A brain-liver circuit regulates glucose homeostasis

Cerebral glutamate metabolism during hypoglycaemia in healthy and type 1 diabetic humans

BACKGROUND

The mechanisms responsible for the progressive failure of hypoglycaemia counterregulation in long-standing type 1 diabetes are poorly understood. Increased brain glucose uptake during hypoglycaemia or alterations of brain energy metabolism could effect glucose sensing by the brain and thus contribute to hypoglycaemia-associated autonomic failure.

MATERIALS AND METHODS

Type 1 diabetic patients (T1DM) and healthy volunteers (CON) were studied before, during and after a hypoglycaemic (50 mg dL(-1)) hyperinsulinaemic (1.5 mU kg(-1) min(-1)) clamp test. The (1)H magnetic resonance spectroscopy of the occipital lobe of the brain was performed employing the STEAM localization technique. The water signal was suppressed by the modified SWAMP method. All spectra were acquired on a 3 Tesla scanner (80 cm MEDSPEC-DBX, Bruker Medical, Ettlingen, Germany) using a 10-cm diameter surface coil.

RESULTS

During hypoglycaemia, T1DM showed blunted endocrine counterregulation. At baseline the brain tissue glucose : creatine ratio was lower in CON than in T1DM (CON 0.13 +/- 0.05 vs. T1DM 0.19 0.11; P < 0.01). During hypoglycaemia glucose : creatine ratios decreased in both groups (CON 0.07 +/- 0.08, P < 0.05; T1DM 0.03 +/- 0.03, P < 0.001). A significant drop in the glutamate : creatine ratio could only be found in CON during hypoglycaemia (CON 1.36 +/- 0.08 vs. 1.26 +/- 0.11; P < 0.01; T1DM 1.32 +/- 0.13 vs. 1.28 +/- 0.15; P = NS). The ratios of glutamine, N-acetylaspartate, choline and myo-inositol : creatine were not different between both groups and did not change throughout the experiment.

CONCLUSIONS

Only in CON does moderate hypoglycaemia reduce intracerebral glutamate concentrations, possibly owing to a slower substrate flux through the tricarboxylic acid cycle in neurones. The maintenance of normal energy metabolism in T1DM during hypoglycaemia might effect glucose sensing in the brain and contribute to hypoglycaemia-associated autonomic failure.

Insulin action in the brain contributes to glucose lowering during insulin treatment of diabetes

To investigate the role of brain insulin action in the pathogenesis and treatment of diabetes, we asked whether neuronal insulin signaling is required for glucose-lowering during insulin treatment of diabetes. Hypothalamic signaling via the insulin receptor substrate-phosphatidylinositol 3-kinase (IRS-PI3K) pathway, a key intracellular mediator of insulin action, was reduced in rats with uncontrolled diabetes induced by streptozotocin (STZ-DM). Further, infusion of a PI3K inhibitor into the third cerebral ventricle of STZ-DM rats prior to peripheral insulin injection attenuated insulin-induced glucose lowering by approximately 35%-40% in both acute and chronic insulin treatment paradigms. Conversely, increased PI3K signaling induced by hypothalamic overexpression of either IRS-2 or protein kinase B (PKB, a key downstream mediator of PI3K action) enhanced the glycemic response to insulin by approximately 2-fold in STZ-DM rats. We conclude that hypothalamic insulin signaling via the IRS-PI3K pathway is a key determinant of the response to insulin in the management of uncontrolled diabetes.

