQQ were found in islets. Low activities of palmitoyl-CoA and stearoyl-CoA desaturases were also detected. Significant activities of glutathione reductase, aldose reductase (EC.1.1.1.21) and aldehyde reductase (EC.1.1.1.2) were present in islets. Potent inhibitors of aldehyde and aldose reductases inhibited neither glucose-induced insulin release nor glucose metabolism in islets indicating that these reductases are not directly involved in glucose-induced insulin reaction. Over 90% of aldose reductase plus aldehyde reductase enzyme activity was present in the cytosol. Kinetic and chromatographic studies indicated that 60-70% of this activity in cytosol was due to aldehyde reductase and the remainder due to aldose reductase. Aldehyde reductase-like enzyme activity, as well as aldose reductase immunoreactivity, was detected in rat islet plasma membrane fractions purified by a polyethylene glycol-Dextran gradient or by a sucrose gradient. This is interesting in view of the fact that voltage-gated potassium channel beta subunits that contain aldehyde and aldose reductase-like NADPH-binding motifs have been detected in plasma membrane fractions of islets [Receptors and Channels 7: 237-243, 2000] and suggests that NADPH might have a yet unknown function in regulating activity of these potassium channels. Reductases may be present in cytosol to protect the insulin cell from molecules that cause oxidative injury.

Lipotoxicity of the pancreatic beta-cell is associated with glucose-dependent esterification of fatty acids into neutral lipids

Prolonged exposure of isolated islets to supraphysiologic concentrations of palmitate decreases insulin gene expression in the presence of elevated glucose levels. This study was designed to determine whether or not this phenomenon is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids. Gene expression of sn-glycerol-3-phosphate acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT), and hormone-sensitive lipase (HSL), three key enzymes of lipid metabolism, was detected in isolated rat islets. Their levels of expression were not affected after a 72-h exposure to elevated glucose and palmitate. To determine the effects of glucose on palmitate-induced neutral lipid synthesis, isolated rat islets were cultured for 72 h with trace amounts of [14C]palmitate with or without 0.5 mmol/l unlabeled palmitate, at 2.8 or 16.7 mmol/l glucose. Glucose increased incorporation of [14C]palmitate into complex lipids. Addition of exogenous palmitate directed lipid metabolism toward neutral lipid synthesis. As a result, neutral lipid mass was increased upon prolonged incubation with elevated palmitate only in the presence of high glucose. The ability of palmitate to increase neutral lipid synthesis in the presence of high glucose was concentration-dependent in HIT cells and was inversely correlated to insulin mRNA levels. 2-Bromopalmitate, an inhibitor of fatty acid mitochondrial beta-oxidation, reproduced the inhibitory effect of palmitate on insulin mRNA levels. In contrast, palmitate methyl ester, which is not metabolized, and the medium-chain fatty acid octanoate, which is readily oxidized, did not affect insulin gene expression, suggesting that fatty-acid inhibition of insulin gene expression requires activation of the esterification pathway. These results demonstrate that inhibition of insulin gene expression upon prolonged exposure of islets to palmitate is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids.

Metabolic engineering with recombinant adenoviruses

Fuel homeostasis in mammals is accomplished by the interplay between tissues and organs with distinct metabolic roles. These regulatory mechanisms are disrupted in obesity and diabetes, leading to a renewed emphasis on discovery of molecular and pharmacologic methods for reversing metabolic disorders. In this chapter, we review the use of recombinant adenoviral vectors as tools for delivering metabolic regulatory genes to cells in culture and to tissues of intact animals. Included are studies on the use of these vectors for gaining insights into the biochemical mechanisms that regulate glucose-stimulated insulin secretion from pancreatic islet beta-cells. We also highlight their use for understanding the function of newly discovered genes that regulate glycogen metabolism in liver and other tissues, and for evaluating "candidate" genes such as glucose-6-phosphatase, which may contribute to development of metabolic dysfunction in pancreatic islets and liver. Finally, we discuss the use of adenoviral and related vectors for causing chronic increases in the levels of circulating hormones. These examples serve to highlight the power of viral gene transfer vectors as tools for understanding metabolic regulatory mechanisms.

