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Evidence continues to emerge detailing the myriad of ways the gut microbiota influences host energy homeostasis. Among the potential mechanisms, short chain fatty acids (SCFAs), the byproducts of microbial fermentation of dietary fibers, exhibit correlative beneficial metabolic effects in humans and rodents, including improvements in glucose homeostasis. The underlying mechanisms, however, remain elusive. We here report that one of the main bacterially produced SCFAs, propionate, activates ileal mucosal free fatty acid receptor 2 to trigger a negative feedback pathway to lower hepatic glucose production in healthy rats in vivo We further demonstrate that an ileal glucagon-like peptide-1 receptor-dependent neuronal network is necessary for ileal propionate and long chain fatty acid sensing to regulate glucose homeostasis. These findings highlight the potential to manipulate fatty acid sensing machinery in the ileum to regulate glucose homeostasis., (© 2016 by The American Society for Biochemistry and Molecular Biology, Inc.)

Emerging evidence suggests the gastrointestinal tract plays an important glucoregulatory role. In this perspective, we first review how the intestine senses ingested nutrients, initiating crucial negative feedback mechanisms through a gut-brain neuronal axis to regulate glycemia, mainly via reduction in hepatic glucose production. We then highlight how intestinal energy sensory mechanisms are responsible for the glucose-lowering effects of bariatric surgery, specifically duodenal-jejunal bypass, and the antidiabetic agents metformin and resveratrol. A better understanding of these pathways lays the groundwork for intestinally targeted drug therapy for the treatment of diabetes., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Metformin is a first-line therapeutic option for the treatment of type 2 diabetes, even though its underlying mechanisms of action are relatively unclear. Metformin lowers blood glucose levels by inhibiting hepatic glucose production (HGP), an effect originally postulated to be due to a hepatic AMP-activated protein kinase (AMPK)-dependent mechanism. However, studies have questioned the contribution of hepatic AMPK to the effects of metformin on lowering hyperglycemia, and a gut-brain-liver axis that mediates intestinal nutrient- and hormone-induced lowering of HGP has been identified. Thus, it is possible that metformin affects HGP through this inter-organ crosstalk. Here we show that intraduodenal infusion of metformin for 50 min activated duodenal mucosal Ampk and lowered HGP in a rat 3 d high fat diet (HFD)-induced model of insulin resistance. Inhibition of duodenal Ampk negated the HGP-lowering effect of intraduodenal metformin, and both duodenal glucagon-like peptide-1 receptor (Glp-1r)-protein kinase A (Pka) signaling and a neuronal-mediated gut-brain-liver pathway were required for metformin to lower HGP. Preabsorptive metformin also lowered HGP in rat models of 28 d HFD-induced obesity and insulin resistance and nicotinamide (NA)-streptozotocin (STZ)-HFD-induced type 2 diabetes. In an unclamped setting, inhibition of duodenal Ampk reduced the glucose-lowering effects of a bolus metformin treatment in rat models of diabetes. These findings show that, in rat models of both obesity and diabetes, metformin activates a previously unappreciated duodenal Ampk-dependent pathway to lower HGP and plasma glucose levels.

Fat-induced hepatic insulin resistance plays a key role in the pathogenesis of type 2 diabetes in obese individuals. Although PKC and inflammatory pathways have been implicated in fat-induced hepatic insulin resistance, the sequence of events leading to impaired insulin signaling is unknown. We used Wistar rats to investigate whether PKCδ and oxidative stress play causal roles in this process and whether this occurs via IKKβ- and JNK-dependent pathways. Rats received a 7-h infusion of Intralipid plus heparin (IH) to elevate circulating free fatty acids (FFA). During the last 2 h of the infusion, a hyperinsulinemic-euglycemic clamp with tracer was performed to assess hepatic and peripheral insulin sensitivity. An antioxidant, N-acetyl-L-cysteine (NAC), prevented IH-induced hepatic insulin resistance in parallel with prevention of decreased IκBα content, increased JNK phosphorylation (markers of IKKβ and JNK activation, respectively), increased serine phosphorylation of IRS-1 and IRS-2, and impaired insulin signaling in the liver without affecting IH-induced hepatic PKCδ activation. Furthermore, an antisense oligonucleotide against PKCδ prevented IH-induced phosphorylation of p47(phox) (marker of NADPH oxidase activation) and hepatic insulin resistance. Apocynin, an NADPH oxidase inhibitor, prevented IH-induced hepatic and peripheral insulin resistance similarly to NAC. These results demonstrate that PKCδ, NADPH oxidase, and oxidative stress play a causal role in FFA-induced hepatic insulin resistance in vivo and suggest that the pathway of FFA-induced hepatic insulin resistance is FFA → PKCδ → NADPH oxidase and oxidative stress → IKKβ/JNK → impaired hepatic insulin signaling., (Copyright © 2014 the American Physiological Society.)

