The autonomic nervous system regulates pancreatic ß-cell proliferation in adult male rats.

The pancreatic ß-cell responds to changes in the nutrient environment to maintain glucose homeostasis by adapting its function and mass. Nutrients can act directly on the ß-cell and also indirectly through the brain via autonomic nerves innervating islets. Despite the importance of the brain-islet axis in insulin secretion, relatively little is known regarding its involvement in ß-cell proliferation. We previously demonstrated that prolonged infusions of nutrients in rats provoke a dramatic increase in ß-cell proliferation in part because of the direct action of nutrients. Here, we addressed the contribution of the autonomic nervous system. In isolated islets, muscarinic stimulation increased, whereas adrenergic stimulation decreased, glucose-induced ß-cell proliferation. Blocking α-adrenergic receptors reversed the effect of epinephrine on glucose + nonesterified fatty acids (NEFA)-induced ß-cell proliferation, whereas activation of ß-adrenergic receptors was without effect. Infusion of glucose + NEFA toward the brain stimulated ß-cell proliferation, and this effect was abrogated following celiac vagotomy. The increase in ß-cell proliferation following peripheral infusions of glucose + NEFA was not inhibited by vagotomy or atropine treatment but was blocked by coinfusion of epinephrine. We conclude that ß-cell proliferation is stimulated by parasympathetic and inhibited by sympathetic signals. Whereas glucose + NEFA in the brain stimulates ß-cell proliferation through the vagus nerve, ß-cell proliferation in response to systemic nutrient excess does not involve parasympathetic signals but may be associated with decreased sympathetic tone.

Glucose and fatty acids synergistically and reversibly promote beta cell proliferation in rats.

AIMS/HYPOTHESIS: The mechanisms underlying pancreatic islet mass expansion have attracted considerable interest as potential therapeutic targets to prevent or delay the onset of type 2 diabetes. While several factors promoting beta cell proliferation have been identified, in the context of nutrient excess the roles of glucose or NEFA in relation to insulin resistance remain unclear. Here we tested the hypothesis that glucose and NEFA synergistically and reversibly promote beta cell proliferation in the context of nutrient-induced insulin resistance. METHODS: Using 72 h infusions of glucose (GLU) or the oleate-enriched lipid emulsion ClinOleic (CLI), singly or in combination, we assessed beta cell proliferation, islet mass and insulin sensitivity in male Lewis rats. The effects of nutrients and endogenous circulating factors were examined in isolated and transplanted islets. Reversibility was studied 3 and 6 days after the end of the infusion. RESULTS: GLU infusions modestly stimulated beta cell proliferation, CLI alone had no effect and GLU+CLI infusions markedly stimulated beta cell proliferation. Insulin sensitivity was equally decreased in GLU and GLU+CLI infusions. GLU+CLI infusions also stimulated beta cell proliferation in islets transplanted under the kidney capsule, albeit to a lesser extent compared with endogenous islets. Ex vivo, the combination of glucose and NEFA enhanced beta cell proliferation in rat and human islets independently from secreted insulin, and serum from GLU+CLI-infused rats potentiated the effect of glucose. Glucose tolerance, beta cell proliferation and islet mass were all restored to normal levels 6 days after termination of the infusion. CONCLUSIONS/INTERPRETATION: Glucose and NEFA synergistically and reversibly promote beta cell proliferation in part via direct action on the beta cell and independently from secreted insulin.

Lipoprotein lipase in hypothalamus is a key regulator of body weight gain and glucose homeostasis in mice.

AIMS/HYPOTHESIS: Regulation of energy balance involves the participation of many factors, including nutrients, among which are circulating lipids, acting as peripheral signals informing the central nervous system of the energy status of the organism. It has been shown that neuronal lipoprotein lipase (LPL) participates in the control of energy balance by hydrolysing lipid particles enriched in triacylglycerols. Here, we tested the hypothesis that LPL in the mediobasal hypothalamus (MBH), a well-known nucleus implicated in the regulation of metabolic homeostasis, could also contribute to the regulation of body weight and glucose homeostasis. METHODS: We injected an adeno-associated virus (AAV) expressing Cre-green fluorescent protein into the MBH of Lpl-floxed mice (and wild-type mice) to specifically decrease LPL activity in the MBH. In parallel, we injected an AAV overexpressing Lpl into the MBH of wild-type mice. We then studied energy homeostasis and hypothalamic ceramide content. RESULTS: The partial deletion of Lpl in the MBH in mice led to an increase in body weight compared with controls (37.72 ± 0.7 g vs 28.46 ± 0.12, p < 0.001) associated with a decrease in locomotor activity. These mice developed hyperinsulinaemia and glucose intolerance. This phenotype also displayed reduced expression of Cers1 in the hypothalamus as well as decreased concentration of several C18 species of ceramides and a 3-fold decrease in total ceramide intensity. Conversely, overexpression of Lpl specifically in the MBH induced a decrease in body weight. CONCLUSIONS/INTERPRETATION: Our study shows that LPL in the MBH is an important regulator of body weight and glucose homeostasis.

