Leptin enhances hypothalamic lactate dehydrogenase A (LDHA)-dependent glucose sensing to lower glucose production in high-fat-fed rats

Insulin and leptin acutely modulate the energy metabolism of primary hypothalamic and cortical astrocytes

Astrocytes constitute a heterogeneous cell population within the brain, contributing crucially to brain homeostasis and playing an important role in overall brain function. Their function and metabolism are not only regulated by local signals, for example, from nearby neurons, but also by long-range signals such as hormones. Thus, two prominent hormones primarily known for regulating the energy balance of the whole organism, insulin, and leptin, have been reported to also impact astrocytes within the brain. In this study, we investigated the acute regulation of astrocytic metabolism by these hormones in cultured astrocytes prepared from the mouse cortex and hypothalamus, a pivotal region in the context of nutritional regulation. Utilizing genetically encoded, fluorescent nanosensors, the cytosolic concentrations of glucose, lactate, and ATP, along with glycolytic rate and the NADH/NAD+ redox state were measured. Under basal conditions, differences between the two populations of astrocytes were observed for glucose and lactate concentrations as well as the glycolytic rate. Additionally, astrocytic metabolism responded to insulin and leptin in both brain regions, with some unique characteristics for each cell population. Finally, both hormones influenced how cells responded to elevated extracellular levels of potassium ions, a common indicator of neuronal activity. In summary, our study provides evidence that insulin and leptin acutely regulate astrocytic metabolism within minutes. Additionally, while astrocytes from the hypothalamus and cortex share similarities in their metabolism, they also exhibit distinct properties, further underscoring the growing recognition of astrocyte heterogeneity.

Regulation of Energy and Glucose Homeostasis by the Nucleus of the Solitary Tract and the Area Postrema

The central nervous system regulates feeding, weight and glucose homeostasis in rodents and humans, but the site-specific mechanisms remain unclear. The dorsal vagal complex in the brainstem that contains the nucleus of the solitary tract (NTS) and area postrema (AP) emerges as a regulatory center that impacts energy and glucose balance by monitoring hormonal and nutrient changes. However, the specific mechanistic metabolic roles of the NTS and AP remain elusive. This mini-review highlights methods to study their distinct roles and recent findings on their metabolic differences and similarities of growth differentiation factor 15 (GDF15) action and glucose sensing in the NTS and AP. In summary, future research aims to characterize hormonal and glucose sensing mechanisms in the AP and/or NTS carries potential to unveil novel targets that lower weight and glucose levels in obesity and diabetes.

Crosstalk between obesity and cancer: a role for adipokines

Adipose tissue is a complex organ that is increasingly being recognised as the largest endocrine organ in the body. Adipocytes among multiple cell types of adipose tissue can secrete a variety of adipokines, which are involved in signalling pathways and these can be changed by obesity and cancer. There are proposed mechanisms to link obesity/adiposity to cancer development including adipocytokine dysregulation. Among these adipokines, leptin acts through multiple pathways including the STAT3, MAPK, and PI3K pathways involved in cell growth. Adiponectin has the opposite action from leptin in tumour growth partly because of increased apoptotic responses of p53 and Bax. Visfatin increases cancer cell proliferation through ERK1/2, PI3K/AKT, and p38 which are stimulated by proinflammatory cytokines. Omentin through the PI3K/Akt-Nos pathway is involved in cancer-tumour development. Apelin might be involved through angiogenesis in tumour progressions. PAI-1 via its anti-fibrinolytic activity on cell adhesion and uPA/uPAR activity influence cancer cell growth.

A glucose-sensing mechanism with glucose transporter-1 and pyruvate kinase in the area postrema regulates hepatic glucose production in rats

The area postrema (AP) of the brain is exposed to circulating metabolites and hormones. However, whether AP detects glucose changes to exert biological responses remains unknown. Its neighboring nuclei, the nucleus tractus solitarius (NTS), responds to acute glucose infusion by inhibiting hepatic glucose production, but the mechanism also remains elusive. Herein, we characterized AP and NTS glucose-sensing mechanisms. Infusion of glucose into the AP, like the NTS, of chow rats suppressed glucose production during the pancreatic (basal insulin)-euglycemic clamps. Glucose transporter-1 or pyruvate kinase lentiviral-mediated knockdown in the AP negated AP glucose infusion to lower glucose production, while the glucoregulatory effect of NTS glucose infusion was also negated by knocking down glucose transporter-1 or pyruvate kinase in the NTS. Furthermore, we determined that high-fat (HF) feeding disrupts glucose infusion to lower glucose production in association with a modest reduction in expression of glucose transporter-1, but not pyruvate kinase, in the AP and NTS. However, pyruvate dehydrogenase activator dichloroacetate infusion into the AP or NTS that enhanced downstream pyruvate metabolism and recapitulated the glucoregulatory effect of glucose in chow rats still failed to lower glucose production in HF rats. We discovered that a glucose transporter-1 and pyruvate kinase-dependent glucose-sensing mechanism in the AP (as well as the NTS) lowers glucose production in chow rats, and that HF disrupts the glucose-sensing mechanism that is downstream of pyruvate metabolism in the AP and NTS. These findings highlight the role of AP and NTS in mediating glucose to regulate hepatic glucose production.

