Leptin signalling pathways in hypothalamic neurons

Appetite Regulation: Hormones, Peptides, and Neurotransmitters and Their Role in Obesity

Understanding body weight regulation will aid in the development of new strategies to combat obesity. This review examines energy homeostasis and food intake behaviors, specifically with regards to hormones, peptides, and neurotransmitters in the periphery and central nervous system, and their potential role in obesity. Dysfunction in feeding signals by the brain is a factor in obesity. The hypothalamic (arcuate nucleus) and brainstem (nucleus tractus solitaris) areas integrate behavioral, endocrine, and autonomic responses via afferent and efferent pathways from and to the brainstem and peripheral organs. Neurons present in the arcuate nucleus express pro-opiomelanocortin, Neuropeptide Y, and Agouti Related Peptide, with the former involved in lowering food intake, and the latter two acutely increasing feeding behaviors. Action of peripheral hormones from the gut, pancreas, adipose, and liver are also involved in energy homeostasis. Vagal afferent neurons are also important in regulating energy homeostasis. Peripheral signals respond to the level of stored and currently available fuel. By studying their actions, new agents maybe developed that disable orexigenic responses and enhance anorexigenic signals. Although there are relatively few medications currently available for obesity treatment, a number of agents are in development that work through these pathways.

New insight into inter-organ crosstalk contributing to the pathogenesis of non-alcoholic fatty liver disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver dysfunction and a significant global health problem with substantial rise in prevalence over the last decades. It is becoming increasingly clear that NALFD is not only predominantly a hepatic manifestation of metabolic syndrome, but also involves extra-hepatic organs and regulatory pathways. Therapeutic options are limited for the treatment of NAFLD. Accordingly, a better understanding of the pathogenesis of NAFLD is critical for gaining new insight into the regulatory network of NAFLD and for identifying new targets for the prevention and treatment of NAFLD. In this review, we emphasize on the current understanding of the inter-organ crosstalk between the liver and peripheral organs that contributing to the pathogenesis of NAFLD.

Translating the biology of adipokines in atherosclerosis and cardiovascular diseases: Gaps and open questions

AIM

Critically discuss the available data, to identify the current gaps and to provide key concepts that will help clinicians in translating the biology of adipokines in the context of atherosclerosis and cardio-metabolic diseases.

DATA SYNTHESIS

Adipose tissue is nowadays recognized as an active endocrine organ, a function related to the ability to secrete adipokines (such as leptin and adiponectin) and pro-inflammatory cytokines (tumor necrosis factor alpha and resistin). Studies in vitro and in animal models have observed that obesity status presents a chronic low-grade inflammation as the consequence of the immune cells infiltrating the adipose tissue as well as adipocytes. This inflammatory signature is often related to the presence of cardiovascular diseases, including atherosclerosis and thrombosis. These links are less clear in humans, where the role of adipokines as prognostic marker and/or player in cardiovascular diseases is not as clear as that observed in experimental models. Moreover, plasma adipokine levels might reflect a condition of adipokine-resistance in which adipokine redundancy occurs. The investigation of the cardio-metabolic phenotype of carriers of single nucleotide polymorphisms affecting the levels or function of a specific adipokine might help determine their relevance in humans. Thus, the aim of the present review is to critically discuss the available data, identify the current gaps and provide key concepts that will help clinicians translate the biology of adipokines in the context of atherosclerosis and cardio-metabolic diseases.

Aminoglycosides, but not PTC124 (Ataluren), rescue nonsense mutations in the leptin receptor and in luciferase reporter genes

