Hyperleptinemia precipitates diet-induced obesity in transgenic mice overexpressing leptin

Allograft inflammatory factor-1-like is a situational regulator of leptin levels, hyperphagia, and obesity

Mouse models enable the study of genetic factors affecting the complex pathophysiology of metabolic disorders. Here, we identify reductions in leptin levels, food intake, and obesity due to high-fat diet, accompanied by increased leptin sensitivity, in mice that harbor the E2a-Cre transgene within Obrq2, an obesity quantitative trait locus (QTL) that includes the leptin gene. Interestingly, loss of allograft inflammatory factor-1-like (AIF1L) protein in these transgenic mice leads to similar leptin sensitivity, yet marked reversal of the obesity phenotype, with accelerated weight gain and increased food intake. Transgenic mice lacking AIF1L also have low circulating leptin, which suggests that benefits of enhanced leptin sensitivity are lost with further impairment of leptin expression due to loss of AIF1L. Together, our results identify AIF1L as a genetic modifier of Obrq2 and leptin that affects leptin levels, food intake, and obesity during the metabolic stress imposed by HFD.

Impaired leptin responsiveness in the nucleus accumbens of leptin-overexpressing transgenic mice with dysregulated sucrose and lipid preference independent of obesity

While hypothalamic leptin resistance can occur prior to establishment of obesity, clarification is needed as to whether the impaired response to leptin in the reward-related nuclei occurs independently of obesity. To answer this question, we attempted to dissociate the normally coexisting leptin resistance from obesity. We investigated phenotypes of leptin-overexpressing transgenic mice fed for 1 week with 60 % high-fat diet (HFD) (LepTg-HFD1W mice). After 1 week, we observed that LepTg-HFD1W mice weighed as same as wild type (WT) mice fed standard chow diet (CD) for 1 week (WT-CD1W mice). However, compared to WT-CD1W mice, LepTg-HFD1W mice exhibited attenuated leptin-induced anorexia, decreased leptin-induced c-fos immunostaining in nucleus accumbens (NAc), one of important site of reward system, decreased leptin-stimulated pSTAT3 immunostaining in hypothalamus. Furthermore, neither sucrose nor lipid preference was suppressed by leptin in LepTg-HFD1W mice. On the contrary, leptin significantly suppressed both preferences in WT mice fed HFD (WT-HFD1 W mice). These results indicate that leptin responsiveness decreases in NAc independently of obesity. Additionally, in this situation, suppressive effect of leptin on the hedonic feeding results in impaired regulation. Such findings suggest the impaired leptin responsiveness in NAc partially contributes to dysregulated hedonic feeding behavior independently of obesity.

Leptin and the Blood-Brain Barrier: Curiosities and Controversies

Leptin for over 25 years has been a central theme in the study of appetite, obesity, and starvation. As the major site of leptin production is peripheral, and the site of action of greatest interest is the hypothalamus, how leptin accesses the central nervous system (CNS) and crosses the blood-brain barrier (BBB) has been of great interest. We review here the ongoing research that addresses fundamental questions such as the sites of leptin resistances in obesity and other conditions, the causes of resistances and their relations to one another, the three barrier sites of entry into the CNS, why recent studies using suprapharmacological doses cannot address these questions but give insight into nonsaturable entry of leptin into the CNS, and how that might be useful in using leptin therapeutically. The current status of the controversy of whether the short form of the leptin receptor acts as the BBB leptin transporter and how obesity may transform leptin transport is reviewed. Review of these and other topics summarizes in a new appreciation of what leptin may have actually evolved to do and what physiological role leptin resistance may play. © 2021 American Physiological Society. Compr Physiol 11:1-19, 2021.

Physiological consequences of transient hyperleptinemia during discrete developmental periods on body weight in mice

Leptin plays a role in central nervous system developmental programs and intercurrent physiological processes related to body fat regulation. The timing and neuromolecular mechanisms for these effects are relevant to the prevention and treatment of obesity. Factors implicated in a body weight "set point" including dietary fat, circulating leptin, and other adipokines tend to covary with adiposity and are difficult to disarticulate experimentally. To dissociate leptin effects from adiposity and diet, we created a transgenic mouse in which leptin expression is regulated by doxycycline exposure. Using this system, we investigated the physiological consequences of developmentally-timed transient hyperleptinemia on subsequent adiposity. We evaluated physiological effects of leptin elevation during adulthood (9 to 29 weeks old), "adolescence" (3 to 8 weeks old), and the immediate postnatal period [postnatal days 0 to 22 (P0 to P22)] on long-term adiposity and susceptibility to gain weight on high-fat diet (HFD) fed ad libitum. We found that inducing chronic hyperleptinemia in adult or "adolescent" mice did not alter body weight when excess leptin was discontinued, and upon later exposure to HFD, weight gain did not differ from controls. However, transient elevation of circulating leptin from P0 to P22 increased weight and fat gain in response to HFD, indicating greater susceptibility to obesity as adults. Thus, transient plasma leptin elevations-mimicking one aspect of transient adiposity-increased later susceptibility to diet-induced obesity, although these effects were restricted to a critical developmental (P0 to P22) time window. These findings may have clinical implications for weight management in infancy.