The fat-derived hormone leptin binds to its hypothalamic receptors to regulate glucose homeostasis. Leptin is also synthesized in the stomach and subsequently binds to its receptors expressed in the intestine, although the functional relevance of such activation remains largely unknown. We report here that intrajejunal leptin administration activates jejunal leptin receptors and signals through a phosphatidylinositol 3-kinase (PI3K)-dependent and signal transducer and activator of transcription 3 (STAT3)-independent signaling pathway to lower glucose production in healthy rodents. Jejunal leptin action is sufficient to lower glucose production in uncontrolled diabetic and high-fat-fed rodents and contributes to the early antidiabetic effect of duodenal-jejunal bypass surgery. These data unveil a glucoregulatory site of leptin action and suggest that enhancing leptin-PI3K signaling in the jejunum lowers plasma glucose concentrations in diabetes., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The small intestine is traditionally viewed as an organ that mediates nutrient digestion and absorption. This view has recently been revised owing to the ability of the duodenum to sense nutrient influx and trigger negative feedback loops to inhibit glucose production and food intake to maintain metabolic homeostasis. Further, duodenal nutrient-sensing defects are acquired in diabetes and obesity, leading to increased glucose production. In contrast, jejunal nutrient sensing inhibits glucose production and mediates the early antidiabetic effect of bariatric surgery, and gut microbiota composition may alter intestinal nutrient-sensing mechanisms to regain better control of glucose homeostasis in diabetes and obesity in the long term. This perspective highlights nutrient-sensing mechanisms in the gut that regulate glucose homeostasis and the potential of targeting gut nutrient-sensing mechanisms as a therapeutic strategy to lower blood glucose concentrations in diabetes.

Glucagon activates hepatic protein kinase A (PKA) to increase glucose production, but the gluco-stimulatory effect is transient even in the presence of continuous intravenous glucagon infusion. Continuous intravenous infusion of insulin, however, inhibits glucose production through its sustained actions in both the liver and the mediobasal hypothalamus (MBH). In a pancreatic clamp setting, MBH infusion with glucagon activated MBH PKA and inhibited hepatic glucose production (HGP) in rats, as did central glucagon infusion in mice. Inhibition of glucagon receptor-PKA signaling in the MBH and hepatic vagotomy each negated the effect of MBH glucagon in rats, whereas the central effect of glucagon was diminished in glucagon receptor knockout mice. A sustained rise in plasma glucagon concentrations transiently increased HGP, and this transiency was abolished in rats with negated MBH glucagon action. In a nonclamp setting, MBH glucagon infusion improved glucose tolerance, and inhibition of glucagon receptor-PKA signaling in the MBH enhanced the ability of intravenous glucagon injection to increase plasma glucose concentrations. We also detected a similar enhancement of glucose concentrations that was associated with a disruption in MBH glucagon signaling in rats fed a high-fat diet. We show that hypothalamic glucagon signaling inhibits HGP and suggest that hypothalamic glucagon resistance contributes to hyperglycemia in diabetes and obesity.

The metabolism of lactate to pyruvate in the mediobasal hypothalamus (MBH) regulates hepatic glucose production. Because astrocytes and neurons are functionally linked by metabolic coupling through lactate transfer via the astrocyte-neuron lactate shuttle (ANLS), we reasoned that astrocytes might be involved in the hypothalamic regulation of glucose metabolism. To examine this possibility, we used the gluconeogenic amino acid proline, which is metabolized to pyruvate in astrocytes. Our results showed that increasing the availability of proline in rats either centrally (MBH) or systemically acutely lowered blood glucose. Pancreatic clamp studies revealed that this hypoglycemic effect was due to a decrease of hepatic glucose production secondary to an inhibition of glycogenolysis, gluconeogenesis, and glucose-6-phosphatase flux. The effect of proline was mimicked by glutamate, an intermediary of proline metabolism. Interestingly, proline's action was markedly blunted by pharmacological inhibition of hypothalamic lactate dehydrogenase (LDH) suggesting that metabolic flux through LDH was required. Furthermore, short hairpin RNA-mediated knockdown of hypothalamic LDH-A, an astrocytic component of the ANLS, also blunted the glucoregulatory action of proline. Thus our studies suggest not only a new role for proline in the regulation of hepatic glucose production but also indicate that hypothalamic astrocytes are involved in the regulatory mechanism as well.