Unbound rather than total concentration and saturation rather than unsaturation determine the potency of fatty acids on insulin secretion

Isolated mouse islets were used to compare the effects of three saturated (myristate, palmitate and stearate) and three unsaturated (oleate, linoleate and linolenate) long-chain fatty acids on insulin secretion. By varying the concentrations of fatty acid (250-1250 micromol/l) and albumin simultaneously or independently, we also investigated whether the insulinotropic effect is determined by the unbound or total concentration of the fatty acids. Only palmitate and stearate slightly increased basal insulin secretion (3 mmol/l glucose). All tested fatty acids potentiated glucose-induced insulin secretion (10-15 mmol/l), and the following rank order of potency was obtained when they were compared at the same total concentrations: palmitate approximately = stearate > myristate > or = oleate > or = linoleate approximately = linolenate. The effect of a given fatty acid varied with the fatty acid to albumin molar ratio, in a way which indicated that the unbound fraction is the important one for the stimulation of beta cells. When the potentiation of insulin secretion was expressed as a function of the unbound concentrations, the following rank order emerged: palmitate > myristate > stearate approximately = oleate > linoleate approximately = linolenate. In conclusion, the acute and direct effects of long-chain fatty acids on insulin secretion are due to their unbound fraction. They are observed only at fatty acid/albumin ratios higher than those normally occurring in plasma. Saturated fatty acids are stronger insulin secretagogues than unsaturated fatty acids. Unbound palmitate is by far the most potent of the six common long-chain fatty acids.

Molecular or pharmacologic perturbation of the link between glucose and lipid metabolism is without effect on glucose-stimulated insulin secretion. A re-evaluation of the long-chain acyl-CoA hypothesis

The mechanism by which glucose stimulates insulin secretion from the pancreatic islets of Langerhans is incompletely understood. It has been suggested that malonyl-CoA plays a regulatory role by inhibiting fatty acid oxidation and promoting accumulation of cytosolic long-chain acyl-CoA (LC-CoA). In the current study, we have re-evaluated this "long-chain acyl-CoA hypothesis" by using molecular and pharmacologic methods to perturb lipid metabolism in INS-1 insulinoma cells or rat islets during glucose stimulation. First, we constructed a recombinant adenovirus containing the cDNA encoding malonyl-CoA decarboxylase (AdCMV-MCD), an enzyme that decarboxylates malonyl-CoA to acetyl-CoA. INS-1 cells treated with AdCMV-MCD had dramatically lowered intracellular malonyl CoA levels compared with AdCMV-betaGal-treated cells at both 3 and 20 mM glucose. Further, at 20 mM glucose, AdCMV-MCD-treated cells were less effective at suppressing [1-14C]palmitate oxidation and incorporated 43% less labeled palmitate and 50% less labeled glucose into cellular lipids than either AdCMV-betaGAL-treated or untreated INS-1 cells. Despite the large metabolic changes caused by expression of MCD, insulin secretion in response to glucose was unaltered relative to controls. The alternative, pharmacologic approach for perturbing lipid metabolism was to use triacsin C to inhibit long-chain acyl-CoA synthetase. This agent caused potent attenuation of palmitate oxidation and glucose or palmitate incorporation into cellular lipids and also caused a 47% decrease in total LC-CoA. Despite this, the drug had no effect on glucose-stimulated insulin secretion in islets or INS-1 cells. We conclude that significant disruption of the link between glucose and lipid metabolism does not impair glucose-stimulated insulin secretion in pancreatic islets or INS-1 cells.

