Physiological role for leptin in the control of thermal conductance

Adipose Tissue Dysfunction Related to Climate Change and Air Pollution: Understanding the Metabolic Consequences

Obesity, a global pandemic, poses a major threat to healthcare systems worldwide. Adipose tissue, the energy-storing organ during excessive energy intake, functions as a thermoregulator, interacting with other tissues to regulate systemic metabolism. Specifically, brown adipose tissue (BAT) is positively associated with an increased resistance to obesity, due to its thermogenic function in the presence of uncoupled protein 1 (UCP1). Recently, studies on climate change and the influence of environmental pollutants on energy homeostasis and obesity have drawn increasing attention. The reciprocal relationship between increasing adiposity and increasing temperatures results in reduced adaptive thermogenesis, decreased physical activity, and increased carbon footprint production. In addition, the impact of climate change makes obese individuals more prone to developing type 2 diabetes mellitus (T2DM). An impaired response to heat stress, compromised vasodilation, and sweating increase the risk of diabetes-related comorbidities. This comprehensive review provides information about the effects of climate change on obesity and adipose tissue, the risk of T2DM development, and insights into the environmental pollutants causing adipose tissue dysfunction and obesity. The effects of altered dietary patterns on adiposity and adaptation strategies to mitigate the detrimental effects of climate change are also discussed.

Hypothalamic Neuromodulation and Control of the Dermal Surface Temperature of Livestock during Hyperthermia

Hyperthermia elicits several physiological and behavioral responses in livestock to restore thermal neutrality. Among these responses, vasodilation and sweating help to reduce core body temperature by increasing heat dissipation by radiation and evaporation. Thermoregulatory behaviors such as increasing standing time, reducing feed intake, shade-seeking, and limiting locomotor activity also increase heat loss. These mechanisms are elicited by the connection between peripheral thermoreceptors and cerebral centers, such as the preoptic area of the hypothalamus. Considering the importance of this thermoregulatory pathway, this review aims to discuss the hypothalamic control of hyperthermia in livestock, including the main physiological and behavioral changes that animals adopt to maintain their thermal stability.

Neural circuits of long-term thermoregulatory adaptations to cold temperatures and metabolic demands

The mammalian brain controls heat generation and heat loss mechanisms that regulate body temperature and energy metabolism. Thermoeffectors include brown adipose tissue, cutaneous blood flow and skeletal muscle, and metabolic energy sources include white adipose tissue. Neural and metabolic pathways modulating the activity and functional plasticity of these mechanisms contribute not only to the optimization of function during acute challenges, such as ambient temperature changes, infection and stress, but also to longitudinal adaptations to environmental and internal changes. Exposure of humans to repeated and seasonal cold ambient conditions leads to adaptations in thermoeffectors such as habituation of cutaneous vasoconstriction and shivering. In animals that undergo hibernation and torpor, neurally regulated metabolic and thermoregulatory adaptations enable survival during periods of significant reduction in metabolic rate. In addition, changes in diet can activate accessory neural pathways that alter thermoeffector activity. This knowledge may be harnessed for therapeutic purposes, including treatments for obesity and improved means of therapeutic hypothermia.

High-fat diet feeding disrupts the coupling of thermoregulation to energy homeostasis

OBJECTIVE

Preserving core body temperature across a wide range of ambient temperatures requires adaptive changes of thermogenesis that must be offset by corresponding changes of energy intake if body fat stores are also to be preserved. Among neurons implicated in the integration of thermoregulation with energy homeostasis are those that express both neuropeptide Y (NPY) and agouti-related protein (AgRP) (referred to herein as AgRP neurons). Specifically, cold-induced activation of AgRP neurons was recently shown to be required for cold exposure to increase food intake in mice. Here, we investigated how consuming a high-fat diet (HFD) impacts various adaptive responses to cold exposure as well as the responsiveness of AgRP neurons to cold.

METHODS

To test this, we used immunohistochemistry, in vivo fiber photometry and indirect calorimetry for continuous measures of core temperature, energy expenditure, and energy intake in both chow- and HFD-fed mice housed at different ambient temperatures.

RESULTS

We show that while both core temperature and the thermogenic response to cold are maintained normally in HFD-fed mice, the increase of energy intake needed to preserve body fat stores is blunted, resulting in weight loss. Using both immunohistochemistry and in vivo fiber photometry, we show that although cold-induced AgRP neuron activation is detected regardless of diet, the number of cold-responsive neurons appears to be blunted in HFD-fed mice.

CONCLUSIONS

We conclude that HFD-feeding disrupts the integration of systems governing thermoregulation and energy homeostasis that protect body fat mass during cold exposure.

Leptin deficiency impairs adipogenesis and browning response in mouse mesenchymal progenitors

Although phenotypically different, brown adipose tissue (BAT) and inguinal white adipose tissue (iWAT) are able to produce heat through non-shivering thermogenesis due to the presence of mitochondrial uncoupling protein 1 (UCP1). The appearance of thermogenically active beige adipocytes in iWAT is known as browning. Both brown and beige cells originate from mesenchymal stem cells (MSCs), and in culture conditions a browning response can be induced with hypothermia (i.e. 32 °C) during which nuclear leptin immunodetection was observed. The central role of leptin in regulating food intake and energy consumption is well recognised, but its importance in the browning process at the cellular level is unclear. Here, immunocytochemical analysis of MSC-derived adipocytes established nuclear localization of both leptin and leptin receptor suggesting an involvement of the leptin pathway in the browning response. In order to elucidate whether leptin modulates the expression of brown and beige adipocyte markers, BAT and iWAT samples from leptin-deficient (ob/ob) mice were analysed and exhibited reduced brown/beige marker expression compared to wild-type controls. When MSCs were isolated and differentiated into adipocytes, leptin deficiency was observed to induce a white phenotype, especially when incubated at 32 °C. These adaptations were accompanied with morphological signs of impaired adipogenic differentiation. Overall, our results indicate that leptin supports adipocyte browning and suggest a potential role for leptin in adipogenesis and browning.