Insulin activates PI3-kinase (PI3K)/AKT to regulate glucose homeostasis in the peripheral tissues and the mediobasal hypothalamus (MBH) of rodents. We report that insulin infusion into the MBH or dorsal vagal complex (DVC) activated insulin receptors. The same dose of insulin that activated MBH PI3K/AKT did not in the DVC. DVC insulin instead activated Erk1/2 and lowered glucose production in rats and mice. Molecular and chemical inhibition of DVC Erk1/2 negated, while activation of DVC Erk1/2 recapitulated, the effects of DVC insulin. Circulating insulin failed to inhibit glucose production when DVC Erk1/2 was inhibited in normal rodents, while DVC insulin action was disrupted in high-fat-fed rodents. Activation of DVC ATP-sensitive potassium channels was necessary for insulin-Erk1/2 and sufficient to inhibit glucose production in normal and high-fat-fed rodents. DVC is a site of insulin action where insulin triggers Erk1/2 signaling to inhibit glucose production and of insulin resistance in high-fat feeding., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Background & Aims: The duodenum senses nutrients to maintain energy and glucose homeostasis, but little is known about the signaling and neuronal mechanisms involved. We tested whether duodenal activation of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase A (PKA) is sufficient and necessary for cholecystokinin (CCK) signaling to trigger vagal afferent firing and regulate glucose production., Methods: In rats, we selectively activated duodenal PKA and evaluated changes in glucose kinetics during the pancreatic (basal insulin) pancreatic clamps and vagal afferent firing. The requirement of duodenal PKA signaling in glucose regulation was evaluated by inhibiting duodenal activation of PKA in the presence of infusion of the intraduodenal PKA agonist (Sp-cAMPS) or CCK1 receptor agonist (CCK-8). We also assessed the involvement of a neuronal network and the metabolic impact of duodenal PKA activation in rats placed on high-fat diets., Results: Intraduodenal infusion of Sp-cAMPS activated duodenal PKA and lowered glucose production, in association with increased vagal afferent firing in control rats. The metabolic and neuronal effects of duodenal Sp-cAMPS were negated by coinfusion with either the PKA inhibitor H89 or Rp-CAMPS. The metabolic effect was also negated by coinfusion with tetracaine, molecular and pharmacologic inhibition of NR1-containing N-methyl-d-aspartate (NMDA) receptors within the dorsal vagal complex, or hepatic vagotomy in rats. Inhibition of duodenal PKA blocked the ability of duodenal CCK-8 to reduce glucose production in control rats, whereas duodenal Sp-cAMPS bypassed duodenal CCK resistance and activated duodenal PKA and lowered glucose production in rats on high-fat diets., Conclusions: We identified a neural glucoregulatory function of duodenal PKA signaling., (Copyright © 2012 AGA Institute. Published by Elsevier Inc. All rights reserved.)

Amino acids profoundly affect insulin action and glucose metabolism in mammals. Here, we investigated the role of the mediobasal hypothalamus (MBH), a key center involved in nutrient-dependent metabolic regulation. Specifically, we tested the novel hypothesis that the metabolism of leucine within the MBH couples the central sensing of leucine with the control of glucose production by the liver. We performed either central (MBH) or systemic infusions of leucine in Sprague-Dawley male rats during basal pancreatic insulin clamps in combination with various pharmacological and molecular interventions designed to modulate leucine metabolism in the MBH. We also examined the role of hypothalamic ATP-sensitive K(+) channels (K(ATP) channels) in the effects of leucine. Enhancing the metabolism of leucine acutely in the MBH lowered blood glucose through a biochemical network that was insensitive to rapamycin but strictly dependent on the hypothalamic metabolism of leucine to α-ketoisocaproic acid and, further, insensitive to acetyl- and malonyl-CoA. Functional K(ATP) channels were also required. Importantly, molecular attenuation of this central sensing mechanism in rats conferred susceptibility to developing hyperglycemia. We postulate that the metabolic sensing of leucine in the MBH is a previously unrecognized mechanism for the regulation of hepatic glucose production required to maintain glucose homeostasis.

Objective: Metabolism of long-chain fatty acids within the duodenum leads to the activation of duodenal mucosal protein kinase C (PKC)-δ and the cholecystokinin (CCK)-A receptor to lower glucose production through a neuronal network. However, the interfunctional relationship between duodenal PKC-δ and CCK remains elusive. Although long-chain fatty acids activate PKC to stimulate the release of CCK in CCK-secreting cells, CCK has also been found to activate PKC-δ in pancreatic acinar cells. We here evaluate whether activation of duodenal mucosal PKC-δ lies upstream (and/or downstream) of CCK signaling to lower glucose production., Research Design and Methods: We first determined with immunofluorescence whether PKC-δ and CCK were colocalized within the duodenal mucosa. We then performed gain- and loss-of-function experiments targeting duodenal PKC-δ and the CCK-A receptor and evaluated the impact on changes in glucose kinetics during pancreatic (basal insulin) clamps in rats in vivo., Results: Immunostaining of PKC-δ was found to colocalize with CCK in the duodenal mucosa. Intraduodenal coinfusion of either the CCK-A receptor antagonist MK-329 or CR-1409 with the PKC activator negated the ability of duodenal mucosal PKC-δ activation to lower glucose production during the pancreatic clamps in normal rats. Conversely, molecular and pharmacological inhibition of duodenal PKC-δ did not negate the ability of the duodenal CCK-A receptor agonist CCK-8 to lower glucose production, indicating that activation of duodenal PKC-δ lies upstream (and not downstream) of CCK signaling. Finally, intraduodenal PKC activator infusion failed to lower glucose production in rats with high-fat diet-induced duodenal CCK resistance., Conclusions: In summary, activation of duodenal PKC-δ leads to the stimulation of CCK release and activation of the CCK-A receptor signaling axis to lower glucose production in normal rats, but fails to bypass duodenal CCK-resistance in high fat-fed rats.