Hyperleptinemia as a contributing factor for the impairment of glucose intolerance in obesity

Obesity has emerged as a major risk factor for insulin resistance leading to the development of type 2 diabetes (T2D). The condition is characterized by high circulating levels of the adipose-derived hormone leptin and a state of chronic low-grade inflammation. Pro-inflammatory signaling in the hypothalamus is associated with a decrease of central leptin- and insulin action leading to impaired systemic glucose tolerance. Intriguingly, leptin not only regulates body weight and glucose homeostasis but also acts as a pro-inflammatory cytokine. Here we demonstrate that increasing leptin levels (62,5 µg/kg/d, PEGylated leptin) in mice fed a high-fat diet (HFD) exacerbated body weight gain and aggravated hypothalamic micro- as well as astrogliosis. In contrast, administration of a predetermined dose of a long-acting leptin antagonist (100 µg/kg/d, PESLAN) chosen to block excessive leptin signaling during diet-induced obesity (DIO) showed the opposite effect and significantly improved glucose tolerance as well as decreased the total number of microglia and astrocytes in the hypothalamus of mice fed HFD. These results suggest that high levels of leptin, such as in obesity, worsen HFD-induced micro-and astrogliosis, whereas the partial reduction of hyperleptinemia in DIO mice may have beneficial metabolic effects and improves hypothalamic gliosis.

Leptin: Less Is More

The successful use of leptin for the treatment of individuals with lipodystrophy and leptin deficiency is well established. However, pharmacological approaches of leptin therapy for the treatment of diet-induced obesity have been ineffective. There is ample room for a better understanding of the much famed "leptin resistance" phenomenon. Our recent data in this area prompt us to call for a conceptual shift. This shift entails a model in which a reduction of bioactive leptin levels in the context of obesity triggers a high degree of leptin sensitization and improved leptin action, both centrally and peripherally. Put another way, hyperleptinemia per se causes leptin resistance and associated metabolic disorders. In this perspective, we briefly discuss the underlying conceptual steps that led us to explore partial leptin reduction as a viable therapeutic avenue. We hope this discussion will contribute to potential future applications of partial leptin reduction therapy for the treatment of obesity and type 2 diabetes.

Partial leptin deficiency confers resistance to diet-induced obesity in mice

Objective

Hyperleptinemia per se is sufficient to promote leptin resistance in the obese state. Leptin sensitivity can be restored by reducing circulating leptin levels within a physiologically healthy range and is a viable antiobesity and antidiabetic strategy. However, a previous study suggests that partial leptin deficiency favors diet-induced obesity and related metabolic disorders in mice, arguing that a lower leptin level may indeed promote diet-induced obesity and its associated metabolic disorders. Here, we aim to elucidate what the impact of partial leptin deficiency is on fat mass and insulin sensitivity.

Methods

We used two different mouse models of partial leptin deficiency: an adipocyte-specific congenital heterozygous leptin knockout mouse line (LepHZ) and the well-established whole body heterozygous leptin knockout mouse (OBHZ). The metabolic studies of OBHZ and LepHZ mice were performed both on normal carbohydrate-rich chow diet and on a high-fat diet (HFD). Male and female mice were included in the study to account for sex-specific differences. Body weight, food intake, glucose tolerance, and insulin tolerance were tested. Histology of adipose tissue and liver tissue allowed insights into adipose tissue inflammation and hepatic triglyceride content. Immunohistochemistry was paired with RT-PCR analysis for expression levels of inflammatory markers.

Results

Both OBHZ and LepHZ mice displayed reduced circulating leptin levels on the chow diet and HFD. On chow diet, male OBHZ and LepHZ mice showed elevated fat mass and body weight, while their glucose tolerance and insulin sensitivity remained unchanged. However, the inability in partially leptin-deficient mice to fully induce circulating leptin during the development of diet-induced obesity results in reduced food intake and leaner mice with lower body weight compared to their littermate controls. Importantly, a strong reduction of adipose tissue inflammation is observed along with improvements in insulin sensitivity and enhanced glucose tolerance. Additionally, partial leptin deficiency protects the mice from fatty liver and liver fibrosis. Chronically HFD-fed OBHZ and LepHZ mice remain more sensitive to exogenous leptin injection, as reflected by their reduced food intake upon an acute leptin treatment.

Conclusion

In response to HFD feeding, the inability to upregulate leptin levels due to partial leptin deficiency protects mice from diet-induced obesity and metabolic dysregulation. Thus, in an obesogenic environment, maintaining lower leptin levels is highly beneficial for both obesity and diabetes management. Chronic leptin reduction represents a viable preventive strategy whose efficacy awaits clinical testing.

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Partial leptin deficiency protects from diet-induced obesity.

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Reduced leptin protects from diet-induced obesity independent of sex.

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Reduction of circulating leptin levels inhibits HFD-induced adipose tissue inflammation.

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Partial leptin deficiency confers resistance to HFD-induced liver fibrosis.

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Partial leptin deficiency enhances leptin sensitivity.

Metabolic and behavioral parameters of mice with reduced expression of Syndecan-1

Energy balance is essential for all species. Ligand-receptor interactions mediate processes that regulate body activities like reproduction and metabolism based on the energy status. Such receptors are the heparan sulfate proteoglycans and specifically the family of syndecans. Therefore we investigated the differences of metabolic parameters of heterozygous Syndecan 1 mice (Sdc1^+/-) with reduced expression of Sdc1 and the corresponding wild type mice. Sdc1^+/- mice have a reduced body weight although they show increased leptin and decreased corticosterone levels. Furthermore, their food and water intake is increased. This is accompanied with less adipose tissue, smaller adipocytes and thus an increased density of adipocytes. For the detailed analysis of the metabolism the automated PhenoMaster system has been used, which allowed continuous and undisturbed recording of food and water intake, energy expenditure and movement. The reason for the lower body weight was the higher energy expenditure of these animals compared to controls. Additionally, female Sdc1^+/- mice showed an increased locomotor activity. Referring to organs, the intestine in Sdc1^+/- mice was heavier and longer, but no differences at the cellular level could be observed. These findings were independent of normal mating or vice versa embryo transfers of Sdc1^+/- and wild type embryos in recipient females of the other genotype. Herein we showed that the reduced expression of Sdc1 led to an altered metabolism on fetal as well as on maternal side, which may play a role in the growth restriction observed in human pregnancy pathologies and in mice lacking Sdc1.