Warm Responsive Neurons in the Hypothalamic Preoptic Area are Potent Regulators of Glucose Homeostasis in Male Mice

When mammals are exposed to a warm environment, overheating is prevented by activation of "warm-responsive" neurons (WRNs) in the hypothalamic preoptic area (POA) that reduce thermogenesis while promoting heat dissipation. Heat exposure also impairs glucose tolerance, but whether this also results from activation of POA WRNs is unknown. To address this question, we sought in the current work to determine if glucose intolerance induced by heat exposure can be attributed to activation of a specific subset of WRNs that express pituitary adenylate cyclase-activating peptide (ie, POAPacap neurons). We report that when mice are exposed to an ambient temperature sufficiently warm to activate POAPacap neurons, the expected reduction of energy expenditure is associated with glucose intolerance, and that these responses are recapitulated by chemogenetic POAPacap neuron activation. Because heat-induced glucose intolerance was not blocked by chemogenetic inhibition of POAPacap neurons, we conclude that POAPacap neuron activation is sufficient, but not required, to explain the impairment of glucose tolerance elicited by heat exposure.

The Role of Selected Adipocytokines in Ovarian Cancer and Endometrial Cancer

Due to their multidirectional influence, adipocytokines are currently the subject of numerous intensive studies. Significant impact applies to many processes, both physiological and pathological. Moreover, the role of adipocytokines in carcinogenesis seems particularly interesting and not fully understood. For this reason, ongoing research focuses on the role of these compounds in the network of interactions in the tumor microenvironment. Particular attention should be drawn to cancers that remain challenging for modern gynecological oncology-ovarian and endometrial cancer. This paper presents the role of selected adipocytokines, including leptin, adiponectin, visfatin, resistin, apelin, chemerin, omentin and vaspin in cancer, with a particular focus on ovarian and endometrial cancer, and their potential clinical relevance.

The Influence of Ambient Temperature on Adipose Tissue Homeostasis, Metabolic Diseases and Cancers

Adipose tissue is a recognized energy storage organ during excessive energy intake and an endocrine and thermoregulator, which interacts with other tissues to regulate systemic metabolism. Adipose tissue dysfunction is observed in most obese mouse models and humans. However, most studies using mouse models were conducted at room temperature (RT), where mice were chronically exposed to mild cold. In this condition, energy use is prioritized for thermogenesis to maintain body temperature in mice. It also leads to the activation of the sympathetic nervous system, followed by the activation of β-adrenergic signaling. As humans live primarily in their thermoneutral (TN) zone, RT housing for mice limits the interpretation of disease studies from mouse models to humans. Therefore, housing mice in their TN zone (~28–30 °C) can be considered to mimic humans physiologically. However, factors such as temperature ranges and TN pre-acclimatization periods should be examined to obtain reliable results. In this review, we discuss how adipose tissue responds to housing temperature and the outcomes of the TN zone in metabolic disease studies. This review highlights the critical role of TN housing in mouse models for studying adipose tissue function and human metabolic diseases.

Leptin as an Antitorpor Hormone: An Explanation for the Increased Metabolic Efficiency and Cold Sensitivity of ob/ob Mice?

AbstractLeptin is recognized as an anorexigenic hormone. In its absence (e.g., in ob/ob mutant mice), mice become obese, primarily as a result of hyperphagia. A recurrent question is whether, additionally, leptin is thermogenic and thus also an antiobesity hormone in this way. We have earlier reviewed available data and have concluded that most articles implying a thermogenic effect of leptin have based this on a misconstrued division by body weight. Here, we have collected evidence that the remaining observations that imply that leptin is a thermogenic hormone are better understood as implying that leptin is an antitorpor hormone. Leptin levels increase in proportion to the body's energy reserves (i.e., stored lipids in the adipose tissue), and leptin thus serves as an indicator of energy availability. In the absence of leptin, ob/ob mice are exceedingly prone to enter daily torpor, since the absence of leptin causes them to perceive a lack of body energy reserves that, in combination with restricted or no food, induces them to enter the torpid state to save energy. This antitorpor effect of leptin probably explains the following earlier observations. First, ob/ob mice have the ability to gain weight even when pair fed with leptin-treated ob/ob mice. This is understood as follows: In the leptin-treated ob/ob mice, food intake is reduced. Untreated pair-fed mice enter daily torpor, and this markedly lowers total daily energy expenditure; the resulting surplus food energy is then accumulated as fat in these mice. However, ob/ob mice fed ad lib. do not enter torpor, so under normal conditions this mechanism does not contribute to the obesity found in the ob/ob mice. Second, neonatal ob/ob mice have the ability to become obese despite eating the same amount as wild-type mice: this is understood as these mice similarly entering daily torpor. Third, ob/ob mice on the C57BL/6J background have a lower metabolic rate: these mice were examined in the absence of food, and torpor was thus probably induced. Fourth, ob/ob mice have apparent high cold sensitivity: these mice experienced cold in the absence of food and would immediately enter deep torpor. It is suggested that this novel explanation of how the antitorpor effects of leptin affect mouse energy metabolism can open new avenues for leptin research.

Roles of leptin on energy balance and thermoregulation in Eothenomys miletus

Leptin is a hormone mainly synthesized and secreted by white adipose tissue (WAT), which regulates various physiological processes. To investigate the role of leptin in energy balance and thermoregulation in Eothenomys miletus, voles were randomly divided into leptin-injected and PBS-injected groups and placed at 25°C ± 1°C with a photoperiod of 12 L:12 D. They were housed under laboratory conditions for 28 days and compared in terms of body mass, food intake, water intake, core body temperature, interscapular skin temperature, resting metabolic rate (RMR), nonshivering thermogenesis (NST), liver and brown adipose tissue (BAT) thermogenic activity, and serum hormone levels. The results showed that leptin injection decreased body mass, body fat, food intake, and water intake. But it had no significant effect on carcass protein. Leptin injection increased core body temperature, interscapular skin temperature, resting metabolic rate, non-shivering thermogenesis, mitochondrial protein content and cytochrome C oxidase (COX) activity in liver and brown adipose tissue, uncoupling protein 1 (UCP1) content and thyroxin 5'-deiodinase (T45'-DII) activity in brown adipose tissue significantly. Serum leptin, triiodothyronine (T3), thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH) concentrations were also increased significantly. Correlation analysis showed that serum leptin levels were positively correlated with core body temperature, body mass loss, uncoupling protein 1 content, thyroxin 5'-deiodinase activity, nonshivering thermogenesis, and negatively correlated with food intake; thyroxin 5'-deiodinase and triiodothyronine levels were positively correlated, suggesting that thyroxin 5'-deiodinase may play an important role in leptin-induced thermogenesis in brown adipose tissue. In conclusion, our study shows that exogenous leptin is involved in the regulation of energy metabolism and thermoregulation in E. miletus, and thyroid hormone may play an important role in the process of leptin regulating energy balance in E. miletus.

Impact of thermogenesis induced by chronic β3-adrenergic receptor agonist treatment on inflammatory and infectious response during bacteremia in mice

White adipocytes store energy differently than brown and brite adipocytes which dissipate energy under the form of heat. Studies have shown that adipocytes are able to respond to bacteria thanks to the presence of Toll-like receptors at their surface. Despite this, little is known about the involvement of each class of adipocytes in the infectious response. We treated mice for one week with a β3-adrenergic receptor agonist to induce activation of brown adipose tissue and brite adipocytes within white adipose tissue. Mice were then injected intraperitoneally with E. coli to generate acute infection. The metabolic, infectious and inflammatory parameters of the mice were analysed during 48 hours after infection. Our results shown that in response to bacteria, thermogenic activity promoted a discrete and local anti-inflammatory environment in white adipose tissue characterized by the increase of the IL-1RA secretion. More generally, activation of brown and brite adipocytes did not modify the host response to infection including no additive effect with fever and an equivalent bacteria clearance and inflammatory response. In conclusion, these results suggest an IL-1RA-mediated immunomodulatory activity of thermogenic adipocytes in response to acute bacterial infection and open a way to characterize their effect along more chronic infection as septicaemia.

Intermittent cold exposure improves glucose homeostasis despite exacerbating diet-induced obesity in mice housed at thermoneutrality

KEY POINTS

Ambient cold exposure is often regarded as a promising anti-obesity treatment in mice. However, most preclinical studies aimed at treating obesity via cold-induced thermogenesis have been confounded by subthermoneutral housing temperatures. Therefore, the ability of ambient cold to combat diet-induced obesity in mice housed under humanized thermoneutral conditions is currently unknown. Moreover, mammals such as mice are rarely exposed to chronic ambient cold without reprieve, yet mice are often subjected to experimental conditions of chronic rather than intermittent cold exposure (ICE), despite ICE being more physiologically relevant. In the present study, we provide novel evidence that thermoneutral housing uncouples the effects of ICE on glucose and energy homeostasis suggesting that ICE, despite improving glucose tolerance, is not an effective obesity treatment when mice are housed under humanized thermoneutral conditions.

ABSTRACT

The present study examines whether a physiologically relevant model of ambient cold exposure, intermittent cold exposure (ICE), could ameliorate the metabolic impairments of diet-induced obesity in male and female mice housed under humanized thermoneutral conditions. Male and female C57BL/6J mice housed at thermoneutrality (29°C) were fed a low-fat diet or high-fat diet for 6 weeks before being weight matched into groups that remained unperturbed or underwent ICE for 4 weeks (4°C for 60 min day^-1 ; 5 days week^-1 ) when being maintained on their respective diets. ICE induced rapid and persistent hyperphagia exacerbating rather than attenuating high-fat diet-induced obesity over time. These ICE-induced increases in adiposity were found to be energy intake-dependent via pair-feeding. Despite exacerbating high-fat diet-induced obesity, ICE improved glucose tolerance, independent of diet, in a sex-specific manner. The effects of ICE on glucose tolerance were not attributed to improvements in whole-body insulin tolerance, tissue specific insulin action, nor differences in markers of hepatic insulin clearance or pancreatic beta cell proliferation. Instead, ICE increased serum concentrations of insulin and C-peptide in response to glucose, suggesting that ICE may improve glucose tolerance by potentiating pancreatic glucose-stimulated insulin secretion. These data suggest that ICE, despite improving glucose tolerance, is not an effective obesity treatment in mice housed under humanized conditions.

The Role of Nonshivering Thermogenesis Genes on Leptin Levels Regulation in Residents of the Coldest Region of Siberia

Leptin plays an important role in thermoregulation and is possibly associated with the microevolutionary processes of human adaptation to a cold climate. In this study, based on the Yakut population (n = 281 individuals) living in the coldest region of Siberia (t°minimum -71.2 °C), we analyze the serum leptin levels and data of 14 single nucleotide polymorphisms (SNPs) of 10 genes (UCP1, UCP2, UCP3, FNDC5, PPARGC1A, CIDEA, PTGS2, TRPV1, LEPR, BDNF) that are possibly involved in nonshivering thermogenesis processes. Our results demonstrate that from 14 studied SNPs of 10 genes, 2 SNPs (the TT rs3811787 genotype of the UCP1 gene and the GG rs6265 genotype of the BDNF gene) were associated with the elevated leptin levels in Yakut females (p < 0.05). Furthermore, of these two SNPs, the rs3811787 of the UCP1 gene demonstrated more indications of natural selection for cold climate adaptation. The prevalence gradient of the T-allele (rs3811787) of UCP1 increased from the south to the north across Eurasia, along the shore of the Arctic Ocean. Thereby, our study suggests the potential involvement of the UCP1 gene in the leptin-mediated thermoregulation mechanism, while the distribution of its allelic variants is probably related to human adaptation to a cold climate.

Cold-induced hyperphagia requires AgRP neuron activation in mice

To maintain energy homeostasis during cold exposure, the increased energy demands of thermogenesis must be counterbalanced by increased energy intake. To investigate the neurobiological mechanisms underlying this cold-induced hyperphagia, we asked whether agouti-related peptide (AgRP) neurons are activated when animals are placed in a cold environment and, if so, whether this response is required for the associated hyperphagia. We report that AgRP neuron activation occurs rapidly upon acute cold exposure, as do increases of both energy expenditure and energy intake, suggesting the mere perception of cold is sufficient to engage each of these responses. We further report that silencing of AgRP neurons selectively blocks the effect of cold exposure to increase food intake but has no effect on energy expenditure. Together, these findings establish a physiologically important role for AgRP neurons in the hyperphagic response to cold exposure.

PGC-1β-expressing POMC neurons mediate the effect of leptin on thermoregulation in the mouse

The arcuate nucleus (ARC) of the hypothalamus is a key regulator of food intake, brown adipose tissue (BAT) thermogenesis, and locomotor activity. Whole-body deficiency of the transcriptional coactivator peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1β (PGC-1β) disrupts mouse circadian locomotor activity and BAT-regulated thermogenesis, in association with altered gene expression at the central level. We examined whether PGC-1β expression in the ARC is required for proper energy balance and locomotor behavior by generating mice lacking the PGC-1β gene specifically in pro-opiomelanocortin (POMC) neurons. POMC neuron-specific deletion of PGC-1β did not impact locomotor behavior, food intake, body composition, energy fuel utilization and metabolic rate in fed, 24-h fasted and 24-h refed conditions. In contrast, in the fed state, deletion of PGC-1β in POMC cells elevated core body temperature during the nighttime period. Importantly, this higher body temperature is not associated with changes in BAT function and gene expression. Conversely, we provide evidence that mice lacking PGC-1β in POMC neurons are more sensitive to the effect of leptin on heat dissipation. Our data indicate that PGC-1β-expressing POMC neurons are part of a circuit controlling body temperature homeostasis and that PGC-1β function in these neurons is involved in the thermoregulatory effect of leptin.

Roles of leptin and resistin in metabolism, reproduction, and leptin resistance

Increased adipose mass can cause insulin resistance and type 2 diabetes mellitus. This phenomenon is related to adipocyte-secreted signaling molecules that affect glucose balance, such as fatty acids, adiponectin, leptin, interleukin-6, tumor necrosis factor-α, and resistin. Among these hormones, leptin and resistin play important roles in regulating weight and glucose metabolism. Leptin and resistin work in both similar and opposite ways, and they interact with each other. Circulating concentrations of leptin and resistin are elevated in models of obesity and rodents fed a high-fat diet. In addition, leptin and resistin are similarly regulated by nutritional status: they are reduced by fasting and increased by feeding. This effect is mediated partially through insulin receptors and glucose transporters. Our latest data provided the first indication that in sheep, intravenous infusion of resistin increases the mean circulating concentrations of leptin and decreases luteinizing hormone in a dose-dependent manner during both the long-day (LD) and short-day seasons. Furthermore, exogenous resistin increased suppressor of cytokine signaling (SOCS)-3 mRNA expression only during the LD season, when the leptin resistance/insensitivity phenomenon was observed in the arcuate nucleus, preoptic area, and anterior pituitary. We concluded that one factor contributing to central leptin resistance is autosuppression, via which leptin and resistin stimulate the expression of SOCS-3, which inhibits leptin signaling. The increased expression of SOCS-3 in response to leptin and resistin may be a pivotal cause of leptin resistance/insensitivity, a pathological situation in obese individuals and a physiological occurrence in sheep during the LD season.

A leptin-BDNF pathway regulating sympathetic innervation of adipose tissue

Mutations in the leptin gene (ob) result in a metabolic disorder that includes severe obesity¹, and defects in thermogenesis² and lipolysis³, both of which are adipose tissue functions regulated by the sympathetic nervous system. However, the basis of these sympathetic-associated abnormalities remains unclear. Furthermore, chronic leptin administration reverses these abnormalities in adipose tissue, but the underlying mechanism remains to be discovered. Here we report that ob/ob mice, as well as leptin-resistant diet-induced obese mice, show significant reductions of sympathetic innervation of subcutaneous white and brown adipose tissue. Chronic leptin treatment of ob/ob mice restores adipose tissue sympathetic innervation, which in turn is necessary to correct the associated functional defects. The effects of leptin on innervation are mediated via agouti-related peptide and pro-opiomelanocortin neurons in the hypothalamic arcuate nucleus. Deletion of the gene encoding the leptin receptor in either population leads to reduced innervation in fat. These agouti-related peptide and pro-opiomelanocortin neurons act via brain-derived neurotropic factor-expressing neurons in the paraventricular nucleus of the hypothalamus (BDNF^PVH). Deletion of BDNF^PVH blunts the effects of leptin on innervation. These data show that leptin signalling regulates the plasticity of sympathetic architecture of adipose tissue via a top-down neural pathway that is crucial for energy homeostasis.

14-3-3ζ mediates an alternative, non-thermogenic mechanism in male mice to reduce heat loss and improve cold tolerance

Objective

Adaptive thermogenesis, which is partly mediated by sympathetic input on brown adipose tissue (BAT), is a mechanism of heat production that confers protection against prolonged cold exposure. Various endogenous stimuli, for example, norepinephrine and FGF-21, can also promote the conversion of inguinal white adipocytes to beige adipocytes, which may represent a secondary cell type that contributes to adaptive thermogenesis. We previously identified an essential role of the molecular scaffold 14-3-3ζ in adipogenesis, but one of the earliest, identified functions of 14-3-3ζ is its regulatory effects on the activity of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of norepinephrine. Herein, we examined whether 14-3-3ζ could influence adaptive thermogenesis via actions on BAT activation or the beiging of white adipocytes.

Methods

Transgenic mice over-expressing a TAP-tagged human 14-3-3ζ molecule or heterozygous mice without one allele of Ywhaz, the gene encoding 14-3-3ζ, were used to explore the contribution of 14-3-3ζ to acute (3 h) and prolonged (3 days) cold (4 °C) exposure. Metabolic caging experiments, PET-CT imaging, and laser Doppler imaging were used to determine the effect of 14-3-3ζ over-expression on thermogenic and vasoconstrictive mechanisms in response to cold.

Results

Transgenic over-expression of 14-3-3ζ (TAP) in male mice significantly improved tolerance to acute and prolonged cold. In response to cold, body temperatures in TAP mice did not decrease to the same extent when compared to wildtype (WT) mice, and this was associated with increased UCP1 expression in beige inguinal white tissue (iWAT) and BAT. Of note was the paradoxical finding that cold-induced changes in body temperatures of TAP mice were associated with significantly decreased energy expenditure. The marked improvements in tolerance to prolonged cold were not due to changes in sensitivity to β-adrenergic stimulation or BAT or iWAT oxidative metabolism; instead, over-expression of 14-3-3ζ significantly decreased thermal conductance and heat loss in mice via increased peripheral vasoconstriction.

Conclusions

Despite being associated with elevations in cold-induced UCP1 expression in brown or beige adipocytes, these findings suggest that 14-3-3ζ regulates an alternative, non-thermogenic mechanism via vasoconstriction to minimize heat loss during cold exposure.

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14-3-3ζ over-expression in male mice improves tolerance to acute and prolonged cold.

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Increasing 14-3-3ζ expression promotes beiging of inguinal white adipose tissue.

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Cold-induced changes in body temperature can be dissociated from energy expenditure.

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14-3-3ζ-dependent decreases in heat loss are associated with vasoconstriction.

Systemic and Central Amylin, Amylin Receptor Signaling, and Their Physiological and Pathophysiological Roles in Metabolism

This article in the Neural and Endocrine Section of Comprehensive Physiology discusses the physiology and pathophysiology of the pancreatic hormone amylin. Shortly after its discovery in 1986, amylin has been shown to reduce food intake as a satiation signal to limit meal size. Amylin also affects food reward, sensitizes the brain to the catabolic actions of leptin, and may also play a prominent role in the development of certain brain areas that are involved in metabolic control. Amylin may act at different sites in the brain in addition to the area postrema (AP) in the caudal hindbrain. In particular, the sensitizing effect of amylin on leptin action may depend on a direct interaction in the hypothalamus. The concept of central pathways mediating amylin action became more complex after the discovery that amylin is also synthesized in certain hypothalamic areas but the interaction between central and peripheral amylin signaling remains currently unexplored. Amylin may also play a dominant pathophysiological role that is associated with the aggregation of monomeric amylin into larger, cytotoxic molecular entities. This aggregation in certain species may contribute to the development of type 2 diabetes mellitus but also cardiovascular disease. Amylin receptor pharmacology is complex because several distinct amylin receptor subtypes have been described, because other neuropeptides [e.g., calcitonin gene-related peptide (CGRP)] can also bind to amylin receptors, and because some components of the functional amylin receptor are also used for other G-protein coupled receptor (GPCR) systems. © 2020 American Physiological Society. Compr Physiol 10:811-837, 2020.

Regulation of adipocyte thermogenesis: mechanisms controlling obesity

Adipocyte biology has been intensely researched in recent years due to the emergence of obesity as a serious global health concern and because of the realization that adipose tissue is more than simply a cell type that stores and releases lipids. The plasticity of adipose tissues, to rapidly adapt to altered physiological states of energy demand, is under neuronal and endocrine control. The capacity for white adipocytes to store chemical energy in lipid droplets is key for protecting other organs from the toxic effects of ectopic lipid deposition. In contrast, thermogenic (brown and beige) adipocytes combust macronutrients to generate heat. The thermogenic activity of adipocytes allows them to protect themselves and other tissues from lipid overaccumulation. Advances in brown fat biology have uncovered key molecular players involved in adipocyte determination, differentiation, and thermogenic activation. It is now, well appreciated that three distinct adipocyte types exist: white, beige, and brown. Moreover, functional differences are present within adipocyte subtypes located in anatomically distinct locations. Adding to this complexity is the recent realization from single-cell sequencing studies that adipocyte progenitors are also heterogeneous. Understanding the molecular details of how to increase the number of thermogenic fat cells and their activation may delineate some of the pathophysiological basis of obesity and obesity-related diseases. Here, we review recent advances that have extended our understanding of the central role that adipose tissue plays in energy balance and the mechanisms that control their amount and function.

Leptin: Is It Thermogenic?

Animals that lack the hormone leptin become grossly obese, purportedly for 2 reasons: increased food intake and decreased energy expenditure (thermogenesis). This review examines the experimental evidence for the thermogenesis component. Analysis of the data available led us to conclude that the reports indicating hypometabolism in the leptin-deficient ob/ob mice (as well as in the leptin-receptor-deficient db/db mice and fa/fa rats) derive from a misleading calculation artefact resulting from expression of energy expenditure per gram of body weight and not per intact organism. Correspondingly, the body weight-reducing effects of leptin are not augmented by enhanced thermogenesis. Congruent with this, there is no evidence that the ob/ob mouse demonstrates atrophied brown adipose tissue or diminished levels of total UCP1 mRNA or protein when the ob mutation is studied on the inbred C57BL/6 mouse background, but a reduced sympathetic nerve activity is observed. On the outbred "Aston" mouse background, brown adipose tissue atrophy is seen, but whether this is of quantitative significance for the development of obesity has not been demonstrated. We conclude that leptin is not a thermogenic hormone. Rather, leptin has effects on body temperature regulation, by opposing torpor bouts and by shifting thermoregulatory thresholds. The central pathways behind these effects are largely unexplored.

Perinatal over- and underfeeding affect hypothalamic leptin and ghrelin neuroendocrine responses in adult rats

BACKGROUND

Changes in the nutritional supply during the perinatal period can lead to metabolic disturbances and obesity in adulthood.

OBJECTIVE

The divergent litter size model was used to investigate the hypothalamic sensitivity to leptin and ghrelin as well as the mechanisms involved in the disruption of food intake and energy expenditure.

METHODS

On postnatal day 3 (P3), male Wistar rats were divided into 3 groups: small litter (SL - 3 pups), normal litter (NL - 10 pups), and large litter (LL - 16 pups). Animals at P60 were intraperitoneally treated with leptin (500 µg/Kg), ghrelin (40 µg/Kg), or vehicle (0.9% NaCl) at 5 pm and the following parameters were assessed: food intake and body weight; immunostaining of p-STAT-3 in the hypothalamus; Western Blotting analysis of p-AMPKα and UCP2 in the mediobasal hypothalamus (MBH), and UCP1 in the interscapular brown adipose tissue (BAT); or heat production, VO₂, VCO₂, and locomotor activity.

RESULTS

SL rats had earlier leptin and ghrelin surges, while LL rats had no variations. At P60, after leptin treatment, LL rats showed hypophagia and increased p-STAT-3 expression in the arcuate nucleus, but SL rats had no response. After ghrelin treatment, LL rats did not have the orexigenic response or AMPKα phosphorylation in the MBH, while SL animals, unexpectedly, decreased body weight gain, without changes in food intake, and increased metabolic parameters and UCP1 expression in the BAT.

CONCLUSIONS

Changes in the nutritional supply at early stages of life modify leptin and ghrelin responsiveness in adulthood, programming metabolic and central mechanisms, which contribute to overweight and obesity in adulthood.

Incendiary Leptin

Leptin is a hormone released by adipose tissue that plays a key role in the control of energy homeostasis through its binding to leptin receptors (LepR), mainly expressed in the hypothalamus. Most scientific evidence points to leptin’s satiating effect being due to its dual capacity to promote the expression of anorexigenic neuropeptides and to reduce orexigenic expression in the hypothalamus. However, it has also been demonstrated that leptin can stimulate (i) thermogenesis in brown adipose tissue (BAT) and (ii) the browning of white adipose tissue (WAT). Since the demonstration of the importance of BAT in humans 10 years ago, its study has aroused great interest, mainly in the improvement of obesity-associated metabolic disorders through the induction of thermogenesis. Consequently, several strategies targeting BAT activation (mainly in rodent models) have demonstrated great potential to improve hyperlipidemias, hepatic steatosis, insulin resistance and weight gain, leading to an overall healthier metabolic profile. Here, we review the potential therapeutic ability of leptin to correct obesity and other metabolic disorders, not only through its satiating effect, but by also utilizing its thermogenic properties.

Adipose PTEN regulates adult adipose tissue homeostasis and redistribution via a PTEN-leptin-sympathetic loop

OBJECTIVE

Despite the large body of work describing the tumor suppressor functions of Phosphatase and tensin homologue deleted on chromosome ten (PTEN), its roles in adipose homeostasis of adult animals are not yet fully understood. Here, we sought to determine the role of PTEN in whole-body adipose homeostasis.

METHODS

We genetically manipulated PTEN in specific fat depots through recombinant adeno-associated viral vector (rAAV)-based gene transfer of Cre recombinase to adult PTEN^flox mice. Additionally, we used a denervation agent, 6OHDA, to assess the role of sympathetic signaling in PTEN-related adipose remodeling. Furthermore, we chemically manipulated AKT signaling via a pan-AKT inhibitor, MK-2206, to assess the role of AKT in PTEN-related adipose remodeling. Finally, to understand the role of leptin and central signaling on peripheral tissues, we knocked down hypothalamic leptin receptor with a microRNA delivered by a rAAV vector.

RESULTS

Knockdown PTEN in individual fat depot resulted in massive expansion of the affected fat depot through activation of AKT signaling associated with suppression of lipolysis and induction of leptin. This hypertrophic expansion of the affected fat depot led to upregulation of PTEN level, higher lipolysis, and induction of white fat browning in other fat depots, and the compensatory reduced fat mass to maintain a set point of whole-body adiposity. Administration of AKT inhibitor MK-2206 prevented the adipose PTEN knockdown-associated effects. 6OHDA-mediated denervation demonstrated that sympathetic innervation was required for the PTEN knockdown-induced adipose redistribution. Knockdown hypothalamic leptin receptor attenuated the adipose redistribution induced by PTEN deficiency in individual fat depot.

CONCLUSIONS

Our results demonstrate the essential role of PTEN in adipose homeostasis, including mass and distribution in adulthood, and reveal an "adipose PTEN-leptin-sympathetic nervous system" feedback loop to maintain a set point of adipose PTEN and whole-body adiposity.

Hypothalamic-Pituitary and Adipose Tissue Responses to the Effect of Resistin in Sheep: The Integration of Leptin and Resistin Signaling Involving a Suppressor of Cytokine Signaling 3 and the Long Form of the Leptin Receptor

We hypothesized that resistin is engaged in the development of leptin central insensitivity/resistance in sheep, which is a unique animal model to explore reversible leptin resistance. Thirty Polish Longwool ewes, which were ovariectomized with estrogen replacement, were used. Treatments consisted of the intravenous injection of control (saline) or recombinant bovine resistin (rbresistin): control (Control; n = 10), a low dose of rbresistin (R1; 1.0 μg/kg body weight (BW); n = 10), and a high dose of rbresistin (R2; 10.0 μg/kg BW; n = 10). The studies were performed during short-day (SD) and long-day (LD) photoperiods. Leptin and resistin concentrations were determined. Expression levels of a suppressor of cytokine signaling (SOCS)-3 and the long form of the leptin receptor (LeptRb) were determined in selected brain regions, including in the anterior pituitary (AP), hypothalamic arcuate nucleus (ARC), preoptic area (POA), and ventro- and dorsomedial nuclei (VMH/DMH). The results indicate that resistin induced a consistent decrease in LeptRb (except in POA) and an increase in SOCS-3 expression during the LD photoperiod in all selected brain regions. In conclusion, the results demonstrate that the action of resistin appears to be strongly associated with photoperiod-driven changes in the leptin signaling pathway, which may underlie the phenomenon of central leptin resistance.

Gsα deficiency in the dorsomedial hypothalamus leads to obesity, hyperphagia, and reduced thermogenesis associated with impaired leptin signaling

OBJECTIVE

G_sα couples multiple receptors, including the melanocortin 4 receptor (MC4R), to intracellular cAMP generation. Germline inactivating G_sα mutations lead to obesity in humans and mice. Mice with brain-specific G_sα deficiency also develop obesity with reduced energy expenditure and locomotor activity, and impaired adaptive thermogenesis, but the underlying mechanisms remain unclear.

METHODS

We created mice (DMHGsKO) with G_sα deficiency limited to the dorsomedial hypothalamus (DMH) and examined the effects on energy balance and thermogenesis.

RESULTS

DMHGsKO mice developed severe, early-onset obesity associated with hyperphagia and reduced energy expenditure and locomotor activity, along with impaired brown adipose tissue thermogenesis. Studies in mice with loss of MC4R in the DMH suggest that defective DMH MC4R/G_sα signaling contributes to abnormal energy balance but not to abnormal locomotor activity or cold-induced thermogenesis. Instead, DMHGsKO mice had impaired leptin signaling along with increased expression of the leptin signaling inhibitor protein tyrosine phosphatase 1B in the DMH, which likely contributes to the observed hyperphagia and reductions in energy expenditure, locomotor activity, and cold-induced thermogenesis.

CONCLUSIONS

DMH G_sα signaling is critical for energy balance, thermogenesis, and leptin signaling. This study provides insight into how distinct signaling pathways can interact to regulate energy homeostasis and temperature regulation.

Validation of an equation for energy expenditure that does not require the respiratory quotient

Background

Energy expenditure (EE) calculated from respirometric indirect calorimetry is most accurate when based on oxygen consumption (VO₂), carbon dioxide production (VCO₂) and estimated protein metabolism (PM). EE has a substantial dependence of ~7% on the respiratory quotient (RQ, VCO₂/VO₂) and a lesser dependence on PM, yet many studies have instead estimated EE from VO₂ only while PM has often been ignored, thus reducing accuracy. In 1949 Weir proposed a method to accurately calculate EE without using RQ, which also adjusts for estimated PM based on dietary composition. This RQ^- method utilizes the calorimeter airflow rate (FR), the change in fractional O₂ concentration (ΔFO₂) and the dietary protein fraction. The RQ^- method has not previously been empirically validated against the standard RQ⁺ method using both VO₂ and RQ. Our aim was to do that.

Methods

VO₂ and VCO₂ were measured repeatedly in 8 mice fed a high protein diet (HPD) during exposure to different temperatures (n = 168 measurements of 24h gas exchange). The HPD-adjusted RQ⁺ equation was: EE [kcal/time] = VO₂ [L/time]×(3.853+1.081RQ) while the corresponding RQ^- equation was: EE = 4.934×FR×ΔFO₂. Agreement was analyzed using the ratios of the RQ^- to RQ⁺ methods along with regression and Bland-Altman agreement analyses. We also evaluated the standard equation using the dietary food quotient (FQ) of 0.91 as a proxy for RQ (FQ⁺ method).

Results

Ratio analysis revealed that the mean error of the RQ^- method was only 0.11 ± 0.042% while the maximum error was only 0.21%. Error using the FQ⁺ method was 4 -and 10-fold greater, respectively. Bland-Altman analysis demonstrated that the RQ^- method very slightly overestimates EE as RQ decreases. Theoretically, this error can be eliminated completely by imposing an incurrent fractional oxygen concentration at a value only slightly greater than the atmospheric level.

Conclusions

The Weir ‘RQ-free’ method for calculating EE is a highly valid alternative to the ‘gold standard’ method that requires RQ. The RQ^- approach permits reduced cost and complexity in studies focused on EE and provides a way to rescue EE measurement in studies compromised by faulty CO₂ measurements. Practitioners of respirometry should consider adjusting EE calculations for estimated protein metabolism based on dietary composition.

Glucoregulatory responses to hypothalamic preoptic area cooling

Normal glucose homeostasis depends on the capacity of pancreatic β-cells to adjust insulin secretion in response to a change of tissue insulin sensitivity. In cold environments, for example, the dramatic increase of insulin sensitivity required to ensure a sufficient supply of glucose to thermogenic tissues is offset by a proportionate reduction of insulin secretion, such that overall glucose tolerance is preserved. That these cold-induced changes of insulin secretion and insulin sensitivity are dependent on sympathetic nervous system (SNS) outflow suggests a key role for thermoregulatory neurons in the hypothalamic preoptic area (POA) in this metabolic response. As these POA neurons are themselves sensitive to changes in local hypothalamic temperature, we hypothesized that direct cooling of the POA would elicit the same glucoregulatory responses that we observed during cold exposure. To test this hypothesis, we used a thermode to cool the POA area, and found that as predicted, short-term (8-h) intense POA cooling reduced glucose-stimulated insulin secretion (GSIS), yet glucose tolerance remained unchanged due to an increase of insulin sensitivity. Longer-term (24-h), more moderate POA cooling, however, failed to inhibit GSIS and improved glucose tolerance, an effect associated with hyperthermia and activation of the hypothalamic-pituitary-adrenal axis, indicative of a stress response. Taken together, these findings suggest that POA cooling is sufficient to recapitulate key glucoregulatory responses to cold exposure.

Epiregulin induces leptin secretion and energy expenditure in high-fat diet-fed mice

Adipokine leptin regulates neuroendocrine circuits that control energy expenditure, thermogenesis and weight loss. However, canonic regulators of leptin secretion, such as insulin and malonyl CoA, do not support these processes. We hypothesize that epiregulin (EREG), a growth factor that is secreted from fibroblasts under thermogenic and cachexia conditions, induces leptin secretion associated with energy dissipation. The effects of EREG on leptin secretion were studied ex vivo, in the intra-abdominal white adipose tissue (iAb WAT) explants, as well as in vivo, in WT mice with diet-induced obesity (DIO) and in ob/ob mice. These mice were pair fed a high-fat diet and treated with intraperitoneal injections of EREG. EREG increased leptin production and secretion in a dose-dependent manner in iAb fat explants via the EGFR/MAPK pathway. After 2 weeks, the plasma leptin concentration was increased by 215% in the EREG-treated group compared to the control DIO group. EREG-treated DIO mice had an increased metabolic rate and core temperature during the active dark cycle and displayed cold-induced thermogenesis. EREG treatment reduced iAb fat mass, the major site of leptin protein production and secretion, but did not reduce the mass of the other fat depots. In the iAb fat, expression of genes supporting mitochondrial oxidation and thermogenesis was increased in EREG-treated mice vs control DIO mice. All metabolic and gene regulation effects of EREG treatment were abolished in leptin-deficient ob/ob mice. Our data revealed a new role of EREG in induction of leptin secretion leading to the energy expenditure state. EREG could be a potential target protein to regulate hypo- and hyperleptinemia, underlying metabolic and immune diseases.

Leptin regulation of core body temperature involves mechanisms independent of the thyroid axis

The ability to maintain core temperature within a narrow range despite rapid and dramatic changes in environmental temperature is essential for the survival of free-living mammals, and growing evidence implicates an important role for the hormone leptin. Given that thyroid hormone plays a major role in thermogenesis and that circulating thyroid hormone levels are reduced in leptin-deficient states (an effect partially restored by leptin replacement), we sought to determine the extent to which leptin's role in thermogenesis is mediated by raising thyroid hormone levels. To this end, we 1) quantified the effect of physiological leptin replacement on circulating levels of thyroid hormone in leptin-deficient ob/ob mice, and 2) determined if the effect of leptin to prevent the fall in core temperature in these animals during cold exposure is mimicked by administration of a physiological replacement dose of triiodothyronine (T₃). We report that, as with leptin, normalization of circulating T₃ levels is sufficient both to increase energy expenditure, respiratory quotient, and ambulatory activity and to reduce torpor in ob/ob mice. Yet, unlike leptin, infusing T₃ at a dose that normalizes plasma T₃ levels fails to prevent the fall of core temperature during mild cold exposure. Because thermal conductance (e.g., heat loss to the environment) was reduced by administration of leptin but not T₃, leptin regulation of heat dissipation is implicated as playing a uniquely important role in thermoregulation. Together, these findings identify a key role in thermoregulation for leptin-mediated suppression of thermal conduction via a mechanism that is independent of the thyroid axis.