Background & Aims: Activation of protein kinase C (PKC) enzymes in liver and brain alters hepatic glucose metabolism, but little is known about their role in glucose regulation in the gastrointestinal tract. We investigated whether activation of PKC-δ in the duodenum is sufficient and necessary for duodenal nutrient sensing and regulates hepatic glucose production through a neuronal network in rats., Methods: In rats, we inhibited duodenal PKC and evaluated whether nutrient-sensing mechanisms, activated by refeeding, have disruptions in glucose regulation. We then performed gain- and loss-of-function pharmacologic and molecular experiments to target duodenal PKC-δ; we evaluated the impact on glucose production regulation during the pancreatic clamping, while basal levels of insulin were maintained., Results: PKC-δ was detected in the mucosal layer of the duodenum; intraduodenal infusion of PKC inhibitors disrupted glucose homeostasis during refeeding, indicating that duodenal activation of PKC-δ is necessary and sufficient to regulate glucose homeostasis. Intraduodenal infusion of the PKC activator 1-oleoyl-2-acetyl-sn-glycerol (OAG) specifically activated duodenal mucosal PKC-δ and a gut-brain-liver neuronal pathway to reduce glucose production. Molecular and pharmacologic inhibition of duodenal mucosal PKC-δ negated the ability of duodenal OAG and lipids to reduce glucose production., Conclusions: In the duodenal mucosa, PKC-δ regulates glucose homeostasis., (Copyright © 2011 AGA Institute. Published by Elsevier Inc. All rights reserved.)

Objective: Sirtuin 1 (SIRT1) and its activator resveratrol are emerging as major regulators of metabolic processes. We investigate the site of resveratrol action on glucose metabolism and the contribution of SIRT1 to these effects. Because the arcuate nucleus in the mediobasal hypothalamus (MBH) plays a pivotal role in integrating peripheral metabolic responses to nutrients and hormones, we examined whether the actions of resveratrol are mediated at the MBH., Research Design and Methods: Sprague Dawley (SD) male rats received acute central (MBH) or systemic injections of vehicle, resveratrol, or SIRT1 inhibitor during basal pancreatic insulin clamp studies. To delineate the pathway(s) by which MBH resveratrol modulates hepatic glucose production, we silenced hypothalamic SIRT1 expression using a short hairpin RNA (shRNA) inhibited the hypothalamic ATP-sensitive potassium (K(ATP)) channel with glibenclamide, or selectively transected the hepatic branch of the vagus nerve while infusing resveratrol centrally., Results: Our studies show that marked improvement in insulin sensitivity can be elicited by acute administration of resveratrol to the MBH or during acute systemic administration. Selective inhibition of hypothalamic SIRT1 using a cell-permeable SIRT1 inhibitor or SIRT1-shRNA negated the effect of central and peripheral resveratrol on glucose production. Blockade of the K(ATP) channel and hepatic vagotomy significantly attenuated the effect of central resveratrol on hepatic glucose production. In addition, we found no evidence for hypothalamic AMPK activation after MBH resveratrol administration., Conclusions: Taken together, these studies demonstrate that resveratrol improves glucose homeostasis mainly through a central SIRT1-dependent pathway and that the MBH is a major site of resveratrol action.

Objective: Circulating glucose inhibits glucose production in normal rodents and humans, but this glucose effectiveness is disrupted in diabetes due partly to sustained hyperglycemia. We hypothesize that hyperglycemia in diabetes impairs hypothalamic glucose sensing to lower glucose production, and changes of glucose transporter-1 (GLUT1) in the hypothalamic glial cells are responsible for the deleterious effects of hyperglycemia in vivo., Research Design and Methods: We tested hypothalamic glucose effectiveness to increase hypothalamic glucose concentration and lower glucose production in rats induced with streptozotocin (STZ) uncontrolled diabetes, STZ and phlorizin, and whole-body and hypothalamic sustained hyperglycemia. We next assessed the content of glial GLUT1 in the hypothalamus, generated an adenovirus expressing GLUT1 driven by a glial fibrillary acidic protein (GFAP) promoter (Ad-GFAP-GLUT1), and injected Ad-GFAP-GLUT1 into the hypothalamus of rats induced with hyperglycemia. Pancreatic euglycemic clamp and tracer-dilution methodologies were used to assess changes in glucose kinetics in vivo., Results: Sustained hyperglycemia, as seen in the early onset of STZ-induced diabetes, disrupted hypothalamic glucose sensing to increase hypothalamic glucose concentration and lower glucose production in association with reduced GLUT1 levels in the hypothalamic glial cells of rats in vivo. Overexpression of hypothalamic glial GLUT1 in STZ-induced rats with reduced GLUT1 acutely normalized plasma glucose levels and in rats with selectively induced hypothalamic hyperglycemia restored hypothalamic glucose effectiveness., Conclusions: Sustained hyperglycemia impairs hypothalamic glucose sensing to lower glucose production through changes in hypothalamic glial GLUT1, and these data highlight the critical role of hypothalamic glial GLUT1 in mediating glucose sensing to regulate glucose production., (© 2011 by the American Diabetes Association.)

The gut plays a unique role in the metabolic defence against energy excess and glucose imbalance. Nutrients, such as lipids, enter the small intestine and activate sensing mechanisms to maintain energy and glucose homeostasis. It is clear that a lipid-induced gut-brain axis exists and that cholecystokinin and a neuronal network are involved, yet the underlying mechanisms in gut lipid sensing that regulate homeostasis remain largely unknown. In parallel, studies underscore the importance of enzymes involved in lipid metabolism within the brain, such as adenosine monophosphate -activated protein kinase, to maintain homeostasis. In this review, we will first examine what is known regarding the mechanisms involved in this lipid-induced gut-brain neuronal axis that regulate food intake and hepatic glucose production. We will also discuss how enzymes that govern brain lipid metabolism could potentially reveal how lipids trigger the gut, and that both the gut and brain may share common biochemical pathways to sense lipids., (Copyright © 2011 John Wiley & Sons, Ltd.)