Regulatory Basis of Adipokines Leptin and Adiponectin in Epilepsy: from Signaling Pathways to Glucose Metabolism

Epilepsy is a common and severe neurological disorder in which impaired glucose metabolism leads to changes in neuronal excitability that slow or promote the development of epilepsy. Leptin and adiponectin are important mediators regulating glucose metabolism in the peripheral and central nervous systems. Many studies have reported a strong association between epilepsy and these two adipokines involved in multiple signaling cascades and glucose metabolism. Due to the complex regulatory mechanisms between them and various signal activation networks, their role in epilepsy involves many aspects, including the release of inflammatory mediators, oxidative damage, and neuronal apoptosis. This paper aims to summarize the signaling pathways involved in leptin and adiponectin and the regulation of glucose metabolism from the perspective of the pathogenesis of epilepsy. In particular, we discuss the dual effects of leptin in epilepsy and the relationship between antiepileptic drugs and changes in the levels of these two adipokines. Clinical practitioners may need to consider these factors in evaluating clinical drugs. Through this review, we can better understand the specific involvement of leptin and adiponectin in the pathogenesis of epilepsy, provide ideas for further exploration, and bring about practical significance for the treatment of epilepsy, especially for the development of personalized treatment according to individual metabolic characteristics.

Mutual regulation of lactate dehydrogenase and redox robustness

The nature of redox is electron transfer; in this way, energy metabolism brings redox stress. Lactate production is associated with NAD regeneration, which is now recognized to play a role in maintaining redox homeostasis. The cellular lactate/pyruvate ratio could be described as a proxy for the cytosolic NADH/NAD ratio, meaning lactate metabolism is the key to redox regulation. Here, we review the role of lactate dehydrogenases in cellular redox regulation, which play the role of the direct regulator of lactate–pyruvate transforming. Lactate dehydrogenases (LDHs) are found in almost all animal tissues; while LDHA catalyzed pyruvate to lactate, LDHB catalyzed the reverse reaction . LDH enzyme activity affects cell oxidative stress with NAD/NADH regulation, especially LDHA recently is also thought as an ROS sensor. We focus on the mutual regulation of LDHA and redox robustness. ROS accumulation regulates the transcription of LDHA. Conversely, diverse post-translational modifications of LDHA, such as phosphorylation and ubiquitination, play important roles in enzyme activity on ROS elimination, emphasizing the potential role of the ROS sensor and regulator of LDHA.

Antipsychotics impair regulation of glucose metabolism by central glucose

Hypothalamic detection of elevated circulating glucose triggers suppression of endogenous glucose production (EGP) to maintain glucose homeostasis. Antipsychotics alleviate symptoms associated with schizophrenia but also increase the risk for impaired glucose metabolism. In the current study, we examined whether two acutely administered antipsychotics from different drug classes, haloperidol (first generation antipsychotic) and olanzapine (second generation antipsychotic), affect the ability of intracerebroventricular (ICV) glucose infusion approximating postprandial levels to suppress EGP. The experimental protocol consisted of a pancreatic euglycemic clamp, followed by kinomic and RNA-seq analyses of hypothalamic samples to determine changes in serine/threonine kinase activity and gene expression, respectively. Both antipsychotics inhibited ICV glucose-mediated increases in glucose infusion rate during the clamp, a measure of whole-body glucose metabolism. Similarly, olanzapine and haloperidol blocked central glucose-induced suppression of EGP. ICV glucose stimulated the vascular endothelial growth factor (VEGF) pathway, phosphatidylinositol 3-kinase (PI3K) pathway, and kinases capable of activating K_ATP channels in the hypothalamus. These effects were inhibited by both antipsychotics. In conclusion, olanzapine and haloperidol impair central glucose sensing. Although results of hypothalamic analyses in our study do not prove causality, they are novel and provide the basis for a multitude of future studies.