The insulinotropic potency of fatty acids is influenced profoundly by their chain length and degree of saturation

Lowering of the elevated plasma FFA concentration in 18- 24-h fasted rats with nicotinic acid (NA) caused complete ablation of subsequent glucose-stimulated insulin secretion (GSIS). Although the effect of NA was reversed when the fasting level of total FFA was maintained by coinfusion of soybean oil or lard oil (plus heparin), the more saturated animal fat proved to be far more potent in enhancing GSIS. We therefore examined the influence of individual fatty acids on insulin secretion in the perfused rat pancreas. When present in the perfusion fluid at 0.5 mM (in the context of 1% albumin), the fold stimulation of insulin release from the fasted pancreas in response to 12.5 mM glucose was as follows: octanoate (C8:0), 3.4; linoleate (C18:2 cis/cis), 5.3; oleate (C18:1 cis), 9.4; palmitate (C16:0), 16. 2; and stearate (C18:0), 21.0. The equivalent value for palmitoleate (C16:1 cis) was 3.1. A cis--> trans switch of the double bond in the C16:1 and C18:1 fatty acids had only a modest, if any, impact on their potency. A similar profile emerged with regard to basal insulin secretion (3 mM glucose). When a subset of these fatty acids was tested in pancreases from fed animals, the same rank order of effectiveness at both basal and stimulatory levels of glucose was seen. The findings reaffirm the essentiality of an elevated plasma FFA concentration for GSIS in the fasted rat. They also show, however, that the insulinotropic effect of individual fatty acids spans a remarkably broad range, increasing and decreasing dramatically with chain length and degree of unsaturation, respectively. Thus, for any given level of glucose, insulin secretion will be influenced greatly not only by the combined concentration of all circulating (unbound) FFA, but also by the makeup of this FFA pool. Both factors will likely be important considerations in understanding the complex interplay between the nature of dietary fat and whole body insulin, glucose, and lipid dynamics.

Modulation of human glucokinase intrinsic activity by SH reagents mirrors post-translational regulation of enzyme activity

The low-affinity glucose phosphorylating enzyme glucokinase plays a key role in the process of glucose recognition in pancreatic B-cells. To evaluate mechanisms of intrinsic regulation of enzyme activity human pancreatic B-cell and liver glucokinase and for comparison rat liver glucokinase were expressed in E. coli bacteria. A one-step purification procedure through metal chelate affinity chromatography revealed 58 kDa proteins with high specific activities in the range of 50 U/mg protein and K(m) values around 8 mM for the substrate D-glucose with a preference for the alpha-anomer. There were no tissue specific differences, no species differences in the electrophoretic mobility, and no differences of the kinetic properties of these well conserved enzymes. The deletion of the 15 tissue-specific NH2-terminal amino acids of the human glucokinase resulted in a catalytically active enzyme whose kinetic properties were not significantly different from those of the wild-type enzymes. The human and rat glucokinase isoforms were non-competitively inhibited by the sulfhydryl group reagents alloxan and ninhydrin with Ki values in the range of 1 microM. The inhibition of glucokinase enzyme activity was reversed by dithiothreitol with an EC50 value of 9 microM for alloxan and of 50 microM for ninhydrin. D-Glucose provided protection against alloxan-induced inhibition of human and rat glucokinase isoenzymes with half-maximal effective concentrations between 11 and 16 mM. The enzyme inhibition by alloxan was accompanied by a change in the electrophoretic mobility with a second lower molecular 49 kDa glucokinase band which can be interpreted as a compact glucokinase molecule locked by disulfide bonds. Quantification of free sulfhydryl groups revealed an average number of 3.6 free sulfhydryl groups per enzyme molecule for the native human glucokinase isoforms. Alloxan decreased the average number of free sulfhydryl groups to 1.9 per enzyme molecule indicating that more than one SH side group is oxidized by this compound. The extraordinary sensitivity of the SH side groups of the glucokinase may be a possible mechanism of enzyme regulation by interconversion of stable (active) and unstable (inactive) conformations of the enzyme. In pancreatic B-cells the glucose-dependent increase of reduced pyridine nucleotides may stabilize the enzyme in the 58 kDa form and provide optimal conditions for glucose recognition and glucose-induced insulin secretion.