Objective: Hypothalamic nutrient sensing regulates glucose production, but the neuronal circuits involved remain largely unknown. Recent studies underscore the importance of N-methyl-d-aspartate (NMDA) receptors in the dorsal vagal complex in glucose regulation. These studies raise the possibility that hypothalamic nutrient sensing activates a forebrain-hindbrain NMDA-dependent circuit to regulate glucose production., Research Design and Methods: We implanted bilateral catheters targeting the mediobasal hypothalamus (MBH) (forebrain) and dorsal vagal complex (DVC) (hindbrain) and performed intravenous catheterizations to the same rat for infusion and sampling purposes. This model enabled concurrent selective activation of MBH nutrient sensing by either MBH delivery of lactate or an adenovirus expressing the dominant negative form of AMPK (Ad-DN AMPK α2 [D¹⁵⁷A]) and inhibition of DVC NMDA receptors by either DVC delivery of NMDA receptor blocker MK-801 or an adenovirus expressing the shRNA of NR1 subunit of NMDA receptors (Ad-shRNA NR1). Tracer-dilution methodology and the pancreatic euglycemic clamp technique were performed to assess changes in glucose kinetics in the same conscious, unrestrained rat in vivo., Results: MBH lactate or Ad-DN AMPK with DVC saline increased glucose infusion required to maintain euglycemia due to an inhibition of glucose production during the clamps. However, DVC MK-801 negated the ability of MBH lactate or Ad-DN AMPK to increase glucose infusion or lower glucose production. Molecular knockdown of DVC NR1 of NMDA receptor via Ad-shRNA NR1 injection also negated MBH Ad-DN AMPK to lower glucose production., Conclusions: Molecular and pharmacological inhibition of DVC NMDA receptors negated hypothalamic nutrient sensing mechanisms activated by lactate metabolism or AMPK inhibition to lower glucose production. Thus, DVC NMDA receptor is required for hypothalamic nutrient sensing to lower glucose production and that hypothalamic nutrient sensing activates a forebrain-hindbrain circuit to lower glucose production.

Objective: The fuel sensor AMP-activated protein kinase (AMPK) in the hypothalamus regulates energy homeostasis by sensing nutritional and hormonal signals. However, the role of hypothalamic AMPK in glucose production regulation remains to be elucidated. We hypothesize that bidirectional changes in hypothalamic AMPK activity alter glucose production., Research Design and Methods: To introduce bidirectional changes in hypothalamic AMPK activity in vivo, we first knocked down hypothalamic AMPK activity in male Sprague-Dawley rats by either injecting an adenovirus expressing the dominant-negative form of AMPK (Ad-DN AMPKα2 [D(157)A]) or infusing AMPK inhibitor compound C directly into the mediobasal hypothalamus. Next, we independently activated hypothalamic AMPK by delivering either an adenovirus expressing the constitutive active form of AMPK (Ad-CA AMPKα1(312) [T172D]) or the AMPK activator AICAR. The pancreatic (basal insulin)-euglycemic clamp technique in combination with the tracer-dilution methodology was used to assess the impact of alternations in hypothalamic AMPK activity on changes in glucose kinetics in vivo., Results: Injection of Ad-DN AMPK into the hypothalamus knocked down hypothalamic AMPK activity and led to a significant suppression of glucose production with no changes in peripheral glucose uptake during the clamps. In parallel, hypothalamic infusion of AMPK inhibitor compound C lowered glucose production as well. Conversely, molecular and pharmacological activation of hypothalamic AMPK negated the ability of hypothalamic nutrients to lower glucose production., Conclusions: These data indicate that changes in hypothalamic AMPK activity are sufficient and necessary for hypothalamic nutrient-sensing mechanisms to alter glucose production in vivo.

Our objective was to investigate whether the direct bilateral infusion of the monounsaturated fatty acid (MUFA) oleic acid (OA) within the mediobasal hypothalamus (MBH) is sufficient to reproduce the effect of administration of OA (30 nmol) in the third cerebral ventricle, which inhibits glucose production (GP) in rats. We used the pancreatic basal insulin clamp technique (plasma insulin ∼20 mU/ml) in combination with tracer dilution methodology to compare the effect of MBH OA on GP to that of a saturated fatty acid (SFA), palmitic acid (PA), and a polyunsaturated fatty acid (PUFA), linoleic acid (LA). The MBH infusion of 200 but not 40 pmol of OA was sufficient to markedly inhibit GP (by 61% from 12.6 ± 0.6 to 5.1 ± 1.6 mg·kg(-1)·min(-1)) such that exogenous glucose had to be infused at the rate of 6.0 ± 1.2 mg·kg(-1)·min(-1) to prevent hypoglycemia. MBH infusion of PA also caused a significant decrease in GP, but only at a total dose of 4 nmol (GP 5.8 ± 1.6 mg·kg(-1)·min(-1)). Finally, MBH LA at a total dose of 0.2 and 4 nmol failed to modify GP compared with rats receiving MBH vehicle. Increased availability of OA within the MBH is sufficient to markedly inhibit GP. LA does not share the effect of OA, whereas PA can reproduce the potent effect of OA on GP, but only at a higher dose. It remains to be determined whether SFAs need to be converted to MUFAs to exert this effect or whether they activate a separate signaling pathway to inhibit GP.

