Lipid environment induces ER stress, TXNIP expression and inflammation in immune cells of individuals with type 2 diabetes

AIMS/HYPOTHESIS

Obesity and type 2 diabetes are concomitant with low-grade inflammation affecting insulin sensitivity and insulin secretion. Recently, the thioredoxin interacting protein (TXNIP) has been implicated in the activation process of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome. In this study, we aim to determine whether the expression of TXNIP is altered in the circulating immune cells of individuals with type 2 vs type 1 diabetes and whether this can be related to specific causes and consequences of inflammation.

METHODS

The expression of TXNIP, inflammatory markers, markers of the unfolded protein response (UPR) to endoplasmic reticulum (ER) stress and enzymes involved in sphingolipid metabolism was quantified by quantitative reverse transcription real-time PCR (qRT-PCR) in peripheral blood mononuclear cells (PBMCs) of 13 non-diabetic individuals, 23 individuals with type 1 diabetes and 81 with type 2 diabetes. A lipidomic analysis on the plasma of 13 non-diabetic individuals, 35 individuals with type 1 diabetes and 94 with type 2 diabetes was performed. The effects of ER stress or of specific lipids on TXNIP and inflammatory marker expression were analysed in human monocyte-derived macrophages (HMDMs) and THP-1 cells.

RESULTS

The expression of TXNIP and inflammatory and UPR markers was increased in the PBMCs of individuals with type 2 diabetes when compared with non-diabetic individuals or individuals with type 1 diabetes. TXNIP expression was significantly correlated with plasma fasting glucose, plasma triacylglycerol concentrations and specific UPR markers. Induction of ER stress in THP-1 cells or cultured HMDMs led to increased expression of UPR markers, TXNIP, NLRP3 and IL-1β. Conversely, a chemical chaperone reduced the expression of UPR markers and TXNIP in PBMCs of individuals with type 2 diabetes. The lipidomic plasma analysis revealed an increased concentration of saturated dihydroceramide and sphingomyelin in individuals with type 2 diabetes when compared with non-diabetic individuals and individuals with type 1 diabetes. In addition, the expression of specific enzymes of sphingolipid metabolism, dihydroceramide desaturase 1 and sphingomyelin synthase 1, was increased in the PBMCs of individuals with type 2 diabetes. Palmitate or C2 ceramide induced ER stress in macrophages as well as increased expression of TXNIP, NLRP3 and IL-1β.

CONCLUSIONS/INTERPRETATION

In individuals with type 2 diabetes, circulating immune cells display an inflammatory phenotype that can be linked to ER stress and TXNIP expression. Immune cell ER stress can in turn be linked to the specific exogenous and endogenous lipid environment found in type 2 diabetes.

Plasma Dihydroceramides Are Diabetes Susceptibility Biomarker Candidates in Mice and Humans

Plasma metabolite concentrations reflect the activity of tissue metabolic pathways and their quantitative determination may be informative about pathogenic conditions. We searched for plasma lipid species whose concentrations correlate with various parameters of glucose homeostasis and susceptibility to type 2 diabetes (T2D). Shotgun lipidomic analysis of the plasma of mice from different genetic backgrounds, which develop a pre-diabetic state at different rates when metabolically stressed, led to the identification of a group of sphingolipids correlated with glucose tolerance and insulin secretion. Quantitative analysis of these and closely related lipids in the plasma of individuals from two population-based prospective cohorts revealed that specific long-chain fatty-acid-containing dihydroceramides were significantly elevated in the plasma of individuals who will progress to diabetes up to 9 years before disease onset. These lipids may serve as early biomarkers of, and help identify, metabolic deregulation in the pathogenesis of T2D.

Molecular phenotyping of multiple mouse strains under metabolic challenge uncovers a role for Elovl2 in glucose-induced insulin secretion

OBJECTIVE

In type 2 diabetes (T2D), pancreatic β cells become progressively dysfunctional, leading to a decline in insulin secretion over time. In this study, we aimed to identify key genes involved in pancreatic beta cell dysfunction by analyzing multiple mouse strains in parallel under metabolic stress.

METHODS

Male mice from six commonly used non-diabetic mouse strains were fed a high fat or regular chow diet for three months. Pancreatic islets were extracted and phenotypic measurements were recorded at 2 days, 10 days, 30 days, and 90 days to assess diabetes progression. RNA-Seq was performed on islet tissue at each time-point and integrated with the phenotypic data in a network-based analysis.

RESULTS

A module of co-expressed genes was selected for further investigation as it showed the strongest correlation to insulin secretion and oral glucose tolerance phenotypes. One of the predicted network hub genes was Elovl2, encoding Elongase of very long chain fatty acids 2. Elovl2 silencing decreased glucose-stimulated insulin secretion in mouse and human β cell lines.

CONCLUSION

Our results suggest a role for Elovl2 in ensuring normal insulin secretory responses to glucose. Moreover, the large comprehensive dataset and integrative network-based approach provides a new resource to dissect the molecular etiology of β cell failure under metabolic stress.

Pancreatic β-cell imaging in humans: fiction or option?

Diabetes mellitus is a growing worldwide epidemic disease, currently affecting 1 in 12 adults. Treatment of disease complications typically consumes ∼10% of healthcare budgets in developed societies. Whilst immune-mediated destruction of insulin-secreting pancreatic β cells is responsible for Type 1 diabetes, both the loss and dysfunction of these cells underly the more prevalent Type 2 diabetes. The establishment of robust drug development programmes aimed at β-cell restoration is still hampered by the absence of means to measure β-cell mass prospectively in vivo, an approach which would provide new opportunities for understanding disease mechanisms and ultimately assigning personalized treatments. In the present review, we describe the progress towards this goal achieved by the Innovative Medicines Initiative in Diabetes, a collaborative public-private consortium supported by the European Commission and by dedicated resources of pharmaceutical companies. We compare several of the available imaging methods and molecular targets and provide suggestions as to the likeliest to lead to tractable approaches. Furthermore, we discuss the simultaneous development of animal models that can be used to measure subtle changes in β-cell mass, a prerequisite for validating the clinical potential of the different imaging tracers.

S 50131 and S 51434, two novel small molecule glucokinase activators, lack chronic efficacy despite potent acute antihyperglycaemic activity in diabetic mice

BACKGROUND AND PURPOSE

Small molecule glucokinase activators (GKAs) have been associated with potent antidiabetic efficacy and hepatic steatosis in rodents. This study reports the discovery of S 50131 and S 51434, two novel GKAs with an original scaffold and an atypical pharmacological profile.

EXPERIMENTAL APPROACH

Activity of the compounds was assessed in vitro by measuring activation of recombinant glucokinase, stimulation of glycogen synthesis in rat hepatocytes and increased insulin secretion from rat pancreatic islets of Langerhans. Efficacy and safety in vivo were evaluated after oral administration in db/db mice by measuring glycaemia, HbA1c and dyslipidaemia-associated events.