Gastric Leptin and Tumorigenesis: Beyond Obesity

Leptin, an adipocyte-derived hormone and its receptor (ObR) expressed in the hypothalamus are well known as an essential regulator of appetite and energy expenditure. Obesity induces abundant leptin production, however, reduced sensitivity to leptin leads to the development of metabolic disorders, so called leptin resistance. The stomach has been identified as an organ that simultaneously expresses leptin and ObR. Accumulating evidence has shown gastric leptin to perform diverse functions, such as those in nutrient absorption and carcinogenesis in the gastrointestinal system, independent of its well-known role in appetite regulation and obesity. Overexpression of leptin and phosphorylated ObR is implicated in gastric cancer in humans and in murine model, and diet-induced obesity causes precancerous lesions in the stomach in mice. While the underlying pathomechanisms remain unclear, leptin signaling can affect gastric mucosal milieu. In this review, we focus on the significant role of the gastric leptin signaling in neoplasia and tumorigenesis in stomach in the context of hereditary and diet-induced obesity.

Temporal and regional onset of leptin resistance in diet-induced obese mice

In common forms of obesity, leptin fails to convey its regulatory effect. This so called "leptin resistance" is not well understood, and solving this puzzle is a key to understanding how obesity develops. In the present study, we investigated the temporal and regional onset of leptin resistance in response to a diet enriched with long-chain saturated fatty acids (high-fat diet; HFD) in mice. Mice were exposed to either a low-fat diet (LFD) or a HFD for 4 hours, 24 hours, 10 days and 28 days. Mice in each group received an i.p. injection of either phosphate-buffered saline or leptin and the number of phosphorylated signal transducer and activator of transcription-3 (pSTAT3) immunoreactive (-IR) cells in the arcuate nucleus (ARC), ventromedial nucleus of the hypothalamus (VMH) and dorsomedial nucleus of the hypothalamus (DMH) was analysed 30 or 120 minutes after treatment. In the ARC, as soon as 24 hours of HFD, the molecular leptin response was reduced by 40% (P≤.01). Compared to at 24 hours, after 10 days, the number of leptin-induced pSTAT3-IR cells was elevated after 120 minutes, suggesting a sustained response and a partial return of leptin sensitivity. After 28 days, leptin failed to induce the number of pSTAT3-IR over control levels, suggesting a markedly reduced sensitivity to leptin. In the VMH after 24 hours, we observed a 50% reduction in leptin-induced pSTAT-3-IR cells, followed by a further decline after 10 days. However, after 28 days, there was a significant increase in pSTAT-3-IR cells (P≤.05), indicating partial recovery of leptin sensitivity. By contrast to these two regions, in the DMH, no loss of leptin sensitivity was observed at any time-point. These findings demonstrate that a loss of sensitivity to leptin occurs rapidly after exposure to HFD in the ARC and VMH but not the DMH. However, there appears to be a biphasic pattern of leptin responsiveness, with a partial return of leptin sensitivity occurring after 10 days in the arcuate nucleus, and after 28 days in the VMH. By 28 days, the response to leptin in the arcuate nucleus was completely lost. These findings suggest that the molecular responses to leptin are altered after high-fat feeding in a time- and region-specific manner.

Manipulation of the Growth Hormone-Insulin-Like Growth Factor (GH-IGF) Axis: A Treatment Strategy to Reverse the Effects of Early Life Developmental Programming

Evidence from human clinical, epidemiological, and experimental animal models has clearly highlighted a link between the early life environment and an increased risk for a range of cardiometabolic disorders in later life. In particular, altered maternal nutrition, including both undernutrition and overnutrition, spanning exposure windows that cover the period from preconception through to early infancy, clearly highlight an increased risk for a range of disorders in offspring in later life. This process, preferentially termed “developmental programming” as part of the developmental origins of health and disease (DOHaD) framework, leads to phenotypic outcomes in offspring that closely resemble those of individuals with untreated growth hormone (GH) deficiency, including increased adiposity and cardiovascular disorders. As such, the use of GH as a potential intervention strategy to mitigate the effects of developmental malprogramming has received some attention in the DOHaD field. In particular, experimental animal models have shown that early GH treatment in the setting of poor maternal nutrition can partially rescue the programmed phenotype, albeit in a sex-specific manner. Although the mechanisms remain poorly defined, they include changes to endothelial function, an altered inflammasome, changes in adipogenesis and cardiovascular function, neuroendocrine effects, and changes in the epigenetic regulation of gene expression. Similarly, GH treatment to adult offspring, where an adverse metabolic phenotype is already manifest, has shown efficacy in reversing some of the metabolic disorders arising from a poor early life environment. Components of the GH-insulin-like growth factor (IGF)-IGF binding protein (GH-IGF-IGFBP) system, including insulin-like growth factor 1 (IGF-1), have also shown promise in ameliorating programmed metabolic disorders, potentially acting via epigenetic processes including changes in miRNA profiles and altered DNA methylation. However, as with the use of GH in the clinical setting of short stature and GH-deficiency, the benefits of treatment are also, in some cases, associated with potential unwanted side effects that need to be taken into account before effective translation as an intervention modality in the DOHaD context can be undertaken.

New insights in leptin resistance mechanisms in mice

Leptin resistance is one of the main challenges of obesity. To date, two levels of resistance have been identified, first a decreased rate of leptin uptake into the brain and secondly a diminished central response to leptin. New findings have identified the mechanisms of leptin transport and demonstrated that it can be rescued in obesity, but it did not overcome the problem of central resistance. Alteration in the actions of leptin following diet-induced obesity (DIO) appears to be a multifactorial condition. Several phosphatases are inhibiting leptin signaling pathways in a pathological way. Besides, hypothalamic inflammation alters the neuronal circuits that control metabolism. Recent studies describing both mechanisms (inhibition of leptin signaling and inflammation), have provided key insights to potential new targets for treatment. However, recent data showing that DIO mice may conserve a cellular and physiological response to endogenous leptin, highlights the need to redefine the concept of "leptin resistance".