Glucose metabolic crosstalk and regulation in brain function and diseases

Brain glucose metabolism, including glycolysis, the pentose phosphate pathway, and glycogen turnover, produces ATP for energetic support and provides the precursors for the synthesis of biological macromolecules. Although glucose metabolism in neurons and astrocytes has been extensively studied, the glucose metabolism of microglia and oligodendrocytes, and their interactions with neurons and astrocytes, remain critical to understand brain function. Brain regions with heterogeneous cell composition and cell-type-specific profiles of glucose metabolism suggest that metabolic networks within the brain are complex. Signal transduction proteins including those in the Wnt, GSK-3β, PI3K-AKT, and AMPK pathways are involved in regulating these networks. Additionally, glycolytic enzymes and metabolites, such as hexokinase 2, acetyl-CoA, and enolase 2, are implicated in the modulation of cellular function, microglial activation, glycation, and acetylation of biomolecules. Given these extensive networks, glucose metabolism dysfunction in the whole brain or specific cell types is strongly associated with neurologic pathology including ischemic brain injury and neurodegenerative disorders. This review characterizes the glucose metabolism networks of the brain based on molecular signaling and cellular and regional interactions, and elucidates glucose metabolism-based mechanisms of neurological diseases and therapeutic approaches that may ameliorate metabolic abnormalities in those diseases.

Hypothalamic glucose-sensing mechanisms

Chronic metabolic diseases, including diabetes and obesity, have become a major global health threat of the twenty-first century. Maintaining glucose homeostasis is essential for survival in mammals. Complex and highly coordinated interactions between glucose-sensing mechanisms and multiple effector systems are essential for controlling glucose levels in the blood. The central nervous system (CNS) plays a crucial role in regulating glucose homeostasis. Growing evidence indicates that disruption of glucose sensing in selective CNS areas, such as the hypothalamus, is closely interlinked with the pathogenesis of obesity and type 2 diabetes mellitus. However, the underlying intracellular mechanisms of glucose sensing in the hypothalamus remain elusive. Here, we review the current literature on hypothalamic glucose-sensing mechanisms and discuss the impact of alterations of these mechanisms on the pathogenesis of diabetes.

Nutrient infusion in the dorsal vagal complex controls hepatic lipid and glucose metabolism in rats

Hypothalamic regulation of lipid and glucose homeostasis is emerging, but whether the dorsal vagal complex (DVC) senses nutrients and regulates hepatic nutrient metabolism remains unclear. Here, we found in rats DVC oleic acid infusion suppressed hepatic secretion of triglyceride-rich very-low-density lipoprotein (VLDL-TG), which was disrupted by inhibiting DVC long-chain fatty acyl-CoA synthetase that in parallel disturbed lipid homeostasis during intravenous lipid infusion. DVC glucose infusion elevated local glucose levels similarly as intravenous glucose infusion and suppressed hepatic glucose production. This was independent of lactate metabolism as inhibiting lactate dehydrogenase failed to disrupt glucose sensing and neither could DVC lactate infusion recapitulate glucose effect. DVC oleic acid and glucose infusion failed to lower VLDL-TG secretion and glucose production in high-fat fed rats, while inhibiting DVC farnesoid X receptor enhanced oleic acid but not glucose sensing. Thus, an impairment of DVC nutrient sensing may lead to the disruption of lipid and glucose homeostasis in metabolic syndrome.

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DVC oleic acid infusion lowers hepatic secretion of VLDL-TG in chow but not HF rats

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Inhibition of ACSL in the DVC negates lipid sensing

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DVC glucose infusion lowers hepatic glucose production in chow but not HF rats

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Inhibition of FXR in the DVC enhances oleic acid but not glucose sensing in HF rats

Tumor Metabolic Reprogramming by Adipokines as a Critical Driver of Obesity-Associated Cancer Progression

Adiposity is associated with an increased risk of various types of carcinoma. One of the plausible mechanisms underlying the tumor-promoting role of obesity is an aberrant secretion of adipokines, a group of hormones secreted from adipose tissue, which have exhibited both oncogenic and tumor-suppressing properties in an adipokine type- and context-dependent manner. Increasing evidence has indicated that these adipose tissue-derived hormones differentially modulate cancer cell-specific metabolism. Some adipokines, such as leptin, resistin, and visfatin, which are overproduced in obesity and widely implicated in different stages of cancer, promote cellular glucose and lipid metabolism. Conversely, adiponectin, an adipokine possessing potent anti-tumor activities, is linked to a more favorable metabolic phenotype. Adipokines may also play a pivotal role under the reciprocal regulation of metabolic rewiring of cancer cells in tumor microenvironment. Given the fact that metabolic reprogramming is one of the major hallmarks of cancer, understanding the modulatory effects of adipokines on alterations in cancer cell metabolism would provide insight into the crosstalk between obesity, adipokines, and tumorigenesis. In this review, we summarize recent insights into putative roles of adipokines as mediators of cellular metabolic rewiring in obesity-associated tumors, which plays a crucial role in determining the fate of tumor cells.