Hypothalamic pathogenesis of type 2 diabetes

There have recently been increasing experimental and clinical evidences suggesting that hypothalamic dysregulation may be one of the underlying mechanisms of abnormal glucose metabolism. First, increased hypothalamic-pituitary-adrenal axis activity induced by uncontrollable excess stress may cause diabetes mellitus as well as dyslipidemia, visceral obesity, and osteoporosis with some resemblance to Cushing's disease. Second, several molecules are known to be expressed both in pancreas and hypothalamus; adenosine triphosphate-sensitive potassium channels, malonyl-CoA, glucokinase, and AMP-activated protein kinase. Those molecules appear to form an integrated hypothalamic system, which may sense hypothalamic fuel status, especially glucose level, and inhibit action of insulin on hepatic gluconeogenesis, thereby forming a brain-liver circuit. Third, hypothalamic resistance to insulin as an adiposity signal may be involved in pathogenesis of peripheral insulin resistance. The results with mice with a neuron-specific disruption of the insulin receptor gene or those lacking insulin receptor substrate 2 in hypothalamus supported this possibility. Finally, it has very recently been suggested that dysregulation of clock genes in hypothalamus may cause abnormal glucose metabolism. Taken together, it is plausible that some hypothalamic abnormality may underlie at least some portion of type 2 diabetes or insulin resistance in humans, and this viewpoint of hypothalamic pathogenesis of type 2 diabetes may lead to the development of new drugs for type 2 diabetes.

Portal sensing of intestinal gluconeogenesis is a mechanistic link in the diminution of food intake induced by diet protein

Protein feeding is known to decrease hunger and subsequent food intake in animals and humans. It has also been suggested that glucose appearance into portal vein, as occurring during meal assimilation, may induce comparable effects. Here, we connect these previous observations by reporting that intestinal gluconeogenesis (i.e., de novo synthesis of glucose) is induced during the postabsorptive time (following food digestion) in rats specifically fed on protein-enriched diet. This results in glucose release into portal blood, counterbalancing the lowering of glycemia resulting from intestinal glucose utilization. Comparable infusions into the portal vein of control postabsorptive rats (fed on starch-enriched diet) decrease food consumption and activate the hypothalamic nuclei regulating food intake. Similar hypothalamic activation occurs on protein feeding. All these effects are absent after denervation of the portal vein. Thus, portal sensing of intestinal gluconeogenesis may be a novel mechanism connecting the macronutrient composition of diet to food intake.

Brain regulation of food intake and appetite: molecules and networks

In the clinic, obesity and anorexia constitute prevalent problems whose manifestations are encountered in virtually every field of medicine. However, as the command centre for regulating food intake and energy metabolism is located in the brain, the basic neuroscientist sees in the same disorders malfunctions of a model network for how integration of diverse sensory inputs leads to a coordinated behavioural, endocrine and autonomic response. The two approaches are not mutually exclusive; rather, much can be gained by combining both perspectives to understand the pathophysiology of over- and underweight. The present review summarizes recent advances in this field including the characterization of peripheral metabolic signals to the brain such as leptin, insulin, peptide YY, ghrelin and lipid mediators as well as the vagus nerve; signalling of the metabolic sensors in the brainstem and hypothalamus via, e.g. neuropeptide Y and melanocortin peptides; integration and coordination of brain-mediated responses to nutritional challenges; the organization of food intake in simple model organisms; the mechanisms underlying food reward and processing of the sensory and metabolic properties of food in the cerebral cortex; and the development of the central metabolic system, as well as its pathological regulation in cancer and infections. Finally, recent findings on the genetics of human obesity are summarized, as well as the potential for novel treatments of body weight disorders.

Enhancing hepatic glycolysis reduces obesity: differential effects on lipogenesis depend on site of glycolytic modulation

Reducing obesity requires an elevation of energy expenditure and/or a suppression of food intake. Here we show that enhancing hepatic glycolysis reduces body weight and adiposity in obese mice. Overexpression of glucokinase or 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is used to increase hepatic glycolysis. Either of the two treatments produces similar increases in rates of fatty acid oxidation in extrahepatic tissues, i.e., skeletal muscle, leading to an elevation of energy expenditure. However, only 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase overexpression causes a suppression of food intake and a decrease in hypothalamic neuropeptide Y expression, contributing to a more pronounced reduction of body weight with this treatment. Furthermore, the two treatments cause differential lipid profiles due to opposite effects on hepatic lipogenesis, associated with distinct phosphorylation states of carbohydrate response element binding protein and AMP-activated protein kinase. The step at which hepatic glycolysis is enhanced dramatically influences overall whole-body energy balance and lipid profiles.

Hypothalamic K(ATP) channels control hepatic glucose production

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.

Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis

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.