Diabetes is characterized by hyperglycemia due partly to increased hepatic glucose production. The hypothalamus regulates hepatic glucose production in rodents. However, it is currently unknown whether other regions of the brain are sufficient in glucose production regulation. The N-methyl-D-aspartate (NMDA) receptor is composed of NR1 and NR2 subunits, which are activated by co-agonist glycine and glutamate or aspartate, respectively. Here we report that direct administration of either co-agonist glycine or NMDA into the dorsal vagal complex (DVC), targeting the nucleus of the solitary tract, lowered glucose production in vivo. Direct infusion of the NMDA receptor blocker MK-801 into the DVC negated the metabolic effect of glycine. To evaluate whether NR1 subunit of the NMDA receptor mediates the effect of glycine, NR1 in the DVC was inhibited by DVC NR1 antagonist 7-chlorokynurenic acid or DVC shRNA-NR1. Pharmacological and molecular inhibition of DVC NR1 negated the metabolic effect of glycine. To evaluate whether the NMDA receptors mediate the effects of NR2 agonist NMDA, DVC NMDA receptors were inhibited by antagonist D-2-amino-5-phosphonovaleric acid (D-APV). DVC D-APV fully negated the ability of DVC NMDA to lower glucose production. Finally, hepatic vagotomy negated the DVC glycine ability to lower glucose production. These findings demonstrate that activation of NR1 and NR2 subunits of the NMDA receptors in the DVC is sufficient to trigger a brain-liver axis to lower glucose production, and suggest that DVC NMDA receptors serve as a therapeutic target for diabetes and obesity.

Emerging studies indicate that hypothalamic hormonal signalling pathways and nutrient metabolism regulate glucose homeostasis in rodents. Although hypothalamic lactate-sensing mechanisms have been described to lower glucose production (GP), it is currently unknown whether the hypothalamus senses lactate in the blood circulation to regulate GP and maintain glucose homeostasis in vivo. To examine whether hypothalamic sensing of circulating lactate is required to regulate GP, we infused intravenous (i.v.) lactate in the absence or presence of inhibition of central/hypothalamic lactate-sensing mechanisms in normal rodents. Inhibition of central/hypothalamic lactate-sensing mechanisms was achieved by three independent approaches. Tracer-dilution methodology in combination with the pancreatic clamp technique was used to assess the effect of i.v. and central/hypothalamic administrations on glucose metabolism in vivo. In the presence of physiologically relevant increases in the levels of plasma lactate, inhibition of central lactate-sensing mechanisms by lactate dehydrogenase inhibitor oxamate (OXA) or ATP-sensitive potassium channels blocker glibenclamide increased GP. Furthermore, direct administration of OXA into the mediobasal hypothalamus increased GP in the presence of similar elevation of circulating lactate. Together, these data indicate that hypothalamic sensing of circulating lactate regulates GP and is required to maintain glucose homeostasis.

Upon the entry of nutrients into the small intestine, nutrient sensing mechanisms are activated to allow the body to adapt appropriately to the incoming nutrients. To date, mounting evidence points to the existence of an upper intestinal lipid-induced gut-brain neuronal axis to regulate energy homeostasis. Moreover, a recent discovery has also revealed an upper intestinal lipid-induced gut-brain-liver neuronal axis involved in the regulation of glucose homeostasis. In this mini-review, we will focus on the mechanisms underlying the activation of these respective neuronal axes by upper intestinal lipids.

Cholecystokinin (CCK) is a peptide hormone that is released from the gut in response to nutrients such as lipids to lower food intake. Here we report that a primary increase of CCK-8, the biologically active form of CCK, in the duodenum lowers glucose production independent of changes in circulating insulin levels. Furthermore, we show that duodenal CCK-8 requires the activation of the gut CCK-A receptor and a gut-brain-liver neuronal axis to lower glucose production. Finally, duodenal CCK-8 fails to lower glucose production in the early onset of high-fat diet-induced insulin resistance. These findings reveal a role for gut CCK that lowers glucose production through a neuronal network and suggest that intestinal CCK resistance may contribute to hyperglycemia in response to high-fat feeding.