Paternal obesity induces metabolic and sperm disturbances in male offspring that are exacerbated by their exposure to an "obesogenic" diet

Obesity and related comorbidities are becoming increasingly prevalent globally. In mice preconception paternal exposure to a high fat diet (HFD) impairs the metabolic and reproductive health of male offspring, despite their control diet (CD) consumption. However, offspring share lifestyle, including diet, with parents. We assessed if male offspring from HFD fathers have a heightened susceptibility to HFD-induced metabolic and reproductive derangements. This 2 × 2 design saw founder males (F0) and their offspring (F1) fed either a HFD or a nutritionally matched CD. Regardless of paternal diet, HFD fed male offspring had greater total body weight and adiposity. Offspring sired by a HFD male and fed a HFD were the heaviest, had the greatest adiposity and had the greatest concentration of serum cholesterol, triglyceride, HDL, and NEFA compared with CD sired/fed littermates. A synergistic increase in serum insulin was unmasked by both father/son HFD consumption, concomitant with increased sera glucose. Either a paternal or offspring HFD was associated with similar reductions to offspring sperm motility. Whereas sperm ROS concentrations and sperm-oocyte binding saw detrimental effects of both F0 HFD and F1 HFD with an interaction evident between both, culminating in the most impaired sperm parameters in this group. This indicates that metabolic and fertility disturbances in male offspring sired by HFD fathers are exacerbated by a "second-hit" of exposure to the same obesogenic environment postnatally. If translatable to human health, this suggests that adverse reproductive and metabolic outcomes may be amplified across generations through a shared calorie dense diet, relevant to the current worldwide obesity epidemic.

Maternal nutritional history modulates the hepatic IGF-IGFBP axis in adult male rat offspring

Alterations in early life nutrition lead to an increased risk of obesity and metabolic syndrome in offspring. We have shown that both relative maternal undernutrition (UN) and maternal obesity result in metabolic derangements in offspring, independent of the postnatal dietary environment. Since insulin-like growth factor binding protein 2 (IGFBP2) has been shown to be independently associated with obesity and diabetes risk, we examined the IGF-IGFBP axis in male rat offspring following either maternal UN or maternal obesity to explain possible common pathways in the development of metabolic disorders. Wistar rats were time-mated and fed either a control diet (CONT), 50 % of CONT (UN) or a high-fat (HF) diet throughout pregnancy. Male offspring were weaned onto a standard chow diet and blood and tissues were collected at postnatal day 160. Plasma and hepatic tissue samples were analysed for key players in the IGF-IGFBP system. Both maternal UN and HF resulted in increased fat mass, hyperinsulinemia, hyperleptinemia and altered blood lipid profiles in offspring compared to CONT. Circulating IGF-1 and IGFBP3 levels and hepatic mRNA expression of IGFBP1 and IGFBP2 were significantly decreased in UN and HF offspring compared to CONT. DNA methylation of the IGFBP2 promotor region was similar between maternal dietary groups. Although chaperone gene heat-shock protein 90 and hepatic IGFBP1 were significantly correlated in CONT offspring this effect was absent in both UN and HF offspring. In conclusion, this study is one of the first to directly compare two experimental models of developmental programming representing both ends of the maternal dietary spectrum. Our data suggest that two disparate nutritional models that elicit similar adverse metabolic phenotypes in offspring are characterised by common alterations in the IGF-IGFBP pathway.

Advances in understanding the interrelations between leptin resistance and obesity

Obesity, which has developed into a global epidemic, is a risk factor in most chronic diseases and some forms of malignancy. The discovery of leptin in 1994 has opened a new field in obesity research. Currently, we know that leptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. However, most individuals with diet-induced obesity (DIO) develop leptin resistance, which is characterized by elevated circulating leptin levels and decreased leptin sensitivity. To date, though various mechanisms have been proposed to explain leptin resistance, the exact mechanisms of leptin resistance in obesity are poorly understood. Consequently, it's an important issue worth discussing regarding what the exact interrelations between leptin resistance and obesity are. Here, we review the latest advancements in the molecular mechanisms of leptin resistance and the exact interrelations between leptin resistance, obesity, and obesity-related diseases, in order to supply new ideas for the study of obesity.

Paternal obesity initiates metabolic disturbances in two generations of mice with incomplete penetrance to the F2 generation and alters the transcriptional profile of testis and sperm microRNA content

Obesity is highly prevalent, and its incidence is increasing. The previous study showing a major effect of paternal obesity on metabolic health of offspring is confounded by comorbidity with diabetes. Therefore, we investigated the effect of diet-induced paternal obesity, in the absence of diabetes, on the metabolic health of two resultant generations and the molecular profiles of the testes and sperm. Founder (F0) male C57BL6 mice were fed either a high-fat diet (HFD) or a control diet (CD); n = 10/diet for a period of 10 wk. Testis expression of mRNA/microRNAs was analyzed by microarray and qPCR and sperm microRNA abundance by qPCR. Two subsequent generations were generated by mating F0 and then F1 mice to CD mice, and their metabolic health was investigated. All mice, other than F0 males, were maintained on a CD. HFD feeding induced paternal obesity with a 21% increase in adiposity, but not overt diabetes, and initiated intergenerational transmission of obesity and insulin resistance in two generations of offspring. This distinct phenotypic constellation is either partially or fully transmitted to both female and male F1 offspring and further transmitted through both parental lineages to the F2 generation, with a heightened effect on female F1 offspring (+67% in adiposity) and their F2 sons (+24% in adiposity). Founder male obesity altered the testes expression of 414 mRNAs by microarray and 11 microRNAs by qPCR, concomitant with alterations in sperm microRNA content and a 25% reduction in global methylation of germ cell DNA. Diet-induced paternal obesity modulates sperm microRNA content and germ cell methylation status, which are potential signals that program offspring health and initiate the transmission of obesity and impaired metabolic health to future generations. This study implicates paternal obesity in the transgenerational amplification of obesity and type 2 diabetes in humans.