In rare cases, monogenetic obesity is caused by nonsense mutations in genes regulating energy balance. A key factor herein is the leptin receptor. Here, we focus on leptin receptor nonsense variants causing obesity, namely the human W31X, murine Y333X and rat Y763X mutations, and explored their susceptibilities to aminoglycoside and PTC124 mediated translational read-through in vitro. In a luciferase based assay, all mutations - when analysed within the mouse receptor - were prone to aminoglycoside mediated nonsense suppression with the highest susceptibility for W31X, followed by Y763X and Y333X. For the latter, the corresponding rodent models appear valuable for in vivo experiments. When W31X was studied in the human receptor, its superior read-through susceptibility – initially observed in the mouse receptor – was eliminated, likely due to the different nucleotide context surrounding the mutation in the two orthologues. The impact of the surrounding context on the read-through opens the possibility to discover novel sequence elements influencing nonsense suppression. As an alternative to toxic aminoglycosides, PTC124 was indicated as a superior nonsense suppressor but inconsistent data concerning its read-through activity are reported. PTC124 failed to rescue W31X as well as different nonsense mutated luciferase reporters, thus, challenging its ability to induce translational read-through.

Reduced body weight gain in ubiquilin-1 transgenic mice is associated with increased expression of energy-sensing proteins

Ubiquilin-1 (Ubqln1), a ubiquitin-like protein, is implicated in a variety of pathophysiological processes, but its role in mediating body weight gain or metabolism has not been determined. Here, we demonstrate that global overexpression of Ubqln1 in a transgenic (Tg) mouse reduces the animal's body weight gain. The decreased body weight gain in Tg mice is associated with lower visceral fat content and higher metabolic rate. The Ubqln1 Tg mice exhibited reduced leptin and insulin levels as well as increased insulin sensitivity manifested by homeostatic model assessment of insulin resistance. Additionally, the reduced body weight in Tg mice was associated with the upregulation of two energy-sensing proteins, sirtuin1 (SIRT1) in the hypothalamus and AMP-activated protein kinase (AMPK) in the skeletal muscle. Consistent with the in vivo results, overexpression of Ubqln1 significantly increased SIRT1 and AMPK levels in the mouse embryonic fibroblast cell culture. Thus, our results not only establish the link between Ubqln1 and body weight regulation but also indicate that the metabolic function of Ubqln1 on body weight may be through regulating energy-sensing proteins.

Non-Neuronal Cells in the Hypothalamic Adaptation to Metabolic Signals

Although the brain is composed of numerous cell types, neurons have received the vast majority of attention in the attempt to understand how this organ functions. Neurons are indeed fundamental but, in order for them to function correctly, they rely on the surrounding "non-neuronal" cells. These different cell types, which include glia, epithelial cells, pericytes, and endothelia, supply essential substances to neurons, in addition to protecting them from dangerous substances and situations. Moreover, it is now clear that non-neuronal cells can also actively participate in determining neuronal signaling outcomes. Due to the increasing problem of obesity in industrialized countries, investigation of the central control of energy balance has greatly increased in attempts to identify new therapeutic targets. This has led to interesting advances in our understanding of how appetite and systemic metabolism are modulated by non-neuronal cells. For example, not only are nutrients and hormones transported into the brain by non-neuronal cells, but these cells can also metabolize these metabolic factors, thus modifying the signals reaching the neurons. The hypothalamus is the main integrating center of incoming metabolic and hormonal signals and interprets this information in order to control appetite and systemic metabolism. Hence, the factors transported and released from surrounding non-neuronal cells will undoubtedly influence metabolic homeostasis. This review focuses on what is known to date regarding the involvement of different cell types in the transport and metabolism of nutrients and hormones in the hypothalamus. The possible involvement of non-neuronal cells, in particular glial cells, in physiopathological outcomes of poor dietary habits and excess weight gain are also discussed.

Leptin-LepRb Expressed in Gastric Cancer Patients and Related to Cancer-Related Depression

Depression is the most common psychiatric disorder among cancer patients. Studies have not only highlighted that leptin and its receptor (LepRb) are independent poor prognostic factors in gastric cancer (GC) patients but also shown that the leptin-LepRb is necessary for antidepressant-like behaviors. In this study, we examined the serum and tissue leptin-LepRb expression in GC patients. Enzyme-linked immunosorbent assay showed that depressive GC patients had significantly higher serum leptin-LepRb than healthy donors. Leptin-LepRb levels in GC tissues were also significantly higher than in matched paracarcinoma tissues using real-time RT-PCR. Moreover, we observed that both serum and tissue leptin-LepRb were significantly higher in depressive GC patients than those in nondepressive GC patients. Further, the patients with high tumor stage tend to have higher leptin-LepRb mRNA levels than that with low tumor stage. Together, our findings suggest that leptin-LepRb plays an important role in the pathogenesis and depression in GC. Leptin-LepRb therefore could be a potential diagnostic marker and therapeutic target in GC patients with depression.