Fatty acids rapidly induce the carnitine palmitoyltransferase I gene in the pancreatic beta-cell line INS-1

Fatty acids are important metabolic substrates for the pancreatic beta-cell, and long term exposure of pancreatic islets to elevated concentrations of fatty acids results in an alteration of glucose-induced insulin secretion. Previous work suggested that exaggerated fatty acid oxidation may be implicated in this process by a mechanism requiring changes in metabolic enzyme expression. We have therefore studied the regulation of carnitine palmitoyltransferase I (CPT I) gene expression by fatty acids in the pancreatic beta-cell line INS-1 since this enzyme catalyzes the limiting step of fatty acid oxidation in various tissues. Palmitate, oleate, and linoleate (0.35 mM) elicited a 4-6-fold increase in CPT I mRNA. The effect was dose-dependent and was similar for saturated and unsaturated fatty acids. It was detectable after 1 h and reached a maximum after 3 h. The induction of CPT I mRNA by fatty acids did not require their oxidation, and 2-bromopalmitate, a nonoxidizable fatty acid, increased CPT I mRNA to the same extent as palmitate. The induction was not prevented by cycloheximide treatment of cells indicating that it was mediated by pre-existing transcription factors. Neither glucose nor pyruvate and various secretagogues had a significant effect except glutamine (7 mM) which slightly induced CPT I mRNA. The half-life of the CPT I transcript was unchanged by fatty acids, and nuclear run-on analysis showed a rapid (less than 45 min) and pronounced transcriptional activation of the CPT I gene by fatty acids. The increase in CPT I mRNA was followed by a 2-3-fold increase in CPT I enzymatic activity measured in isolated mitochondria. The increase in activity was time-dependent, detectable after 4 h, and close to maximal after 24 h. Fatty acid oxidation by INS-1 cells, measured at low glucose, was also 2-3-fold higher in cells cultured with fatty acid in comparison with control cells. Long term exposure of INS-1 cells to fatty acid was associated with elevated secretion of insulin at a low (5 mM) concentration of glucose and a decreased effect of higher glucose concentrations. It also resulted in a decreased oxidation of glucose. The results indicate that the CPT I gene is an early response gene induced by fatty acids at the transcriptional level in beta- (INS-1) cells. It is suggested that exaggerated fatty acid oxidation caused by CPT-1 induction is implicated in the process whereby fatty acids alter glucose-induced insulin secretion.

Inhibitory effects of fatty acids on glucose-regulated B-cell function: association with increased islet triglyceride stores and altered effect of fatty acid oxidation on glucose metabolism

Long-term exposure to fatty acids (FA) inhibits B-cell function. We tested whether the inhibitory effects are associated with increased islet triglycerides (TG). Rat pancreatic islets were cultured for 48 hours in RPMI 1640 medium with 10% fetal calf serum (FCS) and 11 mmol/L glucose in the presence or absence of the long-chain FA, palmitate. Palmitate (0.125 mmol/L) exposure successively increased islet TG 70% after 6 hours and 200% after 48 hours of culture. The dose-response for palmitate was similar for the increase in TG and inhibition of glucose-induced insulin secretion. Reversal of elevated islet TG in RPMI medium (after 48 hours of palmitate) was 29% after 6 hours and 84% after 24 hours. A more rapid decline of TG was observed in Krebs-Ringer bicarbonate (KRB) medium in the absence of nutrients. This decline was totally prevented by 1 mumol/L of the carnitine palmitoyl transferase-I (CPT-I) inhibitor, etomoxir. Etomoxir enhanced glucose-induced insulin secretion from palmitate-cultured islets; however, this effect was lost when TG were normalized. Under conditions when oxidation of FA from islet TG stores was blocked with etomoxir, we tested the effects of octanoate, the oxidation of which is not blocked by etomoxir. Oxidation of [1-14C]octanoate from islets precultured with palmitate (48 hours) did not differ from that in control islets. Conversely, after palmitate, octanoate inhibited glucose oxidation (14CO2 production from [U-14C]glucose, 613 +/- 41 pmol/10 islets/90 min v 1,129 +/- 87 after control conditions, P < .01). In conclusion, (1) palmitate induces increases in islet TG that are associated with inhibition of B-cell function, and (2) long-term exposure to palmitate also induces an inhibitory effect of FA oxidation on glucose metabolism that is independent of TG.