Metabolic transceivers: in tune with the central melanocortin system

The central melanocortin system plays an essential role in the regulation of energy metabolism. Key to this regulation are the responses of neurons expressing proopiomelanocortin (POMC) and agouti-related protein (AgRP) to blood-borne metabolic signals. Recent evidence has demonstrated that POMC and AgRP neurons are not simply mirror opposites of each other in function and responsiveness to metabolic signals, nor are they exclusively first-order neurons. These neurons act as central transceivers, integrating both hormonal and neural signals, and then transmitting this information to peripheral tissues via the autonomic nervous system to coordinate whole-body energy metabolism. This review focuses on most recent developments obtained from rodent studies on the function, metabolic regulation, and circuitry of the central melanocortin system.

AgRP innervation onto POMC neurons increases with age and is accelerated with chronic high-fat feeding in male mice

In many mammals, body weight increases continuously throughout adulthood until late middle age. The hormone leptin is necessary for maintaining body weight, in that high levels of leptin promote negative energy balance. As animals age, however, their increase in body weight is accompanied by a steady rise in circulating leptin levels, indicating the progressive development of counterregulatory mechanisms to antagonize leptin's anorexigenic effects. Hypothalamic neurons coexpressing agouti-related peptide (AgRP) and neuropeptide Y are direct leptin targets. These neurons promote positive energy balance, and they inhibit anorexigenic proopiomelanocortin (POMC) neurons via direct neuropeptide action and release of γ-aminobutyric acid. We show here that AgRP and neuropeptide Y innvervation onto POMC neurons increases dramatically with age in male mice. This is associated with progressive increase of inhibitory postsynaptic currents and decrease of POMC firing rate with age. Neuronal activity is significantly attenuated in POMC neurons that receive a high density of AgRP puncta. These high-density AgRP inputs correlate with leptin levels in normal mice and are nearly absent in mice lacking leptin. The progression of increased AgRP innervation onto POMC somas is accelerated in hyperleptinemic, diet-induced obese mice. Together our study suggests that modulation of hypothalamic AgRP innervation constitutes one mechanism to counter the effects of the age-associated rise in leptin levels, thus sustaining body weight and fat mass at an elevated level in adulthood.

Over-expression of leptin receptors in hypothalamic POMC neurons increases susceptibility to diet-induced obesity

Diet-induced obesity (DIO) in rodents is characterized by impaired activation of signal-transducer and activator of transcription 3 (STAT3) by leptin receptors (LepRb) within the hypothalamic arcuate nucleus. This signaling defect likely plays an important role in development of DIO. However, the neuro-chemical identity of the leptin-STAT3 resistant arcuate neurons has not been determined and the underlying mechanisms responsible for development of cellular leptin resistance remain unclear. To investigate this, we first measured arcuate gene expression of known key signaling components of the LepRb signaling pathway and tested whether specifically the critical arcuate pro-opiomelanocortin (POMC) neurons are resistant to LepRb-STAT3 signaling in mice given a high-fat-diet (HFD) compared to mice provided a low-fat control diet (LFD). We found that leptin-dependent STAT3 phosphorylation was decreased within POMC neurons of HFD mice. In addition, Leprb mRNA and suppressor of cytokine signaling 3 (Socs3) mRNA were elevated in the arcuate of HFD mice. To investigate whether increased LepRb expression per se in POMC neurons can influence development of DIO and Socs3 expression, we created mice that over-express LepRb selectively in POMC neurons (POMC-LepRb). No differences in body weight, fat mass or food intake were found between LFD POMC-LepRb mice and LFD controls. Surprisingly, body weight, fat mass and caloric intake of HFD POMC-LepRb mice was markedly higher than HFD control mice. In addition, arcuate Socs3 mRNA was increased in HFD POMC-LepRb mice compared to HFD controls. These data show that specifically POMC neurons of DIO mice are resistant to STAT3 activation by leptin, indicating that those cells might play a role in development of DIO. Furthermore, over-expression of LepRb selectively in POMC neurons increases susceptibility to the development of DIO. We propose a model where over-reactivity of the leptin-LepRb signaling system in arcuate neurons may play causal a role in development of diet-induced obesity.

Sixteen years and counting: an update on leptin in energy balance

Cloned in 1994, the ob gene encodes the protein hormone leptin, which is produced and secreted by white adipose tissue. Since its discovery, leptin has been found to have profound effects on behavior, metabolic rate, endocrine axes, and glucose fluxes. Leptin deficiency in mice and humans causes morbid obesity, diabetes, and various neuroendocrine anomalies, and replacement leads to decreased food intake, normalized glucose homeostasis, and increased energy expenditure. Here, we provide an update on the most current understanding of leptin-sensitive neural pathways in terms of both anatomical organization and physiological roles.

Obesity and leptin resistance: distinguishing cause from effect

Because leptin reduces food intake and body weight, the coexistence of elevated leptin levels with obesity is widely interpreted as evidence of 'leptin resistance.' Indeed, obesity promotes a number of cellular processes that attenuate leptin signaling (referred to here as 'cellular leptin resistance') and amplify the extent of weight gain induced by genetic and environmental factors. As commonly used, however, the term 'leptin resistance' embraces a range of phenomena that are distinct in underlying mechanisms and pathophysiological implications. Moreover, the induction of cellular leptin resistance by obesity complicates efforts to distinguish the mechanisms that predispose to weight gain from those that result from it. We suggest a framework for approaching these issues and important avenues for future investigation.