Changes in the levels and phosphorylation status of Akt, AMPK, CREB, and FoxO1 in hypothalamus of rainbow trout under conditions of enhanced glucosensing activity

There is no available information in fish about mechanisms linking glucosensing activation and changes in the expression of brain neuropeptides controlling food intake. Therefore, we assessed in rainbow trout hypothalamus the effects of raised levels of glucose on the levels and phosphorylation status of two transcription factors, FoxO1 and CREB, possibly involved in linking those processes. Moreover, we also aimed to assess the changes in the levels and phosphorylation status of two proteins possibly involved in the modulation of these transcription factors such as Akt and AMPK. Therefore, we evaluated in pools of hypothalamus incubated for 3h and 6h at 15 °C in modified Hanks’ medium containing 2, 4, or 8 mM D-glucose the response of parameters related to glucosensing mechanisms, neuropeptide expression, and levels and phosphorylation status of proteins of interest. The activation of hypothalamic glucosensing systems and the concomitant enhanced anorectic potential occurred in parallel with activation of Akt and inhibition of AMPK. The changes in these proteins would relate to neuropeptide expression through changes in the levels and phosphorylation status of transcription factors under their control, such as CREB and FoxO1, which displayed inhibitory (CREB) or activatory (FoxO1) responses to increased glucose.

Diet-Induced Obesity and the Mechanism of Leptin Resistance

Leptin signaling blockade by chronic overstimulation of the leptin receptor or hypothalamic pro-inflammatory responses due to elevated levels of saturated fatty acid can induce leptin resistance by activating negative feedback pathways. Although, long form leptin receptor (Ob-Rb) initiates leptin signaling through more than seven different signal transduction pathways, excessive suppressor of cytokine signaling-3 (SOCS-3) activity is a potential mechanism for the leptin resistance that characterizes human obesity. Because the leptin-responsive metabolic pathways broadly integrate with other neurons to control energy balance, the methods used to counteract the leptin resistance has extremely limited effect. In this chapter, besides the impairment of central and peripheral leptin signaling pathways, limited access of leptin to central nervous system (CNS) through blood-brain barrier, mismatch between high leptin and the amount of leptin receptor expression, contradictory effects of cellular and circulating molecules on leptin signaling, the connection between leptin signaling and endoplasmic reticulum (ER) stress and self-regulation of leptin signaling has been discussed in terms of leptin resistance.

PI3K signaling: A molecular pathway associated with acute hypophagic response during inflammatory challenges

Energy balance has in the hypothalamus a central component of integration of food intake and energy expenditure. An accumulating body of evidence indicates that energy homeostasis is largely affected by inflammatory challenges. Severe undernutrition caused by exacerbated inflammatory response may lead to cachexia. On the other hand, prolonged low-grade inflammation such as that observed in obesity and metabolic syndrome, raises the risk for the development of diabetes and heart diseases. Changes in circulating insulin and cytokines such as leptin, interleukins and tumor necrosis factor, as well as changes in their action in the hypothalamus drive the inhibition of food consumption during inflammation. The molecular pathways associated with these responses have only started to be unraveled. One potential candidate is the PI3K signaling, an important player in distinct hypothalamic neurons that control food intake. This study presents an overview of the current knowledge about PI3K role on cytokines and insulin signaling in the hypothalamic regulation of feeding during inflammation.

Epac2a-null mice exhibit obesity-prone nature more susceptible to leptin resistance

BACKGROUND

The exchange protein directly activated by cAMP (Epac), which is primarily involved in cAMP signaling, has been known to be essential for controlling body energy metabolism. Epac has two isoforms: Epac1 and Epac2. The function of Epac1 on obesity was unveiled using Epac1 knockout (KO) mice. However, the role of Epac2 in obesity remains unclear.

