Leptin-induced changes in body composition in high fat-fed mice

The Role of Cdc42 in the Insulin and Leptin Pathways Contributing to the Development of Age-Related Obesity

Age-related obesity significantly increases the risk of chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, and certain cancers. The insulin-leptin axis is crucial in understanding metabolic disturbances associated with age-related obesity. Rho GTPase Cdc42 is a member of the Rho family of GTPases that participates in many cellular processes including, but not limited to, regulation of actin cytoskeleton, vesicle trafficking, cell polarity, morphology, proliferation, motility, and migration. Cdc42 functions as an integral part of regulating insulin secretion and aging. Some novel roles for Cdc42 have also been recently identified in maintaining glucose metabolism, where Cdc42 is involved in controlling blood glucose levels in metabolically active tissues, including skeletal muscle, adipose tissue, pancreas, etc., which puts this protein in line with other critical regulators of glucose metabolism. Importantly, Cdc42 plays a vital role in cellular processes associated with the insulin and leptin signaling pathways, which are integral elements involved in obesity development if misregulated. Additionally, a change in Cdc42 activity may affect senescence, thus contributing to disorders associated with aging. This review explores the complex relationships among age-associated obesity, the insulin-leptin axis, and the Cdc42 signaling pathway. This article sheds light on the vast molecular web that supports metabolic dysregulation in aging people. In addition, it also discusses the potential therapeutic implications of the Cdc42 pathway to mitigate obesity since some new data suggest that inhibition of Cdc42 using antidiabetic drugs or antioxidants may promote weight loss in overweight or obese patients.

The Association Between an Addictive Tendency Toward Food and Metabolic Characteristics in the General Newfoundland Population

Background: Our previous study of 29 obese food addiction (FA) patients found that FA is associated with lipid profiles and hormones which may be a factor in cardiovascular disease (CVD) and insulin resistance (IR). However, there is currently no data available regarding the relationship between FA symptoms and metabolic characteristics of CVD and IR in the general population. We designed this study to investigate the correlation between FA symptoms with lipid profiles and IR in men and women of the general Newfoundland population. Methods: 710 individuals (435 women and 275 men) recruited from the general Newfoundland population were used in analysis. FA symptoms were evaluated using the Yale Food Addiction Scale (YFAS). Glucose, insulin, HDL, LDL, total cholesterol and triglycerides levels were measured. IR was evaluated using the homeostatic model of assessment (HOMA). Participants were grouped by sex and menopausal status. Age, physical activity, calories and total % body fat were controlled. Results: Partial correlation analysis revealed that in men, YFAS symptom counts were significantly correlated with HOMA-β (r = 0.196, p = 0.021), triglycerides (r = 0.140, p = 0.025) and inversely correlated with HDL (r = -0.133, p = 0.033). After separating by menopausal status, pre-menopausal women exhibited no correlations and post-menopausal women had a significantcorrelation with triglycerides (r = 0.198, p = 0.016). Conclusion: FA is significantly correlated with several markers of metabolic disturbance in men and to a lesser extent, post-menopausal women, in the general population. Further research is required to explain sex specific associations and elucidate any potentially causal mechanisms behind this correlation.

Differential physiological responses to central leptin overexpression in male and female rats

Brains of females are more sensitive to the acute catabolic actions of leptin. However, sex differences in the long-term physiological responses to central leptin receptor modulation are unknown. Accordingly, we centrally delivered a viral vector to overexpress leptin (Leptin), a neutral leptin receptor antagonist (Leptin-Antagonist) or a green fluorescence protein (GFP) (Control). We examined chronic changes in body weight and composition in male and female rats. Females displayed greater and sustained responses to Leptin, whereas males rapidly lost physiological effects and developed leptin resistance as confirmed by lower acute leptin-mediated phosphorylation of signal transducer and activator of transcription 3 (P-STAT3). Surprisingly, despite persistent physiological responses, Leptin-females also exhibited reduced acute leptin-mediated P-STAT3, suggesting an onset of leptin resistance near time of death. In line with this interpretation, Leptin-females and Control-females consumed the same amount of food on the last day of the experiment. Both Leptin-Antagonist groups gained similar percentages of their initial body weight and fat mass, whereas only Leptin-Antagonist-females gained lean body mass. Consequently, the lean/fat mass ratio with Leptin-Antagonist was preserved in females and decreased in males, suggesting a deterioration of body composition in males. In summary, the present study establishes that females are more responsive to long-term central leptin overexpression than males and that leptin antagonism has a greater physiological impact in males. The hormone environment may have played a role in these processes; however, future studies are needed to establish whether such physiological responses are mediated by female or male sex hormones.

Effects of chronic leptin infusion on subsequent body weight and composition in mice: Can body weight set point be reset?