High-fat diet-induced ß-cell proliferation occurs prior to insulin resistance in C57Bl/6J male mice.

Both short- (1 wk) and long-term (2-12 mo) high-fat diet (HFD) studies reveal enhanced ß-cell mass due to increased ß-cell proliferation. ß-Cell proliferation following HFD has been postulated to occur in response to insulin resistance; however, whether HFD can induce ß-cell proliferation independent of insulin resistance has been controversial. To examine the kinetics of HFD-induced ß-cell proliferation and its correlation with insulin resistance, we placed 8-wk-old male C57Bl/6J mice on HFD for different lengths of time and assayed the following: glucose tolerance, insulin secretion in response to glucose, insulin tolerance, ß-cell mass, and ß-cell proliferation. We found that ß-cell proliferation was significantly increased after only 3 days of HFD feeding, weeks before an increase in ß-cell mass or peripheral insulin resistance was detected. These results were confirmed by hyperinsulinemic euglycemic clamps and measurements of α-hydroxybutyrate, a plasma biomarker of insulin resistance in humans. An increase in expression of key islet-proliferative genes was found in isolated islets from 1-wk HFD-fed mice compared with chow diet (CD)-fed mice. These data indicate that short-term HFD feeding enhances ß-cell proliferation before insulin resistance becomes apparent.

[Role of brain lipid sensing in nervous regulation of energy balance]. / Rôle de la détection centrale des lipides dans le contrôle nerveux de la balance énergétique.

Fatty acid sensitive neurons located in hypothalamus, hippocampus or striatum are able to detect daily variations of plasma fatty acid levels. Thus, these neurons play a role to regulate energy balance by controling food intake, insulin secretion or hepatic glucose production. Molecular mechanisms that mediate fatty acid effects include receptor FAT (fatty acid transporter)/CD36. Deregulation of this brain lipid sensing may be an early event leading to further dysfunction of energy balance leading to obesity and type 2 diabetes.

Maternal Protein Restriction in Rats Alters Postnatal Growth and Brain Lipid Sensing in Female Offspring.

Perinatal nutrition is a key player in the susceptibility to developing metabolic diseases in adulthood, leading to the concept of "metabolic programming". The aim of this study was to assess the impact of maternal protein restriction during gestation and lactation on glucose homeostasis and eating behaviour in female offspring. Pregnant rats were fed a normal or protein-restricted (PR) diet and followed throughout gestation and lactation. Body weight, glucose homeostasis, and eating behaviour were evaluated in offspring, especially in females. Body weight gain was lower in PR dams during lactation only, despite different food and water intakes throughout gestation and lactation. Plasma concentration of leptin, adiponectin and triglycerides increased drastically before delivery in PR dams in relation to fat deposits. Although all pups had identical birth body weight, PR offspring body weight differed from control offspring around postnatal day 10 and remained lower until adulthood. Offspring glucose homeostasis was mildly impacted by maternal PR, although insulin secretion was reduced for PR rats at adulthood. Food intake, satiety response, and cerebral activation were examined after a lipid preload and demonstrated some differences between the two groups of rats. Maternal PR during gestation and lactation does induce extrauterine growth restriction, accompanied by alterations in maternal plasma leptin and adiponectin levels, which may be involved in programming the alterations in eating behaviour observed in females at adulthood.

Antimicrobial protein REG3A regulates glucose homeostasis and insulin resistance in obese diabetic mice.

Innate immune mediators of pathogen clearance, including the secreted C-type lectins REG3 of the antimicrobial peptide (AMP) family, are known to be involved in the regulation of tissue repair and homeostasis. Their role in metabolic homeostasis remains unknown. Here we show that an increase in human REG3A improves glucose and lipid homeostasis in nutritional and genetic mouse models of obesity and type 2 diabetes. Mice overexpressing REG3A in the liver show improved glucose homeostasis, which is reflected in better insulin sensitivity in normal weight and obese states. Delivery of recombinant REG3A protein to leptin-deficient ob/ob mice or wild-type mice on a high-fat diet also improves glucose homeostasis. This is accompanied by reduced oxidative protein damage, increased AMPK phosphorylation and insulin-stimulated glucose uptake in skeletal muscle tissue. Oxidative damage in differentiated C2C12 myotubes is greatly attenuated by REG3A, as is the increase in gp130-mediated AMPK activation. In contrast, Akt-mediated insulin action, which is impaired by oxidative stress, is not restored by REG3A. These data highlight the importance of REG3A in controlling oxidative protein damage involved in energy and metabolic pathways during obesity and diabetes, and provide additional insight into the dual function of host-immune defense and metabolic regulation for AMP.