METHODS

To evaluate the role of Epac2 in obesity, we used Epac2a KO mice, which is dominantly expressed in neurons and endocrine tissues. Physiological factors related to obesity were analyzed: body weight, fat mass, food intake, plasma leptin and adiponectin levels, energy expenditure, glucose tolerance, and insulin and leptin resistance. To determine the mechanism of Epac2a, mice received exogenous leptin and then hypothalamic leptin signaling was analyzed.

RESULTS

Epac2a KO mice appeared to have normal glucose tolerance and insulin sensitivity until 12 weeks of age, but an early onset increase of plasma leptin levels and decrease of plasma adiponectin levels compared with wild-type mice. Acute leptin injection revealed impaired hypothalamic leptin signaling in KO mice. Consistently, KO mice fed a high-fat diet (HFD) were significantly obese, presenting greater food intake and lower energy expenditure. HFD-fed KO mice were also characterized by greater impairment of hypothalamic leptin signaling and by weaker leptin-induced decrease in food consumption compared with HFD-fed wild-type mice. In wild-type mice, acute exogenous leptin injection or chronic HFD feeding tended to induce hypothalamic Epac2a expression.

CONCLUSIONS

Considering that HFD is an inducer of hypothalamic leptin resistance and that Epac2a functions in pancreatic beta cells during demands of greater work load, hypothalamic Epac2a may have a role in facilitating leptin signaling, at least in response to higher metabolic demands. Thus, our data indicate that Epac2a is critical for preventing obesity and thus Epac2a activators may be used to manage obesity and obesity-mediated metabolic disorders.

Investigating the role of adipokines in chronic migraine

Background and aims

Adiponectin, leptin, and resistin are adipocyte-derived secretory factors involved in endothelial function, weight, inflammation, and insulin resistance. Recent studies suggested a role for adipokines in episodic migraine as mediators of inflammatory processes. The aim of this study was to investigate plasma concentrations of adiponectin, leptin, and resistin in patients with chronic migraine.

Materials and methods

Twenty-seven chronic migraineurs (20 females, 7 males; mean age 49.0 ± 9.0 yrs) and 37 healthy controls (23 females, 14 males; mean age 49.8 ± 15.0 yrs) were selected for the study. Fasting plasmatic levels of total adiponectin, leptin, and resistin were measured using ELISA kits during a pain-free period. Fasting glucose, insulin, total and HDL-cholesterol, triglycerides, and ESR were also determined.

Results

Serum levels of adiponectin and resistin were significantly increased in chronic migraineurs in comparison with controls ( p = 0.001 and p = 0.032, respectively). After correction for BMI, sex and age, leptin levels were significantly increased in chronic migraineurs ( p = 0.007). A positive correlation between leptin concentrations and both indices of insulin resistance and markers of inflammation was found.

Discussion

Our data suggest that adiponectin and resistin are altered in non-obese chronic migraineurs. Further studies are needed to elucidate the neurobiological mechanisms underlying adipokine dysfunction in migraine.

Effects of cereal fiber on leptin resistance and sensitivity in C57BL/6J mice fed a high-fat/cholesterol diet

BACKGROUND

Cereal fiber is reported to be associated with obesity and metabolic diseases. However, whether cereal fiber improves leptin resistance and sensitivity remains unclear.

DESIGN

For 24 weeks, 48 male C57BL/6J mice were randomly given a normal chow diet (Chow), high-fat/cholesterol diet (HFD), HFD with 0.8% oat fiber (H-oat) or HFD with 0.8% wheat bran fiber (H-wheat). At the end of feeding period, both the serum insulin and leptin levels were determined by ELISA kits. Western blotting was used to assess the protein expressions of the leptin receptor (LepR) and the leptin-signaling pathway in the adipose tissues.

RESULTS

Our results suggested that mice fed oat or wheat bran fiber exhibited lower body weight, serum lipids, as well as insulin and leptin levels. The two cereal fibers potently increased the protein expressions of LepR in the adipose tissue. In addition, protein expressions of Janus kinase 2 (JAK2) and transcription 3 (STAT3) (induced by LepR), which enhances leptin signaling, were significantly higher and the expression of cytokine signaling-3 (SOCS3), which inhibits leptin signaling, was significantly lower in the two cereal fiber groups than in the HFD group.