Objective: A selective rise in hypothalamic lipid metabolism and the subsequent activation of SUR1/Kir6.2 ATP-sensitive K(+) (K(ATP)) channels inhibit hepatic glucose production. The mechanisms that link the ability of hypothalamic lipid metabolism to the activation of K(ATP) channels remain unknown., Research Design and Methods: To examine whether hypothalamic protein kinase C (PKC) mediates the ability of central nervous system lipids to activate K(ATP) channels and regulate glucose production in normal rodents, we first activated hypothalamic PKC in the absence or presence of K(ATP) channel inhibition. We then inhibited hypothalamic PKC in the presence of lipids. Tracer-dilution methodology in combination with the pancreatic clamp technique was used to assess the effect of hypothalamic administrations on glucose metabolism in vivo., Results: We first reported that direct activation of hypothalamic PKC via direct hypothalamic delivery of PKC activator 1-oleoyl-2-acetyl-sn-glycerol (OAG) suppressed glucose production. Coadministration of hypothalamic PKC-delta inhibitor rottlerin with OAG prevented the ability of OAG to activate PKC-delta and lower glucose production. Furthermore, hypothalamic dominant-negative Kir6.2 expression or the delivery of the K(ATP) channel blocker glibenclamide abolished the glucose production-lowering effects of OAG. Finally, inhibition of hypothalamic PKC eliminated the ability of lipids to lower glucose production., Conclusions: These studies indicate that hypothalamic PKC activation is sufficient and necessary for lowering glucose production.

Energy and glucose homeostasis are regulated by food intake and liver glucose production, respectively. The upper intestine has a critical role in nutrient digestion and absorption. However, studies indicate that upper intestinal lipids inhibit food intake as well in rodents and humans by the activation of an intestine-brain axis. In parallel, a brain-liver axis has recently been proposed to detect blood lipids to inhibit glucose production in rodents. Thus, we tested the hypothesis that upper intestinal lipids activate an intestine-brain-liver neural axis to regulate glucose homeostasis. Here we demonstrate that direct administration of lipids into the upper intestine increased upper intestinal long-chain fatty acyl-coenzyme A (LCFA-CoA) levels and suppressed glucose production. Co-infusion of the acyl-CoA synthase inhibitor triacsin C or the anaesthetic tetracaine with duodenal lipids abolished the inhibition of glucose production, indicating that upper intestinal LCFA-CoAs regulate glucose production in the preabsorptive state. Subdiaphragmatic vagotomy or gut vagal deafferentation interrupts the neural connection between the gut and the brain, and blocks the ability of upper intestinal lipids to inhibit glucose production. Direct administration of the N-methyl-d-aspartate ion channel blocker MK-801 into the fourth ventricle or the nucleus of the solitary tract where gut sensory fibres terminate abolished the upper-intestinal-lipid-induced inhibition of glucose production. Finally, hepatic vagotomy negated the inhibitory effects of upper intestinal lipids on glucose production. These findings indicate that upper intestinal lipids activate an intestine-brain-liver neural axis to inhibit glucose production, and thereby reveal a previously unappreciated pathway that regulates glucose homeostasis.

Objective: Hypothalamic lactate metabolism lowers hepatic glucose production and plasma glucose levels in normal rodents. However, it remains unknown whether activation of hypothalamic lactate metabolism lowers glucose production and plasma glucose levels in rodents with diabetes and obesity., Research Design and Methods: We performed intracerebroventricular (ICV) administration of lactate to enhance central lactate metabolism in 1) early-onset streptozotocin-induced uncontrolled diabetic rodents, 2) experimentally induced hypoinsulinemic normal rodents, and 3) early-onset diet-induced insulin-resistant rodents. Tracer-dilution methodology was used to assess the impact of ICV lactate on the rate of glucose production in all three models., Results: We first report that in the absence of insulin treatment, ICV lactate administration lowered glucose production and glucose levels in rodents with uncontrolled diabetes. Second, ICV lactate administration lowered glucose production and glucose levels in normal rodents with experimentally induced hypoinsulinemia. Third, and finally, ICV lactate administration lowered glucose production in normal rodents with diet-induced insulin resistance., Conclusions: Central lactate metabolism lowered glucose production in uncontrolled diabetic and normal rodents with hypoinsulinemia and in rodents with diet-induced insulin resistance. These data suggest that insulin signaling is not required for central lactate to lower glucose production and that the activation of hypothalamic lactate metabolism could consequently bypass insulin resistance and lower glucose levels in early-onset diabetes and obesity.

Increased production of very low-density lipoprotein (VLDL) is a critical feature of the metabolic syndrome. Here we report that a selective increase in brain glucose lowered circulating triglycerides (TG) through the inhibition of TG-VLDL secretion by the liver. We found that the effect of glucose required its conversion to lactate, leading to activation of ATP-sensitive potassium channels and to decreased hepatic activity of stearoyl-CoA desaturase-1 (SCD1). SCD1 catalyzed the synthesis of oleyl-CoA from stearoyl-CoA. Curtailing the liver activity of SCD1 was sufficient to lower the hepatic levels of oleyl-CoA and to recapitulate the effects of central glucose administration on VLDL secretion. Notably, portal infusion of oleic acid restored hepatic oleyl-CoA to control levels and negated the effects of both central glucose and SCD1 deficiency on TG-VLDL secretion. These central effects of glucose (but not those of lactate) were rapidly lost in diet-induced obesity. These findings indicate that a defect in brain glucose sensing could play a critical role in the etiology of the metabolic syndrome.