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 improves intestinal flora dysfunction in mice with high-fat diet-induced obesity

OBJECTIVE

This study investigated the effects of leptin on intestinal flora and inflammation in mice with high-fat diet (HFD)-induced obesity.

METHODS

Mice were fed an HFD for 8 weeks; some were concurrently administered oral leptin for 4 weeks. Pathological changes in adipose tissue were detected using hematoxylin-eosin staining; endotoxin content in adipose tissue was measured by enzyme-linked immunosorbent assay. Intestinal flora were characterized by 16S bacterial rDNA sequencing. Levels of Toll-like receptor 4 (TLR4), nuclear factor-κB inhibitor α (IκB-α), and phosphorylated c-Jun N-terminal kinase (p-JNK) were detected by western blotting.

RESULTS

Mice in the HFD group exhibited weight gain, elevated endotoxin content, and adipocyte hypertrophy, compared with the non-obese control group. Moreover, abundance of bacteria in the Bacteroides genus and community diversity were both reduced in the HFD group; reductions also were observed at corresponding phylum, class, and order levels. Levels of TLR4, IκB-α, and p-JNK were also elevated in the HFD group. Compared with the model group, leptin administration reduced the weight gain and endotoxin content, while increasing Bacteroides abundance and community diversity; it also reduced the levels of TLR4, IκB-α, and p-JNK.

CONCLUSION

Leptin administration improved intestinal flora dysfunction and inflammation in mice with HFD-induced obesity.

Interaction of glucose sensing and leptin action in the brain

Background

In response to energy abundant or deprived conditions, nutrients and hormones activate hypothalamic pathways to maintain energy and glucose homeostasis. The underlying CNS mechanisms, however, remain elusive in rodents and humans.

Scope of review

Here, we first discuss brain glucose sensing mechanisms in the presence of a rise or fall of plasma glucose levels, and highlight defects in hypothalamic glucose sensing disrupt in vivo glucose homeostasis in high-fat fed, obese, and/or diabetic conditions. Second, we discuss brain leptin signalling pathways that impact glucose homeostasis in glucose-deprived and excessed conditions, and propose that leptin enhances hypothalamic glucose sensing and restores glucose homeostasis in short-term high-fat fed and/or uncontrolled diabetic conditions.

Major conclusions

In conclusion, we believe basic studies that investigate the interaction of glucose sensing and leptin action in the brain will address the translational impact of hypothalamic glucose sensing in diabetes and obesity.

iTRAQ-Based Proteomics to Reveal the Mechanism of Hypothalamus in Kidney-Yin Deficiency Rats Induced by Levothyroxine

Kidney-yin deficiency syndrome (KYDS) is a typical syndrome encountered in traditional Chinese medicine (TCM) and is characterized by impaired lipid and glucose homeostasis. The hypothalamus acts as an important regulatory organ by controlling lipid and glucose metabolism in the body. Therefore, proteins in the hypothalamus could play important roles in KYDS development; however, the mechanisms responsible for KYDS remain unclear. Herein, iTRAQ-based proteomics was performed to analyze the protein expression in the hypothalamus of KYDS rats induced by levothyroxine (L-T₄). Results revealed a total of 44 downregulated and 18 upregulated proteins in KYDS group relative to the control group. Gene Ontology (GO) analysis revealed that the differently expressed proteins (DEPs) were related to single-organism metabolism process under the biological process (BP), extracellular region part and organelle under the cellular component (CC), and oxidoreductase activity under the molecular function (MF). Kyoto Encyclopedia of Gene and Genomes (KEGG) analysis showed that fatty acid degradation and pyruvate metabolism participated in the metabolism regulation in KYDS rats. RT-PCR validation of five distinctly expressed proteins related to the two pathways was consistent with the results of proteomics analysis. Taken together, the inhibition of fatty acid degradation and pyruvate metabolism in hypothalamus could potentially cause the dysfunction of the lipid and glucose metabolism in KYDS rats. This current study identified some novel potential biomarkers of KYDS and provided the basis for further research of KYDS.