CONCLUSION

Taken together, our findings suggest that cereal fiber can improve leptin resistance and sensitivity by the JAK2/STAT3 pathway in C57BL/6J mice fed a HFD; furthermore, oat fiber is more effective in the improvement of leptin sensitivity than wheat bran fiber, in this murine model.

Antioxidant Activity of γ-Oryzanol: A Complex Network of Interactions

γ-oryzanol (Orz), a steryl ferulate extracted from rice bran layer, exerts a wide spectrum of biological activities. In addition to its antioxidant activity, Orz is often associated with cholesterol-lowering, anti-inflammatory, anti-cancer and anti-diabetic effects. In recent years, the usefulness of Orz has been studied for the treatment of metabolic diseases, as it acts to ameliorate insulin activity, cholesterol metabolism, and associated chronic inflammation. Previous studies have shown the direct action of Orz when downregulating the expression of genes that encode proteins related to adiposity (CCAAT/enhancer binding proteins (C/EBPs)), inflammatory responses (nuclear factor kappa-B (NF-κB)), and metabolic syndrome (peroxisome proliferator-activated receptors (PPARs)). It is likely that this wide range of beneficial activities results from a complex network of interactions and signals triggered, and/or inhibited by its antioxidant properties. This review focuses on the significance of Orz in metabolic disorders, which feature remarkable oxidative imbalance, such as impaired glucose metabolism, obesity, and inflammation.

PI3K p110β subunit in leptin receptor expressing cells is required for the acute hypophagia induced by endotoxemia

Objective

Hypophagia and increased energy expenditure under inflammatory conditions, such as that observed after bacterial lipopolysaccharide (LPS) administration, are associated with leptin secretion. The hypophagic effect of leptin depends in part on the activation of PI3K signaling pathway. However, the role of PI3K in the endotoxemia-induced hypophagia has not been determined.

Methods

In an attempt to examine the functional contribution of the PI3K pathway in hypophagia and weight loss induced by LPS (100 ug/Kg, ip), we performed a central pharmacological PI3K inhibition (LY294002). Additionally, to gain mechanistic insights on the role of the catalytic PI3K p110α subunit in leptin responsive cells, mice expressing Cre-recombinase driven by the Lepr promoter (LepR-Cre) were crossed with mice carrying a loxP-modified p110α allele (Pi3kca gene) (LepR^Δp110α). As studies have suggested that the PI3K p110β subunit has a dominant role over p110α in energy homeostasis, we further crossed LepR-Cre mice with loxP-modified p110α and p110β (Pi3kcb gene) alleles (LepR^Δp110α+β). In order to verify the requirement of leptin in PI3K effects on food intake, we also used leptin-deficient ob/ob mice.

Results

We found that LPS stimulates PI3K and STAT3 signaling pathways in cells expressing the leptin receptor. Central PI3K inhibition prevented LPS-induced hypophagia and weight loss. Genetic deletion of p110α subunit selectively in LepR cells had no effect on LPS-induced hypophagia and weight loss. However, p110α and p110β double deletion in LepR cells prevented LPS-induced hypophagia and partially reversed the weight loss. Leptin deficiency blunted LPS-induced acute pAKT and pSTAT3 phosphorylation and the acute suppression of food intake.

Conclusions

Our studies show that the PI3K p110β subunit in LepR cells is required for acute endotoxemic hypophagia. The data provide promising approaches for PI3K inhibition in preventing low energy balance and cachectic states during inflammatory challenges.

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Bacterial lipopolyssacharide (LPS) stimulates PI3K pathway in hypothalamic LepR expressing cells.

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LPS-induced hypophagia is prevented by central PI3K inhibition.

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PI3K p110α subunit in LepR cells is not required for LPS-induced hypophagia.

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PI3K p110α and p110β double deletion in LepR cells prevents LPS-induced hypophagia.