Circulating leptin concentrations correlate with fat mass and signal the status of somatic energy stores to the brain. Previous studies suggest that diet-induced elevations of body weight increase body weight “set-point”. To assess whether chronic hyperleptinemia is responsible for this shift in defended body weight, we elevated circulating leptin concentrations in lean mice to those comparable to diet-induced obese mice for eighteen weeks. We hypothesized that following cessation of leptin infusion, a higher body weight would be defended. Compared to saline-infused controls, leptin-infused mice had elevated circulating leptin concentrations, gained less weight, yet had similar metabolic rates. Following cessation of leptin administration, leptin-infused mice gained some weight yet plateaued at 5–10% below controls. These results suggest that, unlike mice rendered hyperleptinemic by diet-induced weight gain, leptin-infused mice do not subsequently “defend” a higher body weight, suggesting that hyperleptinemia per se does not mimic the CNS consequences of chronic weight gain.

Global O-GlcNAc Levels Modulate Transcription of the Adipocyte Secretome during Chronic Insulin Resistance

Increased flux through the hexosamine biosynthetic pathway and the corresponding increase in intracellular glycosylation of proteins via O-linked β-N-acetylglucosamine (O-GlcNAc) is sufficient to induce insulin resistance (IR) in multiple systems. Previously, our group used shotgun proteomics to identify multiple rodent adipocytokines and secreted proteins whose levels are modulated upon the induction of IR by indirectly and directly modulating O-GlcNAc levels. We have validated the relative levels of several of these factors using immunoblotting. Since adipocytokines levels are regulated primarily at the level of transcription and O-GlcNAc alters the function of many transcription factors, we hypothesized that elevated O-GlcNAc levels on key transcription factors are modulating secreted protein expression. Here, we show that upon the elevation of O-GlcNAc levels and the induction of IR in mature 3T3-F442a adipocytes, the transcript levels of multiple secreted proteins reflect the modulation observed at the protein level. We validate the transcript levels in male mouse models of diabetes. Using inguinal fat pads from the severely IR db/db mouse model and the mildly IR diet-induced mouse model, we have confirmed that the secreted proteins regulated by O-GlcNAc modulation in cell culture are likewise modulated in the whole animal upon a shift to IR. By comparing the promoters of similarly regulated genes, we determine that Sp1 is a common cis-acting element. Furthermore, we show that the LPL and SPARC promoters are enriched for Sp1 and O-GlcNAc modified proteins during insulin resistance in adipocytes. Thus, the O-GlcNAc modification of proteins bound to promoters, including Sp1, is linked to adipocytokine transcription during insulin resistance.

Leptin's role in lipodystrophic and nonlipodystrophic insulin-resistant and diabetic individuals

Leptin is an adipocyte-secreted hormone that has been proposed to regulate energy homeostasis as well as metabolic, reproductive, neuroendocrine, and immune functions. In the context of open-label uncontrolled studies, leptin administration has demonstrated insulin-sensitizing effects in patients with congenital lipodystrophy associated with relative leptin deficiency. Leptin administration has also been shown to decrease central fat mass and improve insulin sensitivity and fasting insulin and glucose levels in HIV-infected patients with highly active antiretroviral therapy (HAART)-induced lipodystrophy, insulin resistance, and leptin deficiency. On the contrary, the effects of leptin treatment in leptin-replete or hyperleptinemic obese individuals with glucose intolerance and diabetes mellitus have been minimal or null, presumably due to leptin tolerance or resistance that impairs leptin action. Similarly, experimental evidence suggests a null or a possibly adverse role of leptin treatment in nonlipodystrophic patients with nonalcoholic fatty liver disease. In this review, we present a description of leptin biology and signaling; we summarize leptin's contribution to glucose metabolism in animals and humans in vitro, ex vivo, and in vivo; and we provide insights into the emerging clinical applications and therapeutic uses of leptin in humans with lipodystrophy and/or diabetes.

The beneficial effects of leptin on REM sleep deprivation-induced cognitive deficits in mice

Leptin, a 167 amino acid peptide, is synthesized predominantly in the adipose tissues and plays a key role in the regulation of food intake and body weight. Recent studies indicate that leptin receptor is expressed with high levels in many brain regions that may regulate synaptic plasticity. Here we show that deprivation of rapid eye movement (REMD) sleep resulted in impairment of both cue and contextual fear memory. In parallel, surface expression of GluR1 was reduced in the amygdala. Intraperitoneal injection of leptin to the REMD mice rescued memory impairment and reversed surface GluR1 reduction. Using whole-cell recording to evaluate the synaptic function of the thalamus-lateral amygdala (LA) pathway, we found a decrease in frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) concomitant with reduced AMPA/NMDA ratios in the REMD mice. By contrast, paired-pulse facilitation (PPF) was increased. The effects of REMD on mEPSCs and AMPA/NMDA ratio could be reversed by leptin treatment, whereas on PPR it could not. Phosphatase and tensin homolog (PTEN), a dual protein/lipid phosphatase, down-regulates the effect of the PI-3 kinase pathway. Fear conditioning increased whereas REMD led to a decrease in the phosphorylated states of PTEN, Akt, and glycogen synthase kinase-3β (GSK3β), and the effects of REMD were reversed by leptin. These results suggest that both pre- and postsynaptic functions of the thalamus-LA pathway were altered by fear conditioning and REMD in opposite directions. Leptin treatment reversed REMD-induced memory deficits primarily by a postsynaptic action by restoring surface expression of GluR1 without affecting PPR.