KEY RESULTS

S 50131 and S 51434 activated GK and stimulated glycogen synthesis in hepatocytes and insulin secretion from pancreatic islets. Unexpectedly, while both compounds effectively lowered glycaemia after acute oral administration, they did not decrease HbA1c after a 4-week treatment in db/db mice. This lack of antidiabetic efficacy was associated with increased plasma free fatty acids (FFAs), contrasting with the effect of GKA50 and N00236460, two GKAs with sustained HbA1c lowering activity but neutral regarding plasma FFAs. S 50131, but not S 51434, also induced hepatic steatosis, as did GKA50 and N00236460. However, a shorter, 4-day treatment resulted in increased hepatic triglycerides without changing the plasma FFA levels, demonstrating dynamic alterations in the lipid profile over time.

CONCLUSIONS AND IMPLICATIONS

In addition to confirming the occurrence of dyslipidaemia with GKAs, these findings provide new insights into understanding how such compounds may sustain or lose efficacy over time.

[Interaction between environment and genetic background in type 2 diabetes: lessons from animal models]

The respective roles of predisposing genetic factors and environmental factors in the development of type 2 diabetes (T2D) in obese subjects is poorly documented. Rodent models have been set up in an attempt to better understand of the differential effect of a prolonged metabolic stress induced by a high fat diet on glycaemic control according to the genetic background. In utero growth retardation resulting from a hypocaloric diet in pregnant rats induces a dramatic alteration of the development of islet cells leading to diabetes and insulin secretory defects in adult age. Experimentally induced diabetes in rodents results in hyperglycaemia and hyperinsulinemia in the fetus related to accelerated endocrine pancreas maturation responsible for the onset of diabetes in the adult. Deranged metabolic environment during fetal life may therefore further contribute to the onset of diabetes in the adult. Normal mouse strains with different genetic backgrounds show a wide range of responses to a high fat diet, with strains resistant to the diet and other more or less sensitive to the diet, the most sensitive exhibiting obesity diabetes and, insulin deficiency. The inability of the β cell to respond to the increased insulin demand related to insulin resistance seems to be pivotal in the pathophysiologic process and a new notion is emerging: "nutritional genetics" which studies the influence of nutrients on gene expression.

Small molecule glucokinase activators disturb lipid homeostasis and induce fatty liver in rodents: a warning for therapeutic applications in humans

BACKGROUND AND PURPOSE

Small-molecule glucokinase activators (GKAs) are currently being investigated as therapeutic options for the treatment of type 2 diabetes (T2D). Because liver overexpression of glucokinase is thought to be associated with altered lipid profiles, this study aimed at assessing the potential lipogenic risks linked to oral GKA administration.

EXPERIMENTAL APPROACH

Nine GKA candidates were qualified for their ability to activate recombinant glucokinase and to stimulate glycogen synthesis in rat hepatocytes and insulin secretion in rat INS-1E cells. In vivo activity was monitored by plasma glucose and HbA1c measurements after oral administration in rodents. Risk-associated effects were assessed by measuring hepatic and plasma triglycerides and free fatty acids, as well as plasma aminotransferases, and alkaline phosphatase.

KEY RESULTS

GKAs, while efficiently decreasing glycaemia in acute conditions and HbA1c levels after chronic administration in hyperglycemic db/db mice, were potent inducers of hepatic steatosis. This adverse outcome appeared as soon as 4 days after daily oral administration at pharmacological doses and was not transient. GKA treatment similarly increased hepatic triglycerides in diabetic and normoglycaemic rats, together with a pattern of metabolic phenotypes including different combinations of increased plasma triglycerides, free fatty acids, alanine and aspartyl aminotransferases, and alkaline phosphatase. GKAs belonging to three distinct structural families induced hepatic steatosis in db/db mice, arguing in favour of a target-mediated, rather than a chemical class-mediated, effect.

CONCLUSION AND IMPLICATIONS

Given the risks associated with fatty liver disease in the general population and furthermore in patients with T2D, these findings represent a serious warning for the use of GKAs in humans.

LINKED ARTICLE

This article is commented on by Rees and Gloyn, pp. 335-338 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2012.02201.x.

Peripheral injections of melanin-concentrating hormone receptor 1 antagonist S38151 decrease food intake and body weight in rodent obesity models

The compound S38151 is a nanomolar antagonist that acts at the melanin-concentrating hormone receptor 1 (MCH(1)). S38151 is more stable than its purely peptide counterpart, essentially because of the blockade of its N-terminus. Therefore, its action on various models of obesity was studied. Acute intra-cerebroventricular (i.c.v.) administration of S38151 in wild-type rats counteracted the effect of the stable precursor of melanin-concentrating hormone (MCH), NEI-MCH, in a dose-dependent manner (from 0.5 to 50 nmol/kg). In genetically obese Zucker fa/fa rats, daily i.c.v. administration of S38151 induced dose-dependent (5, 10, and 20 nmol/kg) inhibition of food intake, water intake, and body weight gain, as well as increased motility (maximal effect observed at 20 nmol/kg). In Zucker fa/fa rats, intraperitoneal injection of S38151 (30 mg/kg) induced complete inhibition of food consumption within 1 h. Daily intraperitoneal injection of S38151 (10 and 30 mg/kg) into genetically obese ob/ob mice or diet-induced obese mice is able to limit body weight gain. Furthermore, S38151 administration (10 and 30 mg/kg) does not affect food intake, water intake, or body weight gain in MCHR1-deleted mice, demonstrating that its effects are linked to its interaction with MCH(1). These results validate MCH(1) as a target of interest in obesity. S38151 cannot progress to the clinical phase because it is still too poorly stable in vivo.

Metabolic imbalance of the insulin-like growth factor-I axis in Zucker diabetic fatty rats

In healthy conditions, insulin-like growth factor-I (IGF-I) acts in a coordinated fashion with insulin to lower glycemia, mainly by increasing insulin sensitivity in peripheral tissues. The aim of this study was to explore the relationship between glucose homeostasis and the endocrine IGF-I axis in Zucker diabetic fatty (ZDF) rats. The plasma levels of glucose, insulin, growth hormone, free IGF-I, total IGF-I (associated to insulin-like growth factor binding proteins plus free), and corticosterone were measured in 13-week-old ZDF rats and in age-matched controls under fasting and postprandial conditions. The plasma IGF-I binding capacity was measured by radioligand binding. In ZDF rats, fasting total and free IGF-I levels were reduced by 22% and 92%, respectively, compared with controls. Postprandial free IGF-I was reduced by 35%, whereas total IGF-I was unaffected. The plasma IGF-I binding capacity in ZDF rats was reduced by 24% after fasting and by 13% under postprandial conditions. A clear correlation between free IGF-I and insulin was observed in postprandial controls but not in ZDF rats. A principal component analysis clearly separated ZDF and control rats into 2 main components under both fasting and postprandial conditions. The first component was determined equally by total IGF-I, bound IGF-I, the free to total IGF-I ratio, and the IGF-I binding capacity. The second component was determined mostly by glucose and insulin. Our results show a marked alteration of the plasma IGF-I levels and of the capacity of plasma to bind IGF-I, and a disturbed relationship between IGF-I and postprandial insulinemia in a rat model of type 2 diabetes mellitus.