Integrating Thyroid Hormone Signaling in Hypothalamic Control of Metabolism: Crosstalk Between Nuclear Receptors

The obesity epidemic is well recognized as a significant global health issue. A better understanding of the energy homeostasis mechanisms could help to identify promising anti-obesity therapeutic strategies. It is well established that the hypothalamus plays a pivotal role governing energy balance. The hypothalamus consists of tightly interconnected and specialized neurons that permit the sensing and integration of several peripheral inputs, including metabolic and hormonal signals for an appropriate physiological response. Current evidence shows that thyroid hormones (THs) constitute one of the key endocrine factors governing the regulation and the integration of metabolic homeostasis at the hypothalamic level. THs modulate numerous genes involved in the central control of metabolism, as TRH (Thyrotropin-Releasing Hormone) and MC4R (Melanocortin 4 Receptor). THs act through their interaction with thyroid hormone receptors (TRs). Interestingly, TH signaling, especially regarding metabolic regulations, involves TRs crosstalk with other metabolically linked nuclear receptors (NRs) including PPAR (Peroxisome proliferator-activated receptor) and LXR (Liver X receptor). In this review, we will summarize current knowledge on the important role of THs integration of metabolic pathways in the central regulation of metabolism. Particularly, we will shed light on the crosstalk between TRs and other NRs in controlling energy homeostasis. This could be an important track for the development of attractive therapeutic compounds.

Endocannabinoids in Body Weight Control

Maintenance of body weight is fundamental to maintain one's health and to promote longevity. Nevertheless, it appears that the global obesity epidemic is still constantly increasing. Endocannabinoids (eCBs) are lipid messengers that are involved in overall body weight control by interfering with manifold central and peripheral regulatory circuits that orchestrate energy homeostasis. Initially, blocking of eCB signaling by first generation cannabinoid type 1 receptor (CB1) inverse agonists such as rimonabant revealed body weight-reducing effects in laboratory animals and men. Unfortunately, rimonabant also induced severe psychiatric side effects. At this point, it became clear that future cannabinoid research has to decipher more precisely the underlying central and peripheral mechanisms behind eCB-driven control of feeding behavior and whole body energy metabolism. Here, we will summarize the most recent advances in understanding how central eCBs interfere with circuits in the brain that control food intake and energy expenditure. Next, we will focus on how peripheral eCBs affect food digestion, nutrient transformation and energy expenditure by interfering with signaling cascades in the gastrointestinal tract, liver, pancreas, fat depots and endocrine glands. To finally outline the safe future potential of cannabinoids as medicines, our overall goal is to address the molecular, cellular and pharmacological logic behind central and peripheral eCB-mediated body weight control, and to figure out how these precise mechanistic insights are currently transferred into the development of next generation cannabinoid medicines displaying clearly improved safety profiles, such as significantly reduced side effects.

Selected Physiological Effects of a Garcinia Gummi-Gutta Extract in Rats Fed with Different Hypercaloric Diets

Garcinia gummi-gutta (GGG) rind extract is effective for reducing appetite, body weight and adiposity of obese rodents fed high-fat (HF), high-sugar (HS) or high fat/sugar (HFS)-based diets, but these effects have not been simultaneously evaluated. Thirty obese (~425 g) male Wistar rats were fed for eleven weeks with six hypercaloric diets (4.1 kcal/g; five rats/diet) non-supplemented (HF, HS, HFS), or supplemented (HF+, HS+, HFS+) with GGG extract (5.9%), while rats from the control group (375 g) were fed a normocaloric diet (3.5 kcal/g). Body weight, dietary intake, body fat distribution, and histological and biochemical parameters were recorded. Compared to control rats, non-supplemented and supplemented groups consumed significantly less food (14.3% and 24.6% (−4.3 g/day), respectively) (p < 0.05). Weight loss was greater in the HF+ group (35⁻52 g), which consumed 1.9 times less food than the HS+ or HFS+ fed groups. The HF and HFS groups showed 40% less plasma triacylglycerides and lower glucose levels compared to the HF+. GGG-supplemented diets were associated with lower ketonuria. The HF+ diet was associated with the best anti-adiposity effect (as measured with the dual X-ray absorptiometry (DXA) and Soxhlet methods). The severity of hepatocyte lipidosis was HF > control > HF+, and no signs of toxicity in the testes were observed. The results indicate that GGG is more effective when co-administered with HF diets in obese rats.