Preoptic leptin signaling modulates energy balance independent of body temperature regulation

The adipokine leptin acts on the brain to regulate energy balance but specific functions in many brain areas remain poorly understood. Among these, the preoptic area (POA) is well known to regulate core body temperature by controlling brown fat thermogenesis, and we have previously shown that glutamatergic, long-form leptin receptor (Lepr)-expressing neurons in the POA are stimulated by warm ambient temperature and suppress energy expenditure and food intake. Here we further investigate the role of POA leptin signaling in body weight regulation and its relationship to body temperature regulation in mice. We show that POA Lepr signaling modulates energy expenditure in response to internal energy state, and thus contributes to body weight homeostasis. However, POA leptin signaling is not involved in ambient temperature-dependent metabolic adaptations. Our study reveals a novel cell population through which leptin regulates body weight.

Feeding Obese Diabetic Mice a Genistein Diet Induces Thermogenic and Metabolic Change

Obesity is associated with elevated plasma levels of glucocorticoids and reduced levels of thyroid hormones, both known to effect food intake and energy expenditure. Furthermore, tissue specific glucocorticoid metabolism is altered in obesity, increasing insulin resistance and cardiometabolic risk. The goal of this study was to examine whether these metabolic disturbances can be prevented with the isoflavone genistein in the ob/ob mouse, a model that resembles the phenotype in human obesity. Male ob/ob mice, aged 5 weeks, were fed either a genistein-rich diet (600 mg/kg) or a genistein-free diet for 4 weeks. ob/ob mice weighed 70% more than lean controls. While there was no effect of genistein on body weight, food consumption during weeks 3 and 4 was significantly increased in genistein-fed mice. This was associated with increases in body temperature and plasma levels of triiodothyronine (T3), suggesting a thermogenic effect. The hypercorticosteronism observed in the ob/ob mouse was reduced with genistein treatment. This effect was accompanied by a decrease in protein expression of renal 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) without changes in hepatic 11β-HSD1. Our results suggest that a diet containing genistein can have beneficial effects on energy expenditure, T3 production, and corticosterone status in the ob/ob mouse model of obesity.

Leptin and brain-adipose crosstalks

Interactions between the brain and distinct adipose depots have a key role in maintaining energy balance, thereby promoting survival in response to metabolic challenges such as cold exposure and starvation. Recently, there has been renewed interest in the specific central neuronal circuits that regulate adipose depots. Here, we review anatomical, genetic and pharmacological studies on the neural regulation of adipose function, including lipolysis, non-shivering thermogenesis, browning and leptin secretion. In particular, we emphasize the role of leptin-sensitive neurons and the sympathetic nervous system in modulating the activity of brown, white and beige adipose tissues. We provide an overview of advances in the understanding of the heterogeneity of the brain regulation of adipose tissues and offer a perspective on the challenges and paradoxes that the community is facing regarding the actions of leptin on this system.

Noni (Morinda citrifolia) Modulates the Hypothalamic Expression of Stress- and Metabolic-Related Genes in Broilers Exposed to Acute Heat Stress

Heat stress (HS) adversely affects growth performance and inflicts heavy economic losses to the poultry industry. There is, therefore, a critical need to identify new alternative strategies to alleviate the negative effects induced by HS. The tropic medicinal plant, Morinda citrifolia (Noni), is being used in livestock nutrition, however the literature is limited and conflicting for its impact on growth performance. The present study aimed to determine the effect of Noni on feeding and drinking behavior as well as on the hypothalamic expression of stress- and metabolic-related genes in broiler chickens exposed to acute HS. A total of 480 1 day-old male broiler chicks were randomly assigned to 12 controlled environmental chambers. Birds were subjected to two environmental conditions (TN, 25°C vs. HS, 35°C for 2 h) and fed two diets (control vs. 0.2% Noni) in a 2 × 2 factorial design. Feed intake and core body temperature (BT) were recorded during HS period. Blood was collected and hypothalamic tissues were harvested for target gene and protein analyses. Acute HS-broilers exhibited higher BT (~1°C), spent less time eating with a significant decrease in feed intake, and spent more time drinking along with higher drinking frequency compared to those maintained under TN conditions. Although Noni supplementation did not improve feed intake, it significantly delayed (~30 min) and reduced the BT-induced by HS. At molecular levels and under HS conditions, Noni supplementation down regulated the hypothalamic expression of HSP90 and its related transcription factors HSF1, 2, and 4, increased orexin mRNA levels, and decreased the phosphorylation levels of AMPKα1/2^Thr172 and mTOR^Ser2481. Together, these data indicated that Noni supplementation might modulate HS response in broilers through central orexin-AMPK-mTOR pathways.

The mouse thermoregulatory system: Its impact on translating biomedical data to humans

The laboratory mouse has become the predominant test species in biomedical research. The number of papers that translate or extrapolate data from mouse to human has grown exponentially since the year 2000. There are many physiological and anatomical factors to consider in the process of extrapolating data from one species to another. Body temperature is, of course, a critical determinant in extrapolation because it has a direct impact on metabolism, cardiovascular function, drug efficacy, pharmacokinetics of toxins and drugs, and many other effects. While most would consider the thermoregulatory system of mice to be sufficiently stable and predictable as to not be a cause for concern, the thermal physiology of mice does in fact present unique challenges to the biomedical researcher. A variable and unstable core temperature, high metabolic rate, preference for warm temperatures, large surface area: body mass ratio, and high rate of thermal conductance, are some of the key factors of mice that can affect the interpretation and translation of data to humans. It is the intent of this brief review to enlighten researchers studying interspecies translation of biomedical data on the salient facets of the mouse thermal physiology and show how extrapolation in fields such as physiology, psychology, nutrition, pharmacology, toxicology, and pathology.

The role of leptin in health and disease

Leptin is a master regulator of energy balance and body adiposity. Additionally, leptin exerts important control on glucose homeostasis, thermogenesis, autonomic nervous system and neuroendocrine axes. In metabolic diseases, such as obesity and diabetes mellitus, leptin signaling may be compromised, indicating the important role of this hormone in the etiology and pathophysiological manifestations of these conditions. In the present manuscript, we reviewed important concepts of leptin signaling, as well as about the effects of leptin on several biologic functions. We also discussed the possible therapeutic use of leptin administration and how our current obesogenic environment contributes to the development of leptin resistance. Our objective was to provide a comprehensive and state-of-the-art review about the importance of leptin to maintain the homeostasis and during pathological conditions.