Chromosomal mapping of pancreatic islet morphological features and regulatory hormones in the spontaneously diabetic (Type 2) Goto-Kakizaki rat

Insulin resistance and altered endocrine pancreas function are central pathophysiological features of type 2 diabetes mellitus (T2DM). The Goto-Kakizaki (GK) rat is a model of spontaneous T2DM characterised by reduced beta cell mass and genetically determined glucose intolerance and altered insulin secretion. To identify genetic determinants of endocrine pancreas histopathology, we carried out quantitative trait locus (QTL) mapping of histological phenotypes (beta cell mass -BCM and insulin-positive cell area -IPCA) and plasma concentration of hormones and growth factors in a F2 cohort derived from GK and normoglycemic Brown Norway rats. Although IPCA and BCM in the duodenal region of the pancreas were highly positively correlated (P < 10⁻⁶), and similarly in the splenic region, both measures were poorly correlated when comparing duodenal and splenic phenotypes. Strongest evidence of linkage to pancreas morphological traits was obtained between BCM and chromosome 10 (LOD 3.2). Evidence of significant linkage (LOD 4.2) to plasma corticosterone was detected in a region of chromosome 1 distal to other QTLs previously identified in the GK. Male-specific genetic effects were detected, including linkages (LOD > 4) to growth hormome (GH) on chromosome 6 and prolactin on chromosome 17. These data suggest independent genetic control of the structure and function of ontologically different regions of the endocrine pancreas. Novel QTLs for corticosterone, prolactin and GH may contribute to diabetes in the GK. The QTLs that we have identified in this, and previous genetic studies collectively underline the complex and multiple mechanisms involved in diabetes in the GK strain.

Proteasomal degradation of retinoid X receptor alpha reprograms transcriptional activity of PPARgamma in obese mice and humans

Obese patients have chronic, low-grade inflammation that predisposes to type 2 diabetes and results, in part, from dysregulated visceral white adipose tissue (WAT) functions. The specific signaling pathways underlying WAT dysregulation, however, remain unclear. Here we report that the PPARgamma signaling pathway operates differently in the visceral WAT of lean and obese mice. PPARgamma in visceral, but not subcutaneous, WAT from obese mice displayed increased sensitivity to activation by its agonist rosiglitazone. This increased sensitivity correlated with increased expression of the gene encoding the ubiquitin hydrolase/ligase ubiquitin carboxyterminal esterase L1 (UCH-L1) and with increased degradation of the PPARgamma heterodimerization partner retinoid X receptor alpha (RXRalpha), but not RXRbeta, in visceral WAT from obese humans and mice. Interestingly, increased UCH-L1 expression and RXRalpha proteasomal degradation was induced in vitro by conditions mimicking hypoxia, a condition that occurs in obese visceral WAT. Finally, PPARgamma-RXRbeta heterodimers, but not PPARgamma-RXRalpha complexes, were able to efficiently dismiss the transcriptional corepressor silencing mediator for retinoid and thyroid hormone receptors (SMRT) upon agonist binding. Increasing the RXRalpha/RXRbeta ratio resulted in increased PPARgamma responsiveness following agonist stimulation. Thus, the selective proteasomal degradation of RXRalpha initiated by UCH-L1 upregulation modulates the relative affinity of PPARgamma heterodimers for SMRT and their responsiveness to PPARgamma agonists, ultimately activating the PPARgamma-controlled gene network in visceral WAT of obese animals and humans.

Exploring functional beta-cell heterogeneity in vivo using PSA-NCAM as a specific marker

Background

The mass of pancreatic β-cells varies according to increases in insulin demand. It is hypothesized that functionally heterogeneous β-cell subpopulations take part in this process. Here we characterized two functionally distinct groups of β-cells and investigated their physiological relevance in increased insulin demand conditions in rats.

Methods

Two rat β-cell populations were sorted by FACS according to their PSA-NCAM surface expression, i.e. β^high and β^low-cells. Insulin release, Ca²⁺ movements, ATP and cAMP contents in response to various secretagogues were analyzed. Gene expression profiles and exocytosis machinery were also investigated. In a second part, β^high and β^low-cell distribution and functionality were investigated in animal models with decreased or increased β-cell function: the Zucker Diabetic Fatty rat and the 48 h glucose-infused rat.

Results

We show that β-cells are heterogeneous for PSA-NCAM in rat pancreas. Unlike β^low-cells, β^high-cells express functional β-cell markers and are highly responsive to various insulin secretagogues. Whereas β^low-cells represent the main population in diabetic pancreas, an increase in β^high-cells is associated with gain of function that follows sustained glucose overload.

Conclusion

Our data show that a functional heterogeneity of β-cells, assessed by PSA-NCAM surface expression, exists in vivo. These findings pinpoint new target populations involved in endocrine pancreas plasticity and in β-cell defects in type 2 diabetes.

Mitochondrial reactive oxygen species are obligatory signals for glucose-induced insulin secretion

OBJECTIVE

Insulin secretion involves complex events in which the mitochondria play a pivotal role in the generation of signals that couple glucose detection to insulin secretion. Studies on the mitochondrial generation of reactive oxygen species (ROS) generally focus on chronic nutrient exposure. Here, we investigate whether transient mitochondrial ROS production linked to glucose-induced increased respiration might act as a signal for monitoring insulin secretion.

RESEARCH DESIGN AND METHODS

ROS production in response to glucose was investigated in freshly isolated rat islets. ROS effects were studied using a pharmacological approach and calcium imaging.

RESULTS

Transient glucose increase from 5.5 to 16.7 mmol/l stimulated ROS generation, which was reversed by antioxidants. Insulin secretion was dose dependently blunted by antioxidants and highly correlated with ROS levels. The incapacity of beta-cells to secrete insulin in response to glucose with antioxidants was associated with a decrease in ROS production and in contrast to the maintenance of high levels of ATP and NADH. Then, we investigated the mitochondrial origin of ROS (mROS) as the triggering signal. Insulin release was mimicked by the mitochondrial-complex blockers, antimycin and rotenone, that generate mROS. The adding of antioxidants to mitochondrial blockers or to glucose was used to lower mROS reversed insulin secretion. Finally, calcium imaging on perifused islets using glucose stimulation or mitochondrial blockers revealed that calcium mobilization was completely reversed using the antioxidant trolox and that it was of extracellular origin. No toxic effects were present using these pharmacological approaches.

CONCLUSIONS

Altogether, these complementary results demonstrate that mROS production is a necessary stimulus for glucose-induced insulin secretion.