Essentiality of circulating fatty acids for glucose-stimulated insulin secretion in the fasted rat

We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acid-treated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal beta cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.

Fasting and decreased B cell sensitivity: important role for fatty acid-induced inhibition of PDH activity

Fasting inhibits glucose-induced insulin secretion. We investigated the role of a glucose fatty acid cycle for such inhibition and its molecular basis in pancreatic islets from 48-h fasted rats. The fasting-impaired insulin response to 27 mM glucose was restored by 41% with a carnitine palmitoyltransferase I inhibitor, etomoxir. Etomoxir also restored (by 50%) impaired glucose oxidation in islets from fasted rats and increased the ratio of oxidation to glycolytic flux from 33 to 43%. Fasting decreased total pyruvate dehydrogenase (PDH) activity (active, unphosphorylated plus inactive, phosphorylated form) by 29%, as well as the percentage of active form (54 +/- 5 vs. 79 +/- 2% in fed rats, P < 0.001). Fasting increased islet PDH kinase activity as follows: PDH-bound activity by 36% and free (not PDH bound) PDH kinase by 70%. Fasting failed to affect PDH kinase content when assayed by an enzyme-linked immunoabsorbent assay with antibodies raised against 45 kDa PDH kinase alpha-chain. We conclude that fasting impairs B cell function to a major extent through the operation of a glucose fatty acid cycle and that decreased PDH activity resulting from increased specific activity of PDH kinase constitutes an important molecular mechanism.

Effect of different sepsis-related cytokines on lipid synthesis by isolated hepatocytes

Cytokines seem to play an important role in the metabolic disturbances that are commonly associated with sepsis. In this study, we analyzed the effect of tumor necrosis factor, interleukin-1 and interleukin-6, as well as that of tumor necrosis factor in combination with interleukin-1 or interleukin-6, both on free fatty acids and on phospholipid synthesis by isolated rat hepatocytes. All three cytokines and combinations caused inhibited D-[U-14C]glucose incorporation into phosphatidylcholine (tumor necrosis factor = 6.39 +/- 1.13 pmol/microgram protein vs. control = 12.90 +/- 0.98 pmol/microgram protein, n = 7; p < 0.001). However, when [U-14C]palmitate was used as radioactive precursor, tumor necrosis factor, either alone or in the presence of the other cytokines, stimulated phosphatidylcholine synthesis. D-[U-14C]glucose incorporation into free fatty acids and triacylglycerol was also significantly stimulated, whereas phosphatidylinositol labeling was found inhibited by the assayed cytokines. Our results demonstrate an effect of sepsis-related cytokines, more evident for tumor necrosis factor, on hepatocyte lipid synthesis either from glucose or palmitate. Also, the findings support the hypothesis that cytokine-induced changes in hepatocyte lipid synthesis can contribute to the impairment in lipidic metabolism seen in patients with sepsis.

Long-term exposure of rat pancreatic islets to fatty acids inhibits glucose-induced insulin secretion and biosynthesis through a glucose fatty acid cycle

We tested effects of long-term exposure of pancreatic islets to free fatty acids (FFA) in vitro on B cell function. Islets isolated from male Sprague-Dawley rats were exposed to palmitate (0.125 or 0.25 mM), oleate (0.125 mM), or octanoate (2.0 mM) during culture. Insulin responses were subsequently tested in the absence of FFA. After a 48-h exposure to FFA, insulin secretion during basal glucose (3.3 mM) was several-fold increased. However, during stimulation with 27 mM glucose, secretion was inhibited by 30-50% and proinsulin biosynthesis by 30-40%. Total protein synthesis was similarly affected. Conversely, previous palmitate did not impair alpha-ketoisocaproic acid (5 mM)-induced insulin release. Induction and reversibility of the inhibitory effect on glucose-induced insulin secretion required between 6 and 24 h. Addition of the carnitine palmitoyltransferase I inhibitor etomoxir (1 microM) partially reversed (by > 50%) FFA-associated decrease in secretory as well as proinsulin biosynthetic responses to 27 mM glucose. The inhibitory effect of previous palmitate was similar when co-culture was performed with 5.5, 11, or 27 mM glucose. Exposure to palmitate or oleate reduced the production of 14CO2 from D-[U-14C]glucose, and of 14CO2 from D-[3,4-14C]-glucose, both effects being reversed by etomoxir.