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

Short-term overfeeding blunts the central effects of fatty acids on food intake and glucose production. This acquired defect in nutrient sensing could contribute to the rapid onset of hyperphagia and insulin resistance in this model. Here we examined whether central inhibition of lipid oxidation is sufficient to restore the hypothalamic levels of long-chain fatty acyl-CoAs (LCFA-CoAs) and to normalize food intake and glucose homeostasis in overfed rats. To this end, we targeted the liver isoform of carnitine palmitoyltransferase-1 (encoded by the CPT1A gene) by infusing either a sequence-specific ribozyme against CPT1A or an isoform-selective inhibitor of CPT1A activity in the third cerebral ventricle or in the mediobasal hypothalamus (MBH). Inhibition of CPT1A activity normalized the hypothalamic levels of LCFA-CoAs and markedly inhibited feeding behavior and hepatic glucose fluxes in overfed rats. Thus central inhibition of lipid oxidation is sufficient to restore hypothalamic lipid sensing as well as glucose and energy homeostasis in this model and may be an effective approach to the treatment of diet-induced obesity and insulin resistance.

The brain keenly depends on glucose for energy, and mammalians have redundant systems to control glucose production. An increase in circulating glucose inhibits glucose production in the liver, but this negative feedback is impaired in type 2 diabetes. Here we report that a primary increase in hypothalamic glucose levels lowers blood glucose through inhibition of glucose production in rats. The effect of glucose requires its conversion to lactate followed by stimulation of pyruvate metabolism, which leads to activation of adenosine triphosphate (ATP)-sensitive potassium channels. Thus, interventions designed to enhance the hypothalamic sensing of glucose may improve glucose homeostasis in diabetes.

Obesity is the driving force behind the worldwide increase in the prevalence of type 2 diabetes mellitus. Hyperglycaemia is a hallmark of diabetes and is largely due to increased hepatic gluconeogenesis. The medial hypothalamus is a major integrator of nutritional and hormonal signals, which play pivotal roles not only in the regulation of energy balance but also in the modulation of liver glucose output. Bidirectional changes in hypothalamic insulin signalling therefore result in parallel changes in both energy balance and glucose metabolism. Here we show that activation of ATP-sensitive potassium (K(ATP)) channels in the mediobasal hypothalamus is sufficient to lower blood glucose levels through inhibition of hepatic gluconeogenesis. Finally, the infusion of a K(ATP) blocker within the mediobasal hypothalamus, or the surgical resection of the hepatic branch of the vagus nerve, negates the effects of central insulin and halves the effects of systemic insulin on hepatic glucose production. Consistent with these results, mice lacking the SUR1 subunit of the K(ATP) channel are resistant to the inhibitory action of insulin on gluconeogenesis. These findings suggest that activation of hypothalamic K(ATP) channels normally restrains hepatic gluconeogenesis, and that any alteration within this central nervous system/liver circuit can contribute to diabetic hyperglycaemia.

Increased glucose production is a hallmark of type 2 diabetes and alterations in lipid metabolism have a causative role in its pathophysiology. Here we postulate that physiological increments in plasma fatty acids can be sensed within the hypothalamus and that this sensing is required to balance their direct stimulatory action on hepatic gluconeogenesis. In the presence of physiologically-relevant increases in the levels of plasma fatty acids, negating their central action on hepatic glucose fluxes through (i) inhibition of the hypothalamic esterification of fatty acids, (ii) genetic deletion (Sur1-deficient mice) of hypothalamic K(ATP) channels or pharmacological blockade (K(ATP) blocker) of their activation by fatty acids, or (iii) surgical resection of the hepatic branch of the vagus nerve led to a marked increase in liver glucose production. These findings indicate that a physiological elevation in circulating lipids can be sensed within the hypothalamus and that a defect in hypothalamic lipid sensing disrupts glucose homeostasis.

Exposure to high concentrations of glucose and insulin results in insulin resistance of metabolic target tissues, a characteristic feature of type 2 diabetes. High glucose has also been associated with oxidative stress, and increased levels of reactive oxygen species have been proposed to cause insulin resistance. To determine whether oxidative stress contributes to insulin resistance induced by hyperglycemia in vivo, nondiabetic rats were infused with glucose for 6 h to maintain a circulating glucose concentration of 15 mM with and without coinfusion of the antioxidant N-acetylcysteine (NAC), followed by a 2-h hyperinsulinemic-euglycemic clamp. High glucose (HG) induced a significant decrease in insulin-stimulated glucose uptake [tracer-determined disappearance rate (Rd), control 41.2 +/- 1.7 vs. HG 32.4 +/- 1.9 mg. kg-1. min-1, P < 0.05], which was prevented by NAC (HG + NAC 45.9 +/- 3.5 mg. kg-1. min-1). Similar results were obtained with the antioxidant taurine. Neither NAC nor taurine alone altered Rd. HG caused a significant (5-fold) increase in soleus muscle protein carbonyl content, a marker of oxidative stress that was blocked by NAC, as well as elevated levels of malondialdehyde and 4-hydroxynonenal, markers of lipid peroxidation, which were reduced by taurine. In contrast to findings after long-term hyperglycemia, there was no membrane translocation of novel isoforms of protein kinase C in skeletal muscle after 6 h. These data support the concept that oxidative stress contributes to the pathogenesis of hyperglycemia-induced insulin resistance.