S26948, a new specific peroxisome proliferator activated receptor gamma modulator improved in vivo hepatic insulin sensitivity in 48 h lipid infused rats

We examined whether S26948, a new specific peroxisome proliferator activated receptor gamma modulator prevented insulin-resistance induced by a 48 h intralipid-infusion in normal rat (IL rats). The effect of S26948 (30 mg/kg) was compared to rosiglitazone (10 mg/kg). Rats were catheterized in the right jugular vein 4 days before the beginning of the 48 h lipid or saline infusions. Animals were intraperitoneally injected once daily with vehicle, S26948 or rosiglitazone. At the end of the infusion the rats underwent either a glucose tolerance test or a euglycemic-hyperinsulinemic clamp. Finally isolation and incubation of hepatocytes in another series of rats were performed. Intralipid infusion leads to a 4-fold increase in plasma free fatty acid concentration compared to controls (C). Both S26948 and rosiglitazone decreased plasma free fatty acid concentration in IL rats compared to vehicle treated IL rats. Glucose-induced insulin secretion was significantly increased in IL compared to C and was associated with insulin resistance. Both S26948 and rosiglitazone treatments normalized glucose-induced insulin secretion and improved insulin action in IL rats. However, S26948 specifically improved hepatic insulin sensitivity whereas rosiglitazone improved both hepatic insulin sensitivity and insulin-stimulated glucose utilization. Finally, studies on isolated hepatocytes showed differential effect of both compounds on gene expression of key enzymes of glucose metabolism. Our data show that non thiazolidinedione S26948 may represent an alternative way for the management of dysregulated hepatic insulin sensitivity.

Anatomical versus functional beta-cell mass in experimental diabetes

The ability of pancreatic beta-cell mass to vary according to insulin requirements is an important component of optimal long-term control of glucose homeostasis. It is generally assumed that alteration of this property largely contributes to the impairment of insulin secretion in type 2 diabetes. However, data in humans are scarce and it is impossible to correlate beta-cell mass and function with the various stages of the disease. Thus, the importance of animal models is obvious. In rodents, increased beta-cell mass associated with an increase in the function of individual beta-cells contributes to the adaptation of the insulin response to insulin resistance in late pregnancy and in obesity. A reduction in beta-cell mass always corresponds to an alteration in insulin secretory capacity of islet tissue (Zucker diabetic fatty and Goto-Kakisaki rats, db/db mice). During regenerative processes following experimental reduction of beta-cell mass [partial pancreatectomy, streptozocin (STZ) injection], beta-cell mass increase is not associated with a corresponding improvement of beta-cell function, thus indicating that regenerative beta-cells did not achieve functional maturity. The main lesson from experimental diabetes is therefore that beta-cell mass cannot always predict functional capacity of the beta-cell tissue and that the functional beta-cell mass rather than the anatomical beta-cell mass must be taken into account at all times.

Pathophysiological, genetic and gene expression features of a novel rodent model of the cardio-metabolic syndrome

BACKGROUND

Complex etiology and pathogenesis of pathophysiological components of the cardio-metabolic syndrome have been demonstrated in humans and animal models.

METHODOLOGY/PRINCIPAL FINDINGS

We have generated extensive physiological, genetic and genome-wide gene expression profiles in a congenic strain of the spontaneously diabetic Goto-Kakizaki (GK) rat containing a large region (110 cM, 170 Mb) of rat chromosome 1 (RNO1), which covers diabetes and obesity quantitative trait loci (QTL), introgressed onto the genetic background of the normoglycaemic Brown Norway (BN) strain. This novel disease model, which by the length of the congenic region closely mirrors the situation of a chromosome substitution strain, exhibits a wide range of abnormalities directly relevant to components of the cardio-metabolic syndrome and diabetes complications, including hyperglycaemia, hyperinsulinaemia, enhanced insulin secretion both in vivo and in vitro, insulin resistance, hypertriglyceridemia and altered pancreatic and renal histological structures. Gene transcription data in kidney, liver, skeletal muscle and white adipose tissue indicate that a disproportionately high number (43-83%) of genes differentially expressed between congenic and BN rats map to the GK genomic interval targeted in the congenic strain, which represents less than 5% of the total length of the rat genome. Genotype analysis of single nucleotide polymorphisms (SNPs) in strains genetically related to the GK highlights clusters of conserved and strain-specific variants in RNO1 that can assist the identification of naturally occurring variants isolated in diabetic and hypertensive strains when different phenotype selection procedures were applied.

CONCLUSIONS

Our results emphasize the importance of rat congenic models for defining the impact of genetic variants in well-characterised QTL regions on in vivo pathophysiological features and cis-/trans- regulation of gene expression. The congenic strain reported here provides a novel and sustainable model for investigating the pathogenesis and genetic basis of risks factors for the cardio-metabolic syndrome.

[Anatomical and functional plasticity of pancreatic beta-cells and type 2 diabetes]

The most common form of diabetes, type 2 diabetes (T2D) is a major Public Health issue which is receiving a great deal of attention both in industrial and public research, in order to develop new and more effective drugs. The hyperglycaemia of T2D is the result of two interdependent defects : decreased biological efficacy of insulin in target tissues (insulin resistance), and a decreased capacity for beta cells to secrete insulin in response to glucose. Furthermore, hyperglycaemia evolves with time and even with rigorous treatment there is a progressive deterioration of glucose homeostasis. Seventy five percent of DT2 patients are obese and show a perturbed lipid profile. beta-cell plasticity is a unique property of these cells to adapt their number and volume (beta-cell mass) and their function to the increased secretory demand linked to insulin resistance. This is well documented in physiological (pregnancy) as well in pathophysiological conditions (obesity, acromegaly). Although the lack of reliable techniques makes it very difficult to document it in humans, this property is likely altered in DT2, mainly as a consequence of the prolonged exposure of islet cells to high plasma levels of glucose and free fatty acids (gluco-lipotoxicity). The mechanisms by which hyperglycaemia and hyperlipidemia exert their deleterious effects on the beta-cell include the generation of Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS) and Advanced Glycosylation End Products (AGE). Altogether the prevailing clinical and experimental data urge us to consider that the pathophysiology of DT2 lies, at least in part, the inability of beta-cells to adapt their functional mass to the prevailing insulin demand. This re-evaluation of the pathophysiology of DT2 stimulates the research of new therapeutic approaches aimed at maintaining and/or restoring the functional beta-cell mass by targeting the mechanisms responsible for its decrease.

Transgenic expression of human INS gene in Ins1/Ins2 double knockout mice leads to insulin underproduction and diabetes in some male mice

We have generated transgenic mouse lines expressing exclusively a human INS transgene on an Ins1/Ins2 double knockout (mIKO) background. The transgene expression was driven by either a 4000 bp or a 353 bp promoter. These transgenic lines, designated mIKO:INS4000 and mIKO:INS353, were viable and fertile. Determination of the amounts of insulin transcripts and total pancreatic insulin content revealed relative insulin underproduction in both lines, from birth to adulthood. Total pancreatic insulin stores in mIKO:INS4000 and mIKO:INS353 mice represented only about 50% and 27%, respectively, as compared to wild-type mice. Morphometric analysis of pancreas did not show any compensatory beta-cell hyperplasia. The majority of animals in both lines remained normoglycemic throughout their lives. Nevertheless, glucose tolerance tests revealed glucose intolerance in nearly half of mIKO:INS4000 male mice, likely due to impaired insulin secretion detected in those animals. In addition, a small fraction (2-4%) of male mice in both lines spontaneously developed diabetes with very distinct pathophysiological features. Diabetes was never seen in female animals. The diabetes developed by mIKO:INS353 mice was rapidly lethal, accompanied by a dramatic depletion of pancreatic insulin stores whereas the mIKO:INS4000 diabetic animals could live for several months. This suggests a possible link between the structure of the human INS gene promoter and the type of diabetes developed in these lines.