Autonomic control of pancreatic beta cells: What is known on the regulation of insulin secretion and beta-cell proliferation in rodents and humans.

Insulin secretion and pancreatic beta-cell proliferation are tightly regulated by several signals such as hormones, nutrients, and neurotransmitters. However, the autonomic control of beta cells is not fully understood. In this review, we describe mechanisms involved in insulin secretion as well as metabolic and mitogenic actions on its target tissues. Since pancreatic islets are physically connected to the brain by nerves, parasympathetic and sympathetic neurotransmitters can directly potentiate or repress insulin secretion and beta-cell proliferation. Finally, we highlight the role of the autonomic nervous system in metabolic diseases such as diabetes and obesity.

Very-Long-Chain Unsaturated Sphingolipids Mediate Oleate-Induced Rat ß-Cell Proliferation.

Fatty acid (FA) signaling contributes to ß-cell mass expansion in response to nutrient excess, but the underlying mechanisms are poorly understood. In the presence of elevated glucose, FA metabolism is shifted toward synthesis of complex lipids, including sphingolipids. Here, we tested the hypothesis that sphingolipids are involved in the ß-cell proliferative response to FA. Isolated rat islets were exposed to FA and 16.7 mmol/L glucose for 48-72 h, and the contribution of the de novo sphingolipid synthesis pathway was tested using the serine palmitoyltransferase inhibitor myriocin, the sphingosine kinase (SphK) inhibitor SKI II, or knockdown of SphK, fatty acid elongase 1 (ELOVL1) and acyl-CoA-binding protein (ACBP). Rats were infused with glucose and the lipid emulsion ClinOleic and received SKI II by gavage. ß-Cell proliferation was assessed by immunochemistry or flow cytometry. Sphingolipids were analyzed by liquid chromatography-tandem mass spectrometry. Among the FAs tested, only oleate increased ß-cell proliferation. Myriocin, SKI II, and SphK knockdown all decreased oleate-induced ß-cell proliferation. Oleate exposure did not increase the total amount of sphingolipids but led to a specific rise in 24:1 species. Knockdown of ACBP or ELOVL1 inhibited oleate-induced ß-cell proliferation. We conclude that unsaturated very-long-chain sphingolipids produced from the available C24:1 acyl-CoA pool mediate oleate-induced ß-cell proliferation in rats.

Maternal Protein Restriction in Rats Alters the Expression of Genes Involved in Mitochondrial Metabolism and Epitranscriptomics in Fetal Hypothalamus.

Fetal brain development is closely dependent on maternal nutrition and metabolic status. Maternal protein restriction (PR) is known to be associated with alterations in the structure and function of the hypothalamus, leading to impaired control of energy homeostasis and food intake. The objective of this study was to identify the cellular and molecular systems underlying these effects during fetal development. We combined a global transcriptomic analysis on the fetal hypothalamus from a rat model of maternal PR with in vitro neurosphere culture and cellular analyses. Several genes encoding proteins from the mitochondrial respiratory chain complexes were overexpressed in the PR group and mitochondrial metabolic activity in the fetal hypothalamus was altered. The level of the N6-methyladenosine epitranscriptomic mark was reduced in the PR fetuses, and the expression of several genes involved in the writing/erasing/reading of this mark was indeed altered, as well as genes encoding several RNA-binding proteins. Additionally, we observed a higher number of neuronal-committed progenitors at embryonic day 17 (E17) in the PR fetuses. Together, these data strongly suggest a metabolic adaptation to the amino acid shortage, combined with the post-transcriptional control of protein expression, which might reflect alterations in the control of the timing of neuronal progenitor differentiation.

Nutrient regulation of pancreatic ß-cell proliferation.