CONCLUSIONS

long-term exposure to FFA inhibits glucose-induced insulin secretion and biosynthesis probably through a glucose fatty acid cycle.

Glucose-induced insulin release in islets of young rats: time-dependent potentiation and effects of 2-bromostearate

The development of glucose-stimulated insulin release and time-dependent potentiation (TDP) has been studied in isolated islets from 7-, 14-, and 21-day-old and 3-mo-old rats. Responses were small at 7 days and changed little at 14 days. At 21 days the amount of insulin released in response to glucose was two times that at 14 days but was still less than one-half that released by 3-mo islets. Glucose-induced TDP was absent at 7 days but was present at 21 days. The second phase response to glucose decreased with perifusion time in 7-, 14-, and 21-day islets. In 7- and 21-day islets, high glucose in the presence of 2-bromostearate, an inhibitor of fatty acid oxidation, prevented the time-dependent decrease in responses; in addition, it induced TDP and enhanced TDP in the 7-day and 21-day islets, respectively. The data suggest that, in the young islet, glucose metabolism fails to inhibit fatty acid oxidation as it does in the mature islet and that this leads to a diminished signal for stimulus-secretion coupling.

Does cyclic guanosine monophosphate mediate noradrenaline-induced inhibition of islet insulin secretion stimulated by glucose and palmitate?

Noradrenaline inhibits in rat islets the stimulation of insulin secretion induced by glucose and its potentiation by palmitate, but the signalling system responsible remains unknown. We have tested the hypothesis that noradrenaline-induced inhibition is mediated by an elevation of cyclic GMP (cGMP) levels. The analogue 8-Br-cGMP decreases dose-dependently the potentiation by palmitate of glucose-induced insulin secretion, whereas it only slightly affects the proper effect of glucose. Similarly, it abolishes palmitate acceleration of glucose-induced 45Ca2+ uptake without modifying the sugar effect. Finally, 8-Br-cGMP completely inhibits the stimulation of the lipid synthesis de novo induced by palmitate, but not that caused by glucose alone. On the other hand, noradrenaline increases dose-dependently islet cGMP content, with alpha 2-adrenergic specificity. As noradrenaline-induced elevation of cGMP is sensitive to pertussis toxin, it probably results from alpha 2-adrenoceptor activation of islet guanylate cyclase through a guanine nucleotide regulatory protein. It is concluded that the elevated cGMP levels mediate noradrenaline inhibition of lipid synthesis de novo, and hence of acceleration by palmitate of 45Ca2+ uptake and insulin secretion in the presence of glucose.

Norepinephrine inhibits islet lipid metabolism, 45Ca2+ uptake, and insulin secretion

We have previously shown that palmitate potentiates, in isolated islets, glucose-induced stimulation of insulin release, "de novo" lipid synthesis, and 45Ca2+ turnover in a correlative manner. Norepinephrine, a known inhibitor of the secretory response, has now been used to further investigate the relationships among the three phenomena. The amine decreased insulin secretion dose dependently in response to glucose and palmitate with alpha 2-adrenergic specificity. It also reduced similarly the oxidation of 1 mmol/l [U-14C]palmitate as well as the incorporation of 20 mmol/l D-[U-14C]glucose into islet phospholipids and neutral lipids through an alpha 2-adrenergic mechanism. These results indirectly suggest that alpha 2-adrenoceptor stimulation inhibits in islets both palmitate oxidation and esterification through an inactivation of long-chain acyl-CoA synthetase and other enzymes of glycerolipid synthesis. Islet uptake of 45Ca2+ was also decreased by norepinephrine with a similar sensitivity to that shown by insulin release and de novo lipid synthesis. Therefore, it is suggested that alpha 2-adrenoceptor-mediated reduction of the potentiation by palmitate of the secretory response to glucose depends on the inhibition of fatty acid metabolism and the resulting impairment of de novo lipid synthesis and 45Ca2+ turnover.