Hervey, Harris, and the parabiotic search for lipostatic signals

This paper is an introduction to the papers by Hervey and Harris that describe their experimental use of parabiosis in rats and mice to search for circulating lipostatic signals. Beginning in 1959 with Hervey's foundational paper, they detected three parabiotic signals: the Hervey signal decreases food intake and fat mass in rats; the antilipogenic factor (ALF) decreased fat mass, but not food intake in rats; and the leptin-dependent signal in lean partners of ob/ob mice decreased fat mass, but not food intake. The known lipostatic signals, leptin and insulin, have been candidates for the Hervey and ALF signals, but insulin has been excluded and the evidence for leptin is inconclusive. The site of production of the three parabiotic signals and their molecular structure are not known and specific mechanisms of their lipostatic control are incompletely understood. Given their potential importance for understanding the physiology of lipostatic controls and for developing new therapies for obesity, Hervey and Harris make a strong argument for further research on the three parabiotic signals.

Effects of leptin replacement alone and with exendin-4 on food intake and weight regain in weight-reduced diet-induced obese rats

Weight loss in obese humans produces a relative leptin deficiency, which is postulated to activate potent orexigenic and energy conservation mechanisms to restrict weight loss and promote weight regain. Here we determined whether leptin replacement alone or with GLP-1 receptor agonist exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced diet-induced obese (DIO) rats. Forty percent restriction in daily intake of a high-fat diet in DIO rats for 4 wk reduced body weight by 12%, body fat by 29%, and plasma leptin by 67% and normalized leptin sensitivity. When food restriction ended, body weight, body fat, and plasma leptin increased rapidly. Daily administration of leptin [3-h intraperitoneal (ip) infusions (4 nmol·kg(-1)·h(-1))] at onset and end of dark period for 3 wk did not attenuate hyperphagia and weight regain, nor did it affect mean daily meal sizes or meal numbers. Exendin-4 (50 pmol·kg(-1)·h(-1)) infusions during the same intervals prevented postrestriction hyperphagia and weight regain by normalizing meal size. Coadministration of leptin and exendin-4 did not reduce body weight more than exendin-4 alone. Instead, leptin began to attenuate the inhibitory effects of exendin-4 on food intake, meal size, and weight regain by the end of the second week of administration. Plasma leptin in rats receiving leptin was sevenfold greater than in rats receiving vehicle and 17-fold greater than in rats receiving exendin-4. Together, these results do not support the hypothesis that leptin replacement alone or with exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced DIO rats.

The relation between dietary fructose, dietary fat and leptin responsiveness in rats

Others reported that rats fed a high-fructose diet for 6 months were leptin resistant. We tested peripheral and/or central leptin responses in rats fed fructose for shorter time periods. Rats fed a diet containing 60% energy (% kcal) fructose and 10% kcal fat diet for 21 days had the same serum triglycerides (TG), gained less weight than controls, decreased their food intake and weight gain in response to central injections of 0.5 or 1.0 ug leptin, but were resistant to an i.p. injection of 2.0 mg leptin/kg. An i.p. injection of 1 mg leptin/kg increased phosphorylation of hypothalamic signal transducer and activator of transcription 3 (PSTAT3) implying resistance was not a failure of leptin to cross the blood brain barrier. The effects of dietary fructose were compared with those of dietary fat. Rats fed a 10%kcal fructose and 30%kcal fat diet for 39 days were leptin resistant whereas rats fed a 40%kcal fructose and 30%kcal fat diet responded to i.p. leptin. Another monosaccharide, glucose, replicated the effects of fructose in the 30% kcal fat diet. Surprisingly, none of the rats showed a reliable response to third ventricle leptin and peripheral leptin failed to stimulate hypothalamic PSTAT3 although it did increase PSTAT3 in the brainstem of rats fed the 40%kcal fructose or glucose diets. Thus a high-fructose, low-fat diet induces peripheral leptin resistance in less than 4 weeks, but high dietary concentrations of fructose or glucose prevent peripheral leptin resistance in rats fed a high-fat diet.

Implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity

Insulin and leptin play complementary roles in regulating the consumption, uptake, oxidation and storage of nutrients. Chronic consumption of diets that contain a high proportion of calories from saturated fat induces a progressive deterioration in function of both hormones. Certain rat lines and strains of mice are particularly sensitive to the obesogenic and diabetogenic effects of high fat diets, and have been used extensively to study the developmental progression of insulin and leptin resistance in relation to the increasing adiposity that is characteristic of their response to these diets. Some aspects of the diminished efficacy of each hormone are secondary to increased adiposity but a consensus is emerging to support the view that direct effects of dietary components or their metabolites, independent of the resulting obesity, play important roles in development of insulin and leptin resistance. In this minireview, we will examine the implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity, emphasizing potential interactions between pathways that occur among target sites, and exploring how these interactions may influence the progression of obesity and diabetes.

Sex-dependent regulation of hypothalamic neuropeptide Y-Y1 receptor gene expression in moderate/high fat, high-energy diet-fed mice

In this study we investigated whether long-term consumption of a moderate/high fat (MHF), high-energy diet can affect the gene expression of the Y(1) receptor (Y(1)R) for neuropeptide Y (NPY) in the dorsomedial (DMH), ventromedial (VMH), arcuate (ARC) and paraventricular (PVN) hypothalamic nuclei of male and female Y(1)R/LacZ transgenic mice, carrying the murine Y(1)R promoter linked to the LacZ gene. MHF diet-fed male mice showed an increased consumption of metabolizable energy that was associated with a significant increase in body weight as compared with chow-fed controls. In parallel, consumption of a MHF diet for 8 weeks significantly decreased Y(1)R/LacZ transgene expression in the DMH and VMH of male mice whereas no changes were found in the ARC and PVN. Leptin treatment reduced body weight of both MHF diet- and chow-fed male mice but failed to prevent the decrease in Y(1)R/LacZ transgene expression apparent in the DMH and VMH of male mice after 8 weeks of MHF diet intake. Conversely, no significant changes of metabolizable energy intake, body weight or hypothalamic beta-galactosidase expression were found in MHF diet-fed female Y(1)R/LacZ transgenic mice. A gender-related difference of Y(1)R/LacZ transgenic mice was also observed in response to leptin treatment that failed to decrease body weight of both MHF diet- and chow-fed female mice. Results herein demonstrate that Y(1)R/LacZ FVB mice show a sexual dimorphism both on energy intake and on nucleus-specific regulation of the NPY Y(1)R system in the hypothalamus. Overall, these results provide new insights into the mechanism by which diet composition affects the hypothalamic circuit that controls energy homeostasis.

High-fat diet-induced juvenile obesity leads to cardiomyocyte dysfunction and upregulation of Foxo3a transcription factor independent of lipotoxicity and apoptosis

BACKGROUND

Obesity is associated with dyslipidemia, which leads to elevated triglyceride and ceramide levels, apoptosis and compromised cardiac function.

METHODS

To determine the role of high-fat diet-induced obesity on cardiomyocyte function, weanling male Sprague-Dawley rats were fed diets incorporating 10% of kcal or 45% of kcal from fat. Mechanical function of ventricular myocytes was evaluated including peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR90) and maximal velocity of shortening and relengthening (+/- dl/dt). Intracellular Ca properties were assessed using fluorescent microscopy.

RESULTS

High-fat diet induced hyperinsulinemic insulin-resistant obesity with depressed PS, +/- dl/dt, prolonged TPS/TR90 reduced intracellular Ca release and Ca clearing rate in the absence of hypertension, diabetes, lipotoxicity and apoptosis. Myocyte responsiveness to increased stimulus frequency and extracellular Ca was compromised. SERCA2a and phospholamban levels were increased, whereas phosphorylated phospholamban and potassium channel (Kv1,2) were reduced in high-fat diet group. High-fat diet upregulated the forkhead transcription factor Foxo3a, and suppressed mitochondrial aconitase activity without affecting expression of the caloric sensitive gene silent information regulator 2 (Sir2), protein nitrotyrosine formation, lipid peroxidation and apoptosis. Levels of endothelial nitric oxide synthase (NOS), inducible NOS, triglycerides and ceramide were similar between the two groups.

CONCLUSIONS

Collectively, our data show that high-fat diet-induced obesity resulted in impaired cardiomyocyte function, upregulated Foxo3a transcription factor and mitochondrial damage without overt lipotoxicity or apoptosis.

Establishment and Pathophysiological Characterization of Type 2 Diabetic Mouse Model Produced by Streptozotocin and Nicotinamide

This study was performed in order to establish a mouse model that represents the non-obese type 2 diabetes reflecting a majority of diabetic patients among Asian races and to show its pathophysiological profiles. Streptozotocin (STZ) was administered to C57BL/6J mice with or without nicotinamide (120 or 240 mg/kg, STZ/NA120 or STZ/NA240), twice with an interval of 2 d, and plasