Glucose-Lowering by Leptin in the Absence of Insulin Does Not Fully Rely on the Central Melanocortin System in Male Mice

VMHdm/cSF-1 neuronal circuits regulate skeletal muscle PGC1-α via the sympathoadrenal drive

OBJECTIVE

To adapt to metabolically challenging environments, the central nervous system (CNS) orchestrates metabolism of peripheral organs including skeletal muscle. The organ-communication between the CNS and skeletal muscle has been investigated, yet our understanding of the neuronal pathway from the CNS to skeletal muscle is still limited. Neurons in the dorsomedial and central parts of the ventromedial hypothalamic nucleus (VMHdm/c) expressing steroidogenic factor-1 (VMHdm/cSF-1 neurons) are key for metabolic adaptations to exercise, including increased basal metabolic rate and skeletal muscle mass in mice. However, the mechanisms by which VMHdm/cSF-1 neurons regulate skeletal muscle function remain unclear. Here, we show that VMHdm/cSF-1 neurons increase the sympathoadrenal activity and regulate skeletal muscle peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) in mice via multiple downstream nodes.

METHODS

Optogenetics was used to specifically manipulate VMHdm/cSF-1 neurons combined with genetically-engineered mice and surgical manipulation of the sympathoadrenal activity.

RESULTS

Optogenetic activation of VMHdm/cSF-1 neurons dramatically elevates mRNA levels of skeletal muscle Pgc-1α, which regulates a spectrum of skeletal muscle function including protein synthesis and metabolism. Mechanistically, the sympathoadrenal drive coupled with β2 adrenergic receptor (β2AdR) is essential for VMHdm/cSF-1 neurons-mediated increases in skeletal muscle PGC1-α. Specifically, both adrenalectomy and β2AdR knockout block augmented skeletal muscle PGC1-α by VMHdm/cSF-1 neuronal activation. Optogenetic functional mapping reveals that downstream nodes of VMHdm/cSF-1 neurons are functionally redundant to increase circulating epinephrine and skeletal muscle PGC1-α.

CONCLUSIONS

Collectively, we propose that VMHdm/cSF-1 neurons-skeletal muscle pathway, VMHdm/cSF-1 neurons→multiple downstream nodes→the adrenal gland→skeletal muscle β2AdR, underlies augmented skeletal muscle function for metabolic adaptations.

TNF-α Antagonizes the Effect of Leptin on Insulin Secretion through FOXO1-Dependent Transcriptional Suppression of LepRb in INS-1 Cells

Proinflammatory cytokines play a causal role in the development of hyperinsulinemia and T2MD. FOXO1, a transcription factor which is known to enhance proinflammation, was recently shown to be involved in obesity-induced β cell dysfunction. However, molecular mechanisms for the association remained elusive. In this study, we first found that both leptin (10 nM) and TNF-α (20 ng/ml) significantly inhibited glucose-stimulated insulin secretion (GSIS) of INS-1E cells. When in combination, the GSIS function of INS-1E cells was significantly increased compared with that of the leptin alone treatment, indicating that TNF-α attenuated the inhibiting effect of leptin on GSIS of INS-1E cells. Similarly, we found that TNF-α has the same inhibitory effect on leptin in regulating insulin synthesis and secretion, and the survival and apoptosis of insulin cells. Further studies showed that TNF-α blocks leptin pathway by reducing the expression of leptin receptor (LepRb, also called OBRb) and inhibiting the activation of STAT3, a key molecule involved in the leptin signaling pathway in INS-1E cells. Besides, the downregulated expression of phosphorylated FOXO1 was found to be involved in the possible mechanism of TNF-α. Overexpression of constitutively active FOXO1 markedly aggravated the LepRb reduction by TNF-α treatment of INS-1E cells, and the endogenous FOXO1 knockdown abolished the effect of TNF-α on INS-1E cells. Furthermore, we have proved that FOXO1 could directly bind to the promoter of LepRb as a negative transcription regulator. Taken together, the results of this study reveal that TNF-α-induced LepRb downregulated in pancreatic β cells and demonstrate that transcriptional reduction of FOXO1 might be the primary mechanism underlying TNF-α promoting INS-1E leptin resistance and β cell dysfunction. Conclusions. Our current studies based on INS-1E cells in vitro indicate that the inflammatory factor TNF-α plays an important role in the development of INS-1E leptin resistance and glucose metabolism disorders, probably through FOXO1-induced transcription reduction of LepRb promoter in pancreatic β cells, and FOXO1 may be a novel target for treating β cell dysfunction in obesity-induced hyperinsulinemia and T2DM.

Central regulation of glucose metabolism in an insulin-dependent and -independent manner

The central nervous system (CNS) contributes significantly to glucose homeostasis. The available evidence indicates that insulin directly acts on the CNS, in particular the hypothalamus, to regulate hepatic glucose production, thereby controlling whole-body glucose metabolism. Additionally, insulin also acts on the brain to regulate food intake and fat metabolism, which may indirectly regulate glucose metabolism. Studies conducted over the last decade have found that the CNS can regulate glucose metabolism in an insulin-independent manner. Enhancement of central leptin signalling reverses hyperglycaemia in insulin-deficient rodents. Here, I review the mechanisms by which central insulin and leptin actions regulate glucose metabolism. Although clinical studies have shown that insulin treatment is currently indispensable for managing diabetes, unravelling the neuronal mechanisms underlying the central regulation of glucose metabolism will pave the way for the design of novel therapeutic drugs for diabetes.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Leptin Receptors in RIP-Cre25Mgn Neurons Mediate Anti-dyslipidemia Effects of Leptin in Insulin-Deficient Mice

Leptin is a potent endocrine hormone produced by adipose tissue and regulates a broad range of whole-body metabolism such as glucose and lipid metabolism, even without insulin. Central leptin signaling can lower hyperglycemia in insulin-deficient rodents via multiple mechanisms, including improvements of dyslipidemia. However, the specific neurons that regulate anti-dyslipidemia effects of leptin remain unidentified. Here we report that leptin receptors (LEPRs) in neurons expressing Cre recombinase driven by a short fragment of a promoter region of Ins2 gene (RIP-Cre25Mgn neurons) are required for central leptin signaling to reverse dyslipidemia, thereby hyperglycemia in insulin-deficient mice. Ablation of LEPRs in RIP-Cre25Mgn neurons completely blocks glucose-lowering effects of leptin in insulin-deficient mice. Further investigations reveal that insulin-deficient mice lacking LEPRs in RIP-Cre25Mgn neurons (RIP-CreΔLEPR mice) exhibit greater lipid levels in blood and liver compared to wild-type controls, and that leptin injection into the brain does not suppress dyslipidemia in insulin-deficient RIP-CreΔLEPR mice. Leptin administration into the brain combined with acipimox, which lowers blood lipids by suppressing triglyceride lipase activity, can restore normal glycemia in insulin-deficient RIP-CreΔLEPR mice, suggesting that excess circulating lipids are a driving-force of hyperglycemia in these mice. Collectively, our data demonstrate that LEPRs in RIP-Cre25Mgn neurons significantly contribute to glucose-lowering effects of leptin in an insulin-independent manner by improving dyslipidemia.

CNS Regulation of Glucose Homeostasis: Role of the Leptin-Melanocortin System

PURPOSE OF REVIEW

In this brief review, we highlight studies that have contributed to our current understanding of glucose homeostasis by the central nervous system (CNS) leptin-melanocortin system, particularly proopiomelanocortin neurons and melanocortin-4 receptors (MC4R).

RECENT FINDINGS

Leptin deficiency is associated with insulin resistance and impaired glucose metabolism whereas leptin administration improves tissue glucose uptake/oxidation and reduces hepatic glucose output. These antidiabetic effects of leptin have been demonstrated in experimental animals and humans, even when circulating insulin levels are barely detectable. Recent evidence suggests that these antidiabetic actions of leptin are mediated, in large part, by stimulation of leptin receptors (LRs) in the CNS and require activation of proopiomelanocortin (POMC) neurons and MC4R. These chronic antidiabetic effects of the CNS leptin-melanocortin system appear to be independent of autonomic nervous system and pituitary-thyroid-adrenal (PTA) axis mechanisms. The powerful antidiabetic actions of the CNS leptin-melanocortin system are capable of normalizing plasma glucose even in the absence of insulin and involve interactions of multiple neuronal populations and intracellular signaling pathways. Although the links between the CNS leptin-melanocortin system and its chronic effects on peripheral tissue glucose metabolism are still uncertain, they are independent of insulin action, activation of the autonomic nervous system, or the PTA axis. Unraveling the pathways that contribute to the powerful antidiabetic effects of the CNS leptin-melanocortin system may provide novel therapeutic approaches for diabetes mellitus.

Neuronal control of peripheral nutrient partitioning

The appropriate utilisation, storage and conversion of nutrients in peripheral tissues, referred to as nutrient partitioning, is a fundamental process to adapt to nutritional and metabolic challenges and is thus critical for the maintenance of a healthy energy balance. Alterations in this process during nutrient excess can have deleterious effects on glucose and lipid homeostasis and contribute to the development of obesity and type 2 diabetes. Nutrient partitioning is a complex integrated process under the control of hormonal and neural signals. Neural control relies on the capacity of the brain to sense circulating metabolic signals and mount adaptive neuroendocrine and autonomic responses. This review aims to discuss the hypothalamic neurocircuits and molecular mechanisms controlling nutrient partitioning and their potential contribution to metabolic maladaptation and disease.

ACE2 and ADAM17 Interaction Regulates the Activity of Presympathetic Neurons

Brain renin angiotensin system within the paraventricular nucleus plays a critical role in balancing excitatory and inhibitory inputs to modulate sympathetic output and blood pressure regulation. We previously identified ACE2 and ADAM17 as a compensatory enzyme and a sheddase, respectively, involved in brain renin angiotensin system regulation. Here, we investigated the opposing contribution of ACE2 and ADAM17 to hypothalamic presympathetic activity and ultimately neurogenic hypertension. New mouse models were generated where ACE2 and ADAM17 were selectively knocked down from all neurons (AC-N) or Sim1 neurons (SAT), respectively. Neuronal ACE2 deletion revealed a reduction of inhibitory inputs to AC-N presympathetic neurons relevant to blood pressure regulation. Primary neuron cultures confirmed ACE2 expression on GABAergic neurons synapsing onto excitatory neurons within the hypothalamus but not on glutamatergic neurons. ADAM17 expression was shown to colocalize with angiotensin-II type 1 receptors on Sim1 neurons, and the pressor relevance of this neuronal population was demonstrated by photoactivation. Selective knockdown of ADAM17 was associated with a reduction of FosB gene expression, increased vagal tone, and prevented the acute pressor response to centrally administered angiotensin-II. Chronically, SAT mice exhibited a blunted blood pressure elevation and preserved ACE2 activity during development of salt-sensitive hypertension. Bicuculline injection in those models confirmed the supporting role of ACE2 on GABAergic tone to the paraventricular nucleus. Together, our study demonstrates the contrasting impact of ACE2 and ADAM17 on neuronal excitability of presympathetic neurons within the paraventricular nucleus and the consequences of this mutual regulation in the context of neurogenic hypertension.