Excess consumption of energy-dense foods combined with a sedentary lifestyle is driving an obesity epidemic. Although obesity is closely associated with insulin resistance, most individuals meet the insulin demand by increasing their functional ß-cell mass. Those who eventually develop type 2 diabetes are distinguished by a failure in this compensatory process. Although a causal role of insulin resistance in compensatory ß-cell responses has received considerable experimental support, precisely how the ß cell senses changes in the metabolic environment is still unknown. As metabolism of glucose, lipids and amino acids is profoundly altered in obesity, it is not surprising that these nutrients are conspicuous among the factors proposed to contribute. In this review we summarise our understanding of the role of nutrients, in particular glucose, fatty acids and amino acids in ß-cell compensation with a particular emphasis on their relation to insulin resistance-induced factors and their underlying mechanism of action. Finally, we describe the concept of epigenetic programming and review recent studies illustrating how the status of the ß cell epigenome is a product of its nutrient environment, and how metabolic programming of the ß cell contributes to diabetes risk.

Disruption of Lipid Uptake in Astroglia Exacerbates Diet-Induced Obesity.

Neuronal circuits in the brain help to control feeding behavior and systemic metabolism in response to afferent nutrient and hormonal signals. Although astrocytes have historically been assumed to have little relevance for such neuroendocrine control, we investigated whether lipid uptake via lipoprotein lipase (LPL) in astrocytes is required to centrally regulate energy homeostasis. Ex vivo studies with hypothalamus-derived astrocytes showed that LPL expression is upregulated by oleic acid, whereas it is decreased in response to palmitic acid or triglycerides. Likewise, astrocytic LPL deletion reduced the accumulation of lipid droplets in those glial cells. Consecutive in vivo studies showed that postnatal ablation of LPL in glial fibrillary acidic protein-expressing astrocytes induced exaggerated body weight gain and glucose intolerance in mice exposed to a high-fat diet. Intriguingly, astrocytic LPL deficiency also triggered increased ceramide content in the hypothalamus, which may contribute to hypothalamic insulin resistance. We conclude that hypothalamic LPL functions in astrocytes to ensure appropriately balanced nutrient sensing, ceramide distribution, body weight regulation, and glucose metabolism.

The regulator of G-protein signaling RGS16 promotes insulin secretion and ß-cell proliferation in rodent and human islets.

OBJECTIVE: G protein-coupled receptor (GPCR) signaling regulates insulin secretion and pancreatic ß cell-proliferation. While much knowledge has been gained regarding how GPCRs are activated in ß cells, less is known about the mechanisms controlling their deactivation. In many cell types, termination of GPCR signaling is controlled by the family of Regulators of G-protein Signaling (RGS). RGS proteins are expressed in most eukaryotic cells and ensure a timely return to the GPCR inactive state upon removal of the stimulus. The aims of this study were i) to determine if RGS16, the most highly enriched RGS protein in ß cells, regulates insulin secretion and ß-cell proliferation and, if so, ii) to elucidate the mechanisms underlying such effects. METHODS: Mouse and human islets were infected with recombinant adenoviruses expressing shRNA or cDNA sequences to knock-down or overexpress RGS16, respectively. 60 h post-infection, insulin secretion and cAMP levels were measured in static incubations in the presence of glucose and various secretagogues. ß-cell proliferation was measured in infected islets after 72 h in the presence of 16.7 mM glucose ± somatostatin and various inhibitors. RESULTS: RGS16 mRNA levels are strongly up-regulated in islets of Langerhans under hyperglycemic conditions in vivo and ex vivo. RGS16 overexpression stimulated glucose-induced insulin secretion in isolated mouse and human islets while, conversely, insulin secretion was impaired following RGS16 knock-down. Insulin secretion was no longer affected by RGS16 knock-down when islets were pre-treated with pertussis toxin to inactivate Gαi/o proteins, or in the presence of a somatostatin receptor antagonist. RGS16 overexpression increased intracellular cAMP levels, and its effects were blocked by an adenylyl cyclase inhibitor. Finally, RGS16 overexpression prevented the inhibitory effect of somatostatin on insulin secretion and ß-cell proliferation. CONCLUSIONS: Our results identify RGS16 as a novel regulator of ß-cell function that coordinately controls insulin secretion and proliferation by limiting the tonic inhibitory signal exerted by δ-cell-derived somatostatin in islets.

Urea impairs ß cell glycolysis and insulin secretion in chronic kidney disease.