Leptin receptor modulation of adiposity and fertility

The leptin receptor was discovered as a leptin binding protein, which is highly expressed in the choroid plexus. Mapping of the gene's chromosomal locations in rodents revealed that mutations in Lepr were the basis of obesity/diabetes mutations in rodents and humans. Genetic manipulations that target Lepr expression in specific neurons or hypothalamic areas have generated insights into the modes by which body composition and reproductive function are modulated by the leptin receptor. These animal models have also been instrumental in identifying diabetes susceptibility genes. In this review we discuss the evidence that supports the concept of networked functions of leptin receptor as it pertains to feeding, substrate utilization and reproduction.

Adult type 3 adenylyl cyclase-deficient mice are obese

Background

A recent study of obesity in Swedish men found that polymorphisms in the type 3 adenylyl cyclase (AC3) are associated with obesity, suggesting the interesting possibility that AC3 may play a role in weight control. Therefore, we examined the weight of AC3 mice over an extended period of time.

Methodology/Principal Findings

We discovered that AC3^−/− mice become obese as they age. Adult male AC3^−/− mice are about 40% heavier than wild type male mice while female AC3^−/− are 70% heavier. The additional weight of AC3^−/− mice is due to increased fat mass and larger adipocytes. Before the onset of obesity, young AC3^−/− mice exhibit reduced physical activity, increased food consumption, and leptin insensitivity. Surprisingly, the obesity of AC3^−/− mice is not due to a loss of AC3 from white adipose and a decrease in lipolysis.

Conclusions/Significance

We conclude that mice lacking AC3 exhibit obesity that is apparently caused by low locomotor activity, hyperphagia, and leptin insensitivity. The presence of AC3 in primary cilia of neurons of the hypothalamus suggests that cAMP signals generated by AC3 in the hypothalamus may play a critical role in regulation of body weight.

Overexpression of the transcription factor Foxo4 is associated with rapid glucose clearance

Leptin treatment ameliorates lipoatrophic diabetes in animal models and humans. Transgenic mice overexpressing leptin (LepTg) are lipoatrophic but not diabetic and thus represent a model for elucidating mechanisms of leptin-mediated glucose homeostasis. In this communication, we show that LepTg mice overexpress the forkhead transcription factor foxo4 in their remnant adipose tissue. To further characterize the role of foxo4 in adipose tissue, we generated transgenic mice overexpressing a constitutive active form of foxo4 (A3foxo4) under the control of the aP2 promoter/enhancer. aP2-A3foxo4 mice are not lipoatrophic but are able to clear glucose rapidly similar to LepTg mice. In addition, both LepTg and A3foxo4 mice show in their adipocytes increased AMP-activated protein kinase (AMPK) phosphorylation, suggesting a link between AMPK, glucose clearance, foxo4 and the leptin axis. These studies shed new light on mechanisms by which leptin treatment improves glucose disposal.

Leptin resistance: a prediposing factor for diet-induced obesity

Obesity is a resilient and complex chronic disease. One potential causative factor in the obesity syndrome is leptin resistance. Leptin behaves as a potent anorexic and energy-enhancing hormone in most young or lean animals, but its effects are diminished or lacking in the obese state associated with a normal genetic background. Emerging evidence suggests that leptin resistance predisposes the animal to exacerbated diet-induced obesity (DIO). Elevation of central leptin in young, lean rats induces a leptin resistance that precludes obesity on a chow diet but accelerates high-fat (HF)-induced obesity. Similarly, chronic dietary fructose consumption evokes a leptin resistance that causes obesity only upon HF exposure. Inherent central leptin insensitivity also contributes to dietary weight gain in certain obesity-prone rats. Conversely, aged, leptin-resistant animals are obese with continuous chow feeding and demonstrate aggravated obesity when challenged with an HF diet. Additionally, a submaximal central blockade with a leptin antagonist leads to obesity on both chow and HF diets, as is the case in rodents with leptin receptor deficiency of genetic origin. Despite the differences in the incidence of obesity on a chow diet, all of these forms of leptin resistance predispose rodents to aggravated HF-mediated obesity. Moreover, once leptin resistance takes hold, it aggravates DIO, and the leptin resistance and obesity compound one another, promoting a vicious cycle of escalating weight gain.

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Adipocytes express the rate-limiting enzymes required for glycogen metabolism and increase glycogen synthesis in response to insulin. However, the physiological function of adipocytic glycogen in vivo is unclear, due in part to the low absolute levels and the apparent biophysical constraints of adipocyte morphology on glycogen accumulation. To further study the regulation of glycogen metabolism in adipose tissue, transgenic mice were generated that overexpressed the protein phosphatase-1 (PP1) glycogen-targeting subunit (PTG) driven by the adipocyte fatty acid binding protein (aP2) promoter. Exogenous PTG was detected in gonadal, perirenal, and brown fat depots, but it was not detected in any other tissue examined. PTG overexpression resulted in a modest redistribution of PP1 to glycogen particles, corresponding to a threefold increase in the glycogen synthase activity ratio. Glycogen synthase protein levels were also increased twofold, resulting in a combined greater than sixfold enhancement of basal glycogen synthase specific activity. Adipocytic glycogen levels were increased 200- to 400-fold in transgenic animals, and this increase was maintained to 1 yr of age. In contrast, lipid metabolism in transgenic adipose tissue was not significantly altered, as assessed by lipogenic rates, weight gain on normal or high-fat diets, or circulating free fatty acid levels after a fast. However, circulating and adipocytic leptin levels were doubled in transgenic animals, whereas adiponectin expression was unchanged. Cumulatively, these data indicate that murine adipocytes are capable of storing far higher levels of glycogen than previously reported. Furthermore, these results were obtained by overexpression of an endogenous adipocytic protein, suggesting that mechanisms may exist in vivo to maintain adipocytic glycogen storage at a physiological set point.

Deletion of the serotonin 2c receptor from transgenic mice overexpressing leptin does not affect their lipodystrophy but exacerbates their diet-induced obesity

The binding of leptin to hypothalamic neurons elicits inhibition of orexigenic NPY/AgRP neurons and stimulation of anorexigenic POMC/CART neurons. Projections of serotonergic neurons onto POMC neurons suggest that leptin and serotonin converge onto POMC neurons to regulate body weight. We probed the interaction of these pathways by generating transgenic mice overexpressing leptin (LepTg) without 5HT2c receptors. On a chow diet, the lean phenotype of LepTg mice was unaffected by the absence of 5HT2c receptors, whereas on a high fat diet, LepTg/5HT2c receptors knockout mice showed an exacerbation of diet-induced obesity. POMC mRNA levels were low in LepTg, 5HT2c receptors knockout and LepTg/5HT2c receptors knockout mice, demonstrating that perturbations of the 5HT2c receptor and leptin pathways, either alone or in combination, negatively impact on POMC expression. Thus, on a chow diet, leptin action is independent of 5HT2c receptors whereas on a high fat diet 5HT2c receptors are required for the attenuation of obesity.

Neuronal PTP1B regulates body weight, adiposity and leptin action

Obesity is a major health problem and a risk factor for type 2 diabetes. Leptin, an adipocyte-secreted hormone, acts on the hypothalamus to inhibit food intake and increase energy expenditure. Most obese individuals develop hyperleptinemia and leptin resistance, limiting the therapeutic efficacy of exogenously administered leptin. Mice lacking the tyrosine phosphatase PTP1B are protected from diet-induced obesity and are hypersensitive to leptin, but the site and mechanism for these effects remain controversial. We generated tissue-specific PTP1B knockout (Ptpn1(-/-)) mice. Neuronal Ptpn1(-/-) mice have reduced weight and adiposity, and increased activity and energy expenditure. In contrast, adipose PTP1B deficiency increases body weight, whereas PTP1B deletion in muscle or liver does not affect weight. Neuronal Ptpn1(-/-) mice are hypersensitive to leptin, despite paradoxically elevated leptin levels, and show improved glucose homeostasis. Thus, PTP1B regulates body mass and adiposity primarily through actions in the brain. Furthermore, neuronal PTP1B regulates adipocyte leptin production and probably is essential for the development of leptin resistance.

Diminished growth and enhanced glucose metabolism in triple knockout mice containing mutations of insulin-like growth factor binding protein-3, -4, and -5

IGF-I and IGF-II are essential regulators of mammalian growth, development and metabolism, whose actions are modified by six high-affinity IGF binding proteins (IGFBPs). New lines of knockout (KO) mice lacking either IGFBP-3, -4, or -5 had no apparent deficiencies in growth or metabolism beyond a modest growth impairment (approximately 85-90% of wild type) when IGFBP-4 was eliminated. To continue to address the roles of these proteins in whole animal physiology, we generated combinational IGFBP KO mice. Mice homozygous for targeted defects in IGFBP-3, -4, and -5 remain viable and at birth were the same size as IGFBP-4 KO mice. Unlike IGFBP-4 KO mice, however, the triple KO mice became significantly smaller by adulthood (78% wild type) and had significant reductions in fat pad accumulation (P < 0.05), circulating levels of total IGF-I (45% of wild type; P < 0.05) and IGF-I bioactivity (37% of wild type; P < 0.05). Metabolically, triple KO mice showed normal insulin tolerance, but a 37% expansion (P < 0.05) of beta-cell number and significantly increased insulin secretion after glucose challenge, which leads to enhanced glucose disposal. Finally, triple KO mice demonstrated a tissue-specific decline in activation of the Erk signaling pathway as well as weight of the quadriceps muscle. Taken together, these data provide direct evidence for combinatorial effects of IGFBP-3, -4, and -5 in both metabolism and at least some soft tissues and strongly suggest overlapping roles for IGFBP-3 and -5 in maintaining IGF-I-mediated postnatal growth in mice.

Targeted overexpression of leptin to keratinocytes in transgenic mice results in lack of skin phenotype but induction of early leptin resistance

The epidermis has a great potential as a bioreactor to produce proteins with systemic action. However, the consequences of ectopic epidermal protein overexpression need to be carefully addressed to avoid both local and systemic adverse effects. Thus, the long-term effects of leptin on skin physiology have not been studied, and the metabolic consequences of sustained keratinocyte-derived leptin overexpression are unknown. Herein we describe that very high serum leptin levels can be achieved from a cutaneous source in transgenic mice in which leptin cDNA overexpression was driven by the keratin K5 gene regulatory sequences. Histopathological analysis including the study of skin differentiation and proliferation markers in these transgenic mice revealed that keratinocyte-derived leptin overexpression appears not to have any impact on cutaneous homeostasis. Although young K5-leptin transgenic mice showed remarkable thinness and high glucose metabolism as shown in other leptin transgenic mouse models, a marked leptin insensitivity become apparent as early as 3-4 months of age as demonstrated by increased weight gain and insulin resistance development. Other signs of leptin/insulin resistance included increased bone mass, organomegaly, and wound healing impairment. In addition, to provide evidence for the lack of untoward effects of leptin on epidermis, this transgenic mouse helps us to establish the safe ranges of keratinocyte-derived leptin overexpression and may be useful as a model to study leptin resistance.

Skeletal muscle AMP-activated protein kinase phosphorylation parallels metabolic phenotype in leptin transgenic mice under dietary modification

Leptin augments glucose and lipid metabolism independent of its effect on satiety. Administration of leptin in rodents increases skeletal muscle beta-oxidation by activating AMP-activated protein kinase (AMPK). We previously reported that, as hyperleptinemic as obese human subjects, transgenic skinny mice overexpressing leptin in liver (LepTg) exhibit enhanced insulin sensitivity and lipid clearance. To assess skeletal muscle AMPK activity in leptin-sensitive and -insensitive states, we examined phosphorylation of AMPK and its target, acetyl CoA carboxylase (ACC), in muscles from LepTg under dietary modification. Here we show that phosphorylation of AMPK and ACC are chronically augmented in LepTg soleus muscle, with a concomitant increase in the AMP-to-ATP ratio and a significant decrease in tissue triglyceride content. Despite preexisting hyperleptinemia, high-fat diet (HFD)-fed LepTg develop obesity, insulin-resistance, and hyperlipidemia. In parallel, elevated soleus AMPK and ACC phosphorylation in regular diet-fed LepTg is attenuated, and tissue triglyceride content is increased in those given HFD. Of note, substitution of HFD with regular diet causes a robust recovery of soleus AMPK and ACC phosphorylation in LepTg, with a higher rate of body weight reduction and a regain of insulin sensitivity. In conclusion, soleus AMPK and ACC phosphorylation in LepTg changes in parallel with its insulin sensitivity under dietary modification, suggesting a close association between skeletal muscle AMPK activity and sensitivity to leptin.

Leptin resistance exacerbates diet-induced obesity and is associated with diminished maximal leptin signalling capacity in rats

AIMS/HYPOTHESIS

Leptin resistance is generally considered a consequence of obesity. We postulated that leptin resistance is associated with diminished hypothalamic leptin signalling capacity and that leptin resistance is causal to obesity. We assessed maximal leptin-mediated binding of the transcription factor signal transducer and activator of transcription 3 (STAT3), and the response to high-fat feeding in lean leptin-resistant rats.

MATERIALS AND METHODS

Recombinant adeno-associated virus encoding rat leptin cDNA (rAAV-leptin) or control vector were administered by intracerebroventricular injection to lean F344 x BN rats for up to 150 days, and food consumption, body weight, serum leptin and glucose tolerance were measured. Leptin-mediated hypothalamic transcription factor binding was assessed at day 150 following an intracerebroventricular injection of 2 mug leptin. Rats pretreated with either control or rAAV-leptin vector for 94 days were given a high-fat diet, and energy intake, body weight gain and adiposity were examined.

RESULTS

The rAAV-leptin-treated rats initially responded to leptin gene delivery then became leptin-resistant. They displayed persistent submaximal hypothalamic leptin signalling and enhanced insulin sensitivity, yet maximal hypothalamic signalling capacity was decreased by more than 50%. On a high-fat diet, the leptin-resistant rats consumed more energy, gained more weight and accumulated greater visceral fat mass than controls.

CONCLUSIONS/INTERPRETATION

The maximal hypothalamic leptin signalling capacity was diminished in leptin-resistant rats receiving central rAAV-leptin gene therapy. Moreover, this leptin-invoked leptin resistance perturbs the regulation of energy homeostasis in response to high fat exposure, producing augmented energy consumption. This, coupled with potential hypersensitivity to insulin, creates a milieu favouring fat deposition. Our data suggest that leptin resistance is both a consequence and cause of obesity.

Screening of stimulatory effects of dietary risk factors on mouse intestinal cell kinetics

AIM

Although epidemiological and experimental studies validate influence of genetic, environmental and dietary factors in the causation of various types of cancers including colon, results from all these sources are inconclusive. Hypothesizing that high fat diet and obesity are among the major predisposing factors in the incidence of colon cancer, we evaluated the role of diet constituted with food material derived from a tropical plant, Tamarindus indica Linn (TI).

METHODS

A two part randomized double-blind study was conducted employing inbred Swiss albino mice from a single generation for the whole investigation. One day-old neonates (n = 12) were subcutaneously administered with monosodium glutamate (MSG) to induce obesity (OB). At weaning these animals were maintained on modified AIN-76 diet supplemented with 10% TI and 10% fat bolus (w/w, TIFB) for 8 wk. Subsequently, in the second part of study, four groups of animals belonging to the same generation, age and gender (n = 12 per group), were maintained on: AIN-76 control diet (CD); AIN-76 mixed with 10% TI extract (TI); and, mixed with 10% TI and 10% FB (TIFB) for 8 wk, to determine intestinal crypt cell proliferation, functionally-specific enzyme activities, fermentation profile, and energy preferences.

RESULTS

We observed a significant increase in the crypt cell production rate in distal colonic segment of experimental animals when compared with the controls. This segment also contained significantly low butyrate levels compared to control and TIFB groups. All the experimental groups showed a gross decrease in the enzyme activities viz., succinate dehydrogenase, acid-galactosidase and dipeptidyl amino peptidase IV demonstrating pathological stress caused by the test regimens, and an altered metabolic flux in the cellular environment.

CONCLUSION

We have demonstrated a cumulative response to the three dietary factors, one of which (TI) is reported, herein, for the first time to modulate kinetics of large intestinal mucosa, contributing to total risk posed by these test agents.

Animal models of nonalcoholic fatty liver disease and steatohepatitis

As occurs in people, nonalcoholic fatty liver disease (NAFLD) is associated strongly with obesity, diabetes, and dyslipidemia in experimental animals. There are many animal models that have been used to investigate the pathogenesis of nonalcoholic fatty liver disease. Most of this work has used mice or rats that are fed diets high in fat or carbohydrates, or mice that exhibit a genetic deficiency of a satiety factor such as leptin, 5-adenosylmethionine,or enzyme deficiencies in fatty acid oxidation. The purpose of this article is to update information regarding animal models in the pathogenesis of NAFLD.