Insulin and glucose modulate protein kinase C activity in rat adipocytes

In the presence of 1 mM glucose, insulin (10 ng/ml) increases both catalytic and receptor-binding properties of adipocyte cytosolic protein kinase C (PKC). Preincubation of adipocytes with 10 mM glucose raises basal PKC catalytic activity and prevents further stimulation of this enzyme by insulin. The effect of hyperglycemia is likely to be mediated by direct conversion of glucose into diacylglycerol. Thus, an incorporation of 14C-glucose into diacylglycerol is enhanced 10-fold in the presence of 10 mM glucose. These observations indicate that, in normal adipocytes, both insulin and glucose activate PKC; hyperglycemia eliminates the ability of insulin to stimulate this enzyme, thereby interfering with insulin action.

Palmitate dependence of insulin secretion, "de novo" phospholipid synthesis and 45Ca2+-turnover in glucose stimulated rat islets

Palmitate ability to modify D-[U-14C]glucose incorporation into different lipids ("de novo" synthesis), as well as sugar-stimulation of insulin release and 45Ca2+-fluxes, was investigated in islets of fed and 48-h starved rats. The fatty-acid induced dose-dependent, correlative increments of insulin secretion, 45Ca2+-influx and the "de novo" synthesis of each phospholipid fraction analysed at 20 mmol/l (but not 3 mmol/l) glucose. Omission of calcium reduced drastically (p less than 0.001) insulin release and the "de novo" synthesis of neutral glycerolipids, leaving unaltered that of acidic phospholipids (phosphatidate and phosphoinositides). The increased synthesis of the latter is therefore not the consequence of stimulated secretion. It could initiate or contribute to maintain an increased turnover of islet phosphoinositides, thus generating some mediators of the calcium signalling system (inositol phosphates). Starvation led to a drastic reduction (p less than 0.001) of both insulin secretion, "de novo" synthesis of each lipid fraction, and 45Ca2+-influx in response to glucose and palmitate. The presence of a fatty-acid oxidation inhibitor (2-bromostearate or 2-tetradecylglycidate) prevented the effect of starvation on 45Ca2+-influx, as it has been shown to do on insulin secretion and palmitate incorporation into islet lipids. It is finally suggested that palmitate might amplify the insulin secretory response of islets to glucose, through the stimulation of the "de novo" synthesis of phosphoinositides and the subsequent generation of inositol phosphates, which would contribute to accelerated calcium turnover.

Glucose stimulation of insulin secretion in islets of fed and starved rats and its dependence on lipid metabolism

The influence of a physiologic range of palmitate concentrations (0, 0.25, 0.5, and 1.0 mmol/L) on glucose ability to modify insulin secretion, (U-14C) palmitate oxidation, and (U-14C) glucose incorporation into lipids has been studied in islets isolated from either fed or 48-hour starved rats. Palmitate potentiated the insulin response of fed islets to glucose in a particular dose-related manner. Glucose stimulated secretion was accompanied by a decreased palmitate oxidation and an increased (U-14C) glucose incorporation into di-, tri-acylglycerols, and predominantly into phospholipids. These metabolic parameters showed also a positive dependence on palmitate concentration. Starvation increased islet capacity to oxidize palmitate, rendered it insensitive to glucose inhibition, and inhibited both (U-14C) glucose incorporation into all lipid fractions and sugar induced insulin release. The stimulation of islet lipid synthesis by glucose seems to be limited by the exogenous supply of fatty acids and their rate of oxidation. As judged from (U-14C) glucose incorporation data, the rate of phospholipid biosynthesis showed a significant and positive correlation with insulin secretion. This metabolic pathway might provide islet cells with some lipid intermediates (diacylglycerol and/or specific phospholipids) that have been considered as possible mediators of the calcium messenger system.