Disorders of glucose homeostasis are common in chronic kidney disease (CKD) and are associated with increased mortality, but the mechanisms of impaired insulin secretion in this disease remain unclear. Here, we tested the hypothesis that defective insulin secretion in CKD is caused by a direct effect of urea on pancreatic ß cells. In a murine model in which CKD is induced by 5/6 nephrectomy (CKD mice), we observed defects in glucose-stimulated insulin secretion in vivo and in isolated islets. Similarly, insulin secretion was impaired in normal mouse and human islets that were cultured with disease-relevant concentrations of urea and in islets from normal mice treated orally with urea for 3 weeks. In CKD mouse islets as well as urea-exposed normal islets, we observed an increase in oxidative stress and protein O-GlcNAcylation. Protein O-GlcNAcylation was also observed in pancreatic sections from CKD patients. Impairment of insulin secretion in both CKD mouse and urea-exposed islets was associated with reduced glucose utilization and activity of phosphofructokinase 1 (PFK-1), which could be reversed by inhibiting O-GlcNAcylation. Inhibition of O-GlcNAcylation also restored insulin secretion in both mouse models. These results suggest that insulin secretory defects associated with CKD arise from elevated circulating levels of urea that increase islet protein O-GlcNAcylation and impair glycolysis.

Hippocampal lipoprotein lipase regulates energy balance in rodents.

Brain lipid sensing is necessary to regulate energy balance. Lipoprotein lipase (LPL) may play a role in this process. We tested if hippocampal LPL regulated energy homeostasis in rodents by specifically attenuating LPL activity in the hippocampus of rats and mice, either by infusing a pharmacological inhibitor (tyloxapol), or using a genetic approach (adeno-associated virus expressing Cre-GFP injected into Lpl (lox/lox) mice). Decreased LPL activity by either method led to increased body weight gain due to decreased locomotor activity and energy expenditure, concomitant with increased parasympathetic tone (unchanged food intake). Decreased LPL activity in both models was associated with increased de novo ceramide synthesis and neurogenesis in the hippocampus, while intrahippocampal infusion of de novo ceramide synthesis inhibitor myriocin completely prevented body weight gain. We conclude that hippocampal lipid sensing might represent a core mechanism for energy homeostasis regulation through de novo ceramide synthesis.

Fatty acid transporter CD36 mediates hypothalamic effect of fatty acids on food intake in rats.

Variations in plasma fatty acid (FA) concentrations are detected by FA sensing neurons in specific brain areas such as the hypothalamus. These neurons play a physiological role in the control of food intake and the regulation of hepatic glucose production. Le Foll et al. previously showed in vitro that at least 50% of the FA sensing in ventromedial hypothalamic (VMH) neurons is attributable to the interaction of long chain FA with FA translocase/CD36 (CD36). The present work assessed whether in vivo effects of hypothalamic FA sensing might be partly mediated by CD36 or intracellular events such as acylCoA synthesis or ß-oxidation. To that end, a catheter was implanted in the carotid artery toward the brain in male Wistar rats. After 1 wk recovery, animals were food-deprived for 5 h, then 10 min infusions of triglyceride emulsion, Intralipid +/- heparin (IL, IL(H), respectively) or saline/heparin (SH) were carried out and food intake was assessed over the next 5 h. Experimental groups included: 1) Rats previously injected in ventromedian nucleus (VMN) with shRNA against CD36 or scrambled RNA; 2) Etomoxir (CPT1 inhibitor) or saline co-infused with IL(H)/S(H); and 3) Triacsin C (acylCoA synthase inhibitor) or saline co-infused with IL(H)/S(H). IL(H) significantly lowered food intake during refeeding compared to S(H) (p<0.001). Five hours after refeeding, etomoxir did not affect this inhibitory effect of IL(H) on food intake while VMN CD36 depletion totally prevented it. Triacsin C also prevented IL(H) effects on food intake. In conclusion, the effect of FA to inhibit food intake is dependent on VMN CD36 and acylCoA synthesis but does not required FA oxidation.

The multiple roles of fatty acid handling proteins in brain.

Lipids are essential components of a living organism as energy source but also as constituent of the membrane lipid bilayer. In addition fatty acid (FA) derivatives interact with many signaling pathways. FAs have amphipathic properties and therefore require being associated to protein for both transport and intracellular trafficking. Here we will focus on several FA handling proteins, among which the fatty acid translocase/CD36 (FAT/CD36), members of fatty acid transport proteins (FATPs), and lipid chaperones fatty acid-binding proteins (FABPs). A decade of extensive studies has helped decipher the mechanism of action of these proteins in peripheral tissue with high lipid metabolism. However, considerably less information is available regarding their role in the brain, despite the high lipid content of this tissue. This review will primarily focus on the recent studies that have highlighted the crucial role of lipid handling proteins in brain FA transport, neuronal differentiation and development, cognitive processes and brain diseases. Finally a special focus will be made on the recent studies that have revealed the role of FAT/CD36 in brain lipid sensing and nervous control of energy balance.