Genetic control of plasma lipid levels in a cross derived from normoglycaemic Brown Norway and spontaneously diabetic Goto-Kakizaki rats

AIMS/HYPOTHESIS

Dyslipidaemia is a main component of the insulin resistance syndrome. The inbred Goto-Kakizaki (GK) rat is a model of spontaneous type 2 diabetes and insulin resistance, which has been used to identify diabetes-related susceptibility loci in genetic crosses. The objective of our study was to test the genetic control of lipid metabolism in the GK rat and investigate a possible relationship with known genetic loci regulating glucose homeostasis in this strain.

MATERIALS AND METHODS

Plasma concentration of triglycerides, phospholipids, total cholesterol, HDL, LDL and VLDL cholesterol were determined in a cohort of 151 hybrids of an F2 cross derived from GK and non-diabetic Brown Norway (BN) rats. Data from the genome-wide scan of the F2 hybrids were used to test for evidence of genetic linkage to the lipid quantitative traits.

RESULTS

We identified statistically significant quantitative trait loci (QTLs) that control the level of plasma phospholipids and triglycerides (chromosome 1), LDL cholesterol (chromosome 3) and total and HDL cholesterol (chromosomes 1 and 5). These QTLs do not coincide with previously identified diabetes susceptibility loci in a similar cross. The significance of lipid QTLs mapped to chromosomes 1 and 5 is strongly influenced by sex.

CONCLUSION/INTERPRETATION

We established that several genetic loci control the quantitative variations of plasma lipid variables in a GKxBN cross. They appear to be distinct from known GK diabetes QTLs, indicating that lipid metabolism and traits directly relevant to glucose and insulin regulation are controlled by different gene variants in this strain combination.

Design and synthesis of novel imidazoline derivatives with potent antihyperglycemic activity in a rat model of type 2 diabetes

Imidazoline derivatives have been reported to show antihyperglycemic activity in vivo. In the present study, we first showed that there was no correlation between the in vivo antidiabetic activity and the in vitro affinities for the I1/I2 binding sites for several substituted aryl imidazolines. Among these compounds, 2-(alpha-cyclohexyl-benzyl)-4,5-dihydro-1H-imidazole 2 exhibited potent antihyperglycemic properties. It was then chosen as lead compound. Thirty-six new derivatives were synthesized by replacing the cyclohexyl/benzyl group by various cyclic systems or the imidazoline ring by isosteric heterocycles. These compounds were evaluated in vivo for their antihyperglycemic activity using an oral glucose tolerance test (OGTT) in a rat model of type-2 diabetes obtained by giving a single intravenous (iv) injection of a low dose of streptozotocin to rats (STZ rats) and in normal rats. Nine compounds with an imidazoline moiety, possibly substituted by a methyl group, had a potent effect on the glucose tolerance in normal or STZ-diabetic rats, after an oral (po) administration of the test compound at a dose of 30 or 10 mg kg(-1), without any hypoglycemia. Replacement of the imidazoline ring by isosteric heterocycles resulted in a total loss of activity.

Mitochondrial reactive oxygen species are required for hypothalamic glucose sensing

The physiological signaling mechanisms that link glucose sensing to the electrical activity in metabolism-regulating hypothalamus are still controversial. Although ATP production was considered the main metabolic signal, recent studies show that the glucose-stimulated signaling in neurons is not totally dependent on this production. Here, we examined whether mitochondrial reactive oxygen species (mROS), which are physiologically generated depending on glucose metabolism, may act as physiological sensors to monitor the glucose-sensing response. Transient increase from 5 to 20 mmol/l glucose stimulates reactive oxygen species (ROS) generation on hypothalamic slices ex vivo, which is reversed by adding antioxidants, suggesting that hypothalamic cells generate ROS to rapidly increase glucose level. Furthermore, in vivo, data demonstrate that both the glucose-induced increased neuronal activity in arcuate nucleus and the subsequent nervous-mediated insulin release might be mimicked by the mitochondrial complex blockers antimycin and rotenone, which generate mROS. Adding antioxidants such as trolox and catalase or the uncoupler carbonyl cyanide m-chlorophenylhydrazone in order to lower mROS during glucose stimulation completely reverses both parameters. In conclusion, the results presented here clearly show that the brain glucose-sensing mechanism involved mROS signaling. We propose that this mROS production plays a key role in brain metabolic signaling.

Psammomys obesus, a model for environment-gene interactions in type 2 diabetes

Type 2 diabetes is characterized by insulin resistance and progressive beta-cell failure. Deficient insulin secretion, with increased proportions of insulin precursor molecules, is a common feature of type 2 diabetes; this could result from inappropriate beta-cell function and/or reduced beta-cell mass. Most studies using tissues from diabetic patients are retrospective, providing only limited information on the relative contribution of beta-cell dysfunction versus decreased beta-cell mass to the "beta-cell failure" of type 2 diabetes. The gerbil Psammomys obesus is a good model to address questions related to the role of insulin resistance and beta-cell failure in nutritionally induced diabetes. Upon a change from its natural low-calorie diet to the calorie-rich laboratory food, P. obesus develops moderate obesity associated with postprandial hyperglycemia. Continued dietary load, superimposed on its innate insulin resistance, results in depletion of pancreatic insulin stores, with increased proportions of insulin precursor molecules in the pancreas and the blood. Inadequate response of the preproinsulin gene to the increased insulin needs is an important cause of diabetes progression. Changes in beta-cell mass do not correlate with pancreatic insulin stores and are unlikely to play a role in disease initiation and progression. The major culprit is the inappropriate insulin production with depletion of insulin stores as a consequence. Similar mechanisms could operate during the evolution of type 2 diabetes in humans.

Role of glucose in IRS signaling in rat pancreatic islets: specific effects and interplay with insulin

We investigated the possible interplay between insulin and glucose signaling pathways in rat pancreatic β-cell with a special focus on the role of glucose in IRS signaling in vivo. Three groups of rats were constituted by combining simultaneous infusion during 48 h either of glucose and/or insulin, or glucose+diazoxide: Hyperglycemic- Hyperinsulinemic (HGHI), euglycemic-Hyperinsulinemic (eGHI), Hyperglycemic-euinsulinemic (HGeI). Control rats were infused with 0,9% NaCl. In HGHI and HGeI rats plasma glucose levels were maintained at 20-22 mmol/l. In eGHI rats, plasma glucose was not different from that of controls, whereas plasma insulin was much higher than in controls. In HGHI rats, IRS-2 mRNA expression, total protein and phosphorylated protein amounts were increased compared to controls. In HGeI rats, only IRS-2 mRNA expression was increased. No change was observed in eGHI rats whatever the parameter considered. In all groups, mRNA concentration of IRS-1 was similar to that of controls. The quantity of total and phosphorylated IRS- 1 protein was dramatically increased in HGHI rats and to a lesser extent in eGHI rats. Neither mRNA nor IRS-1 protein expression were modified in HGeI rats. The data suggest that glucose and insulin play at once a specific and a complementary role in islet IRSs signaling. Especially, glucose stimulates IRS-2 mRNA expression whatever the insulin status and independently of the secretory process. The differential regulation of IRS-1 and IRS-2 expressions is in agreement with their supposed different involvement in the control of β-cell growth and function.

Improvement of metabolic state in an animal model of nutrition-dependent type 2 diabetes following treatment with S 23521, a new glucagon-like peptide 1 (GLP-1) analogue

Glucagon-like peptide 1 (GLP-1) analogues are considered potential drugs for type 2 diabetes. We studied the effect of a novel GLP-1 analogue, S 23521 ([a8-des R36] GLP-1-[7-37]-NH2), on the metabolic state and beta-cell function, proliferation and survival in the Psammomys obesus model of diet-induced type 2 diabetes. Animals with marked hyperglycaemia after 6 days of high-energy diet were given twice-daily s.c. injection of 100 microg/kg S 23521 for 15 days. Food intake was significantly decreased in S 23251-treated P. obesus; however, there was no significant difference in body weight from controls. Progressive worsening of hyperglycaemia was noted in controls, as opposed to maintenance of pre-treatment glucose levels in the S 23521 group. Prevention of diabetes progression was associated with reduced mortality. In addition, the treated group had higher serum insulin, insulinogenic index and leptin, whereas plasma triglyceride and non-esterified fatty acid levels were decreased. S 23521 had pronounced effect on pancreatic insulin, which was 5-fold higher than the markedly depleted insulin reserve of control animals. Immunohistochemical analysis showed islet degranulation with disrupted morphology in untreated animals, whereas islets from S 23521-treated animals appeared intact and filled with insulin; beta-cell apoptosis was approximately 70% reduced, without a change in beta-cell proliferation. S 23521 treatment resulted in a 2-fold increase in relative beta-cell volume. Overall, S 23521 prevented the progression of diabetes in P. obesus with marked improvement of the metabolic profile, including increased pancreatic insulin reserve, beta-cell viability and mass. These effects are probably due to actions of S 23521 both directly on islets and via reduced food intake, and emphasize the feasibility of preventing blood glucose deterioration over time in type 2 diabetes.

Structural requirements of the N-terminal region of GLP-1-[7-37]-NH2 for receptor interaction and cAMP production

A series of GLP-1-[7-36]-NH(2) (tGLP-1) and GLP-1-[7-37] analogs modified in position 7, 8, 9 and 36, have been designed and evaluated on murine GLP-1 receptors expressed in RIN T3 cells for both their affinity and activity. Ten of the synthesized peptides were found full agonists with activities superior or at least equal to that of the native hormone. Five of them were investigated for their plasmatic stability and the most stable, [a(8)-desR(36)]GLP-1-[7-37]- NH(2) (Compound 8), evaluated in vivo in a glucose tolerance test which confirmed a clearly longer activity than that of the native hormone. We also performed circular dichroism study and propose a hypothetical structural model explaining the most part of observed activities of GLP-1 analogs on RIN T3 cells.

Review: pancreatic beta-cell neogenesis revisited

β-cell neogenesis triggers the generation of new β-cells from precursor cells. Neogenesis from duct epithelium is the most currently described and the best documented process of differentiation of precursor cells into β-cells. It is contributes not only to β-cell mass expansion during fetal and nonatal life but it is also involved in the maintenance of the β-cell mass in adults. It is also required for the increase in β-cell mass in situations of increase insulin demand (obesity, pregnancy). A large number of factors controlling the differentiation of β-cells has been identified. They are classified into the following main categories: growth factors, cytokine and inflamatory factors, and hormones such as PTHrP and GLP-1. The fact that intestinal incretin hormone GLP-1 exerts a major trophic role on pancreatic β-cells provides insights into the possibility to pharmacologically stimulate β-cell neogenesis. This could have important implications for the of treatment of type 1 and type 2 diabetes. Transdifferentiation, that is, the differentiation of already differentiated cells into β-cells, remains controversial.However, more and more studies support this concept. The cells, which can potentially “transdifferentiate” into β-cells, can belong to the pancreas (acinar cells) and even islets, or originate from extra-pancreatic tissues such as the liver. Neogenesis from intra-islet precursors also have been proposed and subpopulations of cell precursors inside islets have been described by some authors. Nestin positive cells, which have been considered as the main candidates, appear rather as progenitors of endothelial cells rather than β-cells and contribute to angiogenesis rather than neogenesis. To take advantage of the different differentiation processes may be a direction for future cellular therapies. Ultimately, a better understanding of the molecular mechanisms involved in β-cell neogenesis will allow us to use any type of differentiated and/or undifferentiated cells as a source of potential cell precursors.

Early changes in insulin secretion and action induced by high-fat diet are related to a decreased sympathetic tone

To evaluate the relationship between the development of obesity, nervous system activity, and insulin secretion and action, we tested the effect of a 2-mo high-fat diet in rats (HF rats) on glucose tolerance, glucose-induced insulin secretion (GIIS), and glucose turnover rate compared with chow-fed rats (C rats). Moreover, we measured pancreatic and hepatic norepinephrine (NE) turnover, as assessment of sympathetic tone, and performed hypothalamic microdialysis to quantify extracellular NE turnover. Baseline plasma triglyceride, free fatty acid, insulin, and glucose concentrations were similar in both groups. After 2 days of diet, GIIS was elevated more in HF than in C rats, whereas plasma glucose time course was similar. There was a significant increase in basal pancreatic NE level of HF rats, and a twofold decrease in the fractional turnover constant was observed, indicating a change in sympathetic tone. In ventromedian hypothalamus of HF rats, the decrease in NE extracellular concentration after a glucose challenge was lower compared with C rats, suggesting changes in overall activity. After 7 days, insulin hypersecretion persisted, and glucose intolerance appeared. Later (2 mo), there was no longer insulin hypersecretion, whereas glucose intolerance worsened. At all times, HF rats also displayed hepatic insulin resistance. On day 2 of HF diet, GIIS returned to normal after treatment with oxymetazoline, an alpha(2A)-adrenoreceptor agonist, thus suggesting the involvement of a low sympathetic tone in insulin hypersecretion in response to glucose in HF rats. In conclusion, the HF diet rapidly results in an increased GIIS, at least in part related to a decreased sympathetic tone, which can be the first step of a cascade of events leading to impaired glucose homeostasis.

Dynamic changes in {beta}-cell mass and pancreatic insulin during the evolution of nutrition-dependent diabetes in psammomys obesus: impact of glycemic control

Recent studies ascribe a major role to pancreatic beta-cell loss in type 2 diabetes. We investigated the dynamics of beta-cell mass during diabetes evolution in Psammomys obesus, a model for nutrition-dependent type 2 diabetes, focusing on the very early and the advanced stages of the disease. P. obesus fed a high-calorie diet for 26 days developed severe hyperglycemia, beta-cell degranulation, and markedly reduced pancreatic insulin content. Reducing calories for 7 days induced normoglycemia in 90% of the animals, restoring beta-cell granulation and insulin content. To dissociate effects of diet from blood glucose reduction, diabetic animals received phlorizin for 2 days, which normalized glycemia and increased the pancreatic insulin reserve to 50% of control, despite a calorie-rich diet. During diabetes progression, beta-cell mass decreased initially but recovered spontaneously to control levels, despite persistent hyperglycemia. Strikingly, however, beta-cell mass did not correlate with degree of hyperglycemia or pancreatic insulin content. We conclude that reduced insulin reserve is the main cause of diabetes progression, whereas irreversible beta-cell mass reduction is a late event in P. obesus. The rapid recovery of the pancreas by phlorizin-induced normoglycemia implies a causal relationship between hyperglycemia and islet dysfunction. Similar mechanisms could be operative during the evolution of type 2 diabetes in humans.

Beta oxidation in the brain is required for the effects of non-esterified fatty acids on glucose-induced insulin secretion in rats

AIMS/HYPOTHESIS

NEFA play a key role in the setting of insulin resistance and hyperinsulinaemia, which are both features of the prediabetic state. In addition to the direct effects on pancreas and peripheral tissues, NEFA have been reported to act via changes in autonomic nervous system activity. The present study was aimed at studying the effects of a local increase in NEFA in the brain on glucose-induced insulin secretion (GIIS) and on insulin action. We hypothesised that cerebral NEFA beta oxidation is a prerequisite for these central effects.

METHODS

Male Wistar rats were infused with Intralipid/heparin for 24 h through the carotid artery towards the brain (IL rats), after which we performed the GIIS test, a euglycaemic-hyperinsulinaemic clamp and c-fos immunochemistry. In another series of experiments, Intralipid/heparin infusion was coupled with lateral ventricular infusion of etomoxir, a CPT1 inhibitor, which was initiated 5 days previously.

RESULTS

During the infusion period, there were no changes in plasma NEFA, insulin or glucose concentrations. IL rats displayed an increased GIIS compared with control rats (C rats) infused with saline/heparin, and their liver insulin sensitivity was decreased. Furthermore, lipid infusion induced a significant decrease in c-fos-like immunoreactive neurons in medial hypothalamic nuclei, and an increase in lateral hypothalamus. Neuronal activation profile was almost normalised in IL rats infused with etomoxir, and GIIS was strongly decreased, possibly because of the concomitant normalisation of hepatic glucose output.

CONCLUSIONS/INTERPRETATION

These results strongly suggest that beta oxidation is required for the central effects of NEFA on GIIS.

Quantitative trait locus dissection in congenic strains of the Goto-Kakizaki rat identifies a region conserved with diabetes loci in human chromosome 1q

Genetic studies in human populations and rodent models have identified regions of human chromosome 1q21–25 and rat chromosome 2 showing evidence of significant and replicated linkage to diabetes-related phenotypes. To investigate the relationship between the human and rat diabetes loci, we fine mapped the rat locus Nidd/ gk2 linked to hyperinsulinemia in an F2 cross derived from the diabetic (type 2) Goto-Kakizaki (GK) rat and the Brown Norway (BN) control rat, and carried out its genetic and pathophysiological characterization in BN.GK congenic strains. Evidence of glucose intolerance and enhanced insulin secretion in a congenic strain allowed us to localize the underlying diabetes gene(s) in a rat chromosomal interval of ∼3–6 cM conserved with an 11-Mb region of human 1q21–23. Positional diabetes candidate genes were tested for transcriptional changes between congenics and controls and sequence variations in a panel of inbred rat strains. Congenic strains of the GK rats represent powerful novel models for accurately defining the pathophysiological impact of diabetes gene(s) at the locus Nidd/ gk2 and improving functional annotations of diabetes candidates in human 1q21–23.

Insulinotropic agent ID-1101 (4-hydroxyisoleucine) activates insulin signaling in rat

ID-1101 (4-hydroxyisoleucine), an amino acid extracted from fenugreek seeds, exhibits an interesting glucose-dependent insulin-stimulating activity. The present study was undertaken to investigate a possible extrapancreatic effect of ID-1101 on insulin signaling and action besides its previously described insulinotropic action. Insulin-sensitizing effects of ID-1101 were investigated in rat in vivo by three different approaches: 1) using euglycemic hyperinsulinemic clamps in two different rat models of insulin resistance, i.e., Zucker fa/fa rats and rats fed a sucrose-lipid diet; 2) measuring liver and muscle phosphatidylinositol (PI) 3-kinase activity after an acute injection of ID-1101 in normal and insulin-resistant diabetic rats; and 3) after chronic treatment in two rat models of insulin resistance. Euglycemic hyperinsulinemic clamp experiments revealed that ID-1101 can improve insulin resistance through an increase of peripheral glucose utilization rate in sucrose-lipid-fed rats and by decreasing hepatic glucose production in Zucker fa/fa rats. Moreover, we demonstrated that a single injection of ID-1101 activates the PI 3-kinase activity in liver and muscle from normal rats but also in muscle from diabetic rats. Finally, chronic ID-1101 treatment significantly reduced insulinemia in type 2 diabetic rats and reduced the progression of hyperinsulinemia in insulin-resistant obese Zucker fa/fa rats. These findings clearly demonstrate that ID-1101 can reduce insulin resistance through activation of the early steps of insulin signaling in peripheral tissues and in liver. In summary, ID-1101, besides its insulinotropic effect, directly improves insulin sensitivity, making it a potentially very valuable therapeutic agent for diabetes treatment.

Intrauterine hyperglycemia increases insulin binding sites but not glucose transporter expression in discrete brain areas in term rat fetuses

Diabetic pregnancy results in several metabolic and hormonal disorders, both in the embryo and the fetus of different species, including humans. Insulin is a potent modulator of brain development and is suggested to promote the differentiation and maturation of hypothalamic or related extrahypothalamic structures, which are directly involved in neural inputs to the pancreas. Because these structures are known to be specifically responsive both to insulin and glucose, we examined the effects of 48-h hyperglycemic clamps in unrestrained pregnant rats on insulin binding and glucose transporter expression in hypothalamic and extrahypothalamic-related areas of their fetal offspring. The main result was an increase in insulin binding in the ventromedial hypothalamic nucleus (VMH), the arcuate nucleus (AN), and the lateral hypothalamus (LH), and in the nucleus of the tractus solitarius (NTS) for extrahypothalamic areas (+30% in the VMH, +37% in the AN, +25.8% in the LH, and +37.3% in the NTS). The deleterious effect of brain hyperinsulinism during the late gestational stage does not seem to act through glucose transporter (GLUT) expression, inasmuch as no relationship between GLUT level and hyperinsulinism in brain areas could be observed. The specific increase in insulin binding in areas involved in the nervous control of metabolism could be a factor in the increased glucose intolerance and impairment of insulin secretion that was previously observed in the adult rats from hyperglycemic mothers.

Specific and combined effects of insulin and glucose on functional pancreatic beta-cell mass in vivo in adult rats

We investigated the specific and associated effects of insulin and glucose on beta-cell growth and function in adult rats. By combining simultaneous infusion either of glucose and/or insulin or glucose and diazoxide, three groups of rats were constituted: hyperglycemic-hyperinsulinemic rats (high glucose-high insulin), hyperglycemic-euinsulinemic rats (high glucose), and euglycemic-hyperinsulinemic rats (high insulin). All the infusions lasted 48 h. Control rats were infused with 0.9% NaCl (saline controls). In all groups, beta-cell mass was significantly increased, compared with controls (by 70% in high glucose-high insulin rats, 65% in high glucose rats, and 50% in high insulin rats). The stimulation of neogenesis was suggested by the high number of islets budding from pancreatic ducts in high glucose-high insulin and high glucose rats and by the presence of numerous clusters of few beta-cells within the exocrine pancreas in high insulin rats. beta-Cell hypertrophy was observed only in high glucose-high insulin rats. The rate of beta-cell proliferation was similar to that of controls in high glucose-high insulin rats after a 48-h glucose infusion, dropped dramatically in high insulin rats, and dropped to a lesser extent in high glucose rats. In high glucose-high insulin and high glucose rats, beta-cell mass increase was related to a higher beta-cell responsiveness to glucose in vitro as measured by islet perifusion studies, whereas in high insulin rats, no significant enhancement of glucose induced insulin secretion could be noticed. The data show that glucose and insulin may have specific stimulating effects on beta-cell growth and function in vivo in adult rats independently of the influence they exert each other on their respective plasma concentration.

Intracerebroventricular infusion of a triglyceride emulsion leads to both altered insulin secretion and hepatic glucose production in rats

We investigated here whether non-esterified fatty acids (NEFA) influence insulin secretion and action through a direct effect on central nervous system sites involved in the control of glucose homeostasis. Normal Wistar rats received a 48-h intracerebroventricular infusion of either a 10% triglyceride (Intralipid, IL)/heparin emulsion (IL/h) or saline/heparin solution (control). At 48 h, insulin secretion as measured by an intravenous glucose tolerance test, was more elevated in IL/h than in control rats. Pancreatic noradrenaline turnover was decreased by 57% in IL/h rats, suggesting low pancreatic sympathetic output that could account partly for the elevated insulin secretion. The time course of glycaemia was similar in both groups, suggesting insulin resistance. Euglycaemic-hyperinsulinaemic clamps were imposed to assess peripheral and hepatic insulin sensitivity. At each insulin concentration glucose utilization was increased to a similar extent in both groups, whereas hepatic glucose production decreased much less in IL/h than in control rats. Hepatic insulin insensitivity could be related partly to activation of the hypothalamic-pituitary-adrenocortical axis, since plasma corticosterone concentration was significantly increased in IL/h rats compared with controls. Our data indicate that lipids may alter both insulin secretion and hepatic sensitivity to insulin through their effect on central nervous system.

Pancreatic beta-cell alpha2A adrenoceptor and phospholipid changes in hyperlipidemic rats

We previously showed that a 48-h intravenous lipid infusion in rats induces pancreatic beta-cell hypersensitivity to catecholamines. Our aim was to study the lipid-related changes that may account for such hypersensitivity in pancreatic islets. We show here that a 48-h increase in plasma FFA alters the binding characteristics of beta-cell alpha2 adrenoceptors in rats. Lipid infusion decreases pancreatic norepinephrine (NE) turnover rate by 28%, reflecting a reduction of pancreatic NE stores. Following lipid infusion, the density of alpha2 adrenoceptor binding sites is significantly lower and receptor affinity higher, both in islet homogenates (by three- and fivefold, respectively) and isolated whole beta-cells (by two- and sixfold, respectively). These changes correlate with the elevated insulin response to glucose found in lipid-infused rats. We also found a modification of islet phospholipid content, particularly in phosphoethanolamine species containing infused FA such as palmitate, oleate, stearate, and linoleate. This may account for the modifications in receptor affinity. These results suggest that hyperlipidemia-associated pathologies such as diabetes and obesity not only may result from alterations of metabolic pathways but also may be a consequence of early modifications in nervous firing rates and signal transduction pathways.

Glucose-induced insulin hypersecretion in lipid-infused healthy subjects is associated with a decrease in plasma norepinephrine concentration and urinary excretion

We investigated the effect of a 48 h triglyceride infusion on the subsequent insulin secretion in response to glucose in healthy men. We measured the variations in plasma concentration and urinary excretion of catecholamines as an indirect estimation of sympathetic tone. For 48 h, 20 volunteers received a triglyceride/heparin or a saline solution, separated by a 1-month interval. At time 48 h, insulin secretion in response to glucose was investigated by a single iv glucose injection (0.5 g/kg(-1)) followed by an hyperglycemic clamp (10 mg.kg(-1).min(-1), during 50 min). The triglyceride infusion resulted in a 3-fold elevation in plasma free fatty acids and an increase in insulin and C-peptide plasma concentrations (1.5- and 2.5-fold, respectively, P < 0.05), compared with saline. At time 48 h of lipid infusion, plasma norepinephrine (NE) concentration and urinary excretion levels were lowered compared with saline (plasma NE: 0.65 +/- 0.08 vs. 0.42 +/- 0.06 ng/ml, P < 0.05; urinary excretion: 800 +/- 70 vs. 620 +/- 25 nmol/24 h, P < 0.05). In response to glucose loading, insulin and C-peptide plasma concentrations were higher in lipid compared with saline infusion (plasma insulin: 600 +/- 98 vs. 310 +/- 45 pM, P < 0.05; plasma C-peptide 3.5 +/- 0.2 vs. 1.7 +/- 0.2 nM, P < 0.05). In conclusion, in healthy subjects, a 48-h lipid infusion induces basal hyperinsulinemia and exaggerated insulin secretion in response to glucose which may be partly related to a decrease in sympathetic tone.

Sialylated form of the neural cell adhesion molecule (NCAM): a new tool for the identification and sorting of beta-cell subpopulations with different functional activity

To clarify the relationship between variations in beta-cell mass and pancreatic function, we investigated the possibility to analyze, quantify, and sort beta-cell subpopulations with different functional maturity. To this aim, we tested the reliability of the sialylated form of neural cell adhesion molecule (NCAM) (PSA-NCAM) as a marker of beta-cell functional activity. Islet cells isolated from adult rats were analyzed for their PSA-NCAM abundance using an anti-PSA-NCAM antibody. We found that PSA-NCAM is expressed only in beta-cells. The PSA-NCAM labeling was also studied with a fluorescence-activated cell sorter. We showed that the beta-cell population is heterogeneous for PSA-NCAM labeling. To directly determine the relationship between PSA-NCAM labeling and beta-cell activity, in vitro insulin secretion studies were performed on sorted beta-cell subpopulations using a perifusion technique. Two beta-cell subpopulations were analyzed: one that was highly labeled for PSA-NCAM and another that was poorly labeled. Insulin secretion from high PSA-NCAM-labeled beta-cells was significantly higher than that in low PSA-NCAM-labeled beta-cells. This differential expression in the beta-cell population was well correlated with differences in glucose responsiveness. PSA-NCAM seems thus suitable for use as a tool to identify beta-cell subpopulations according to their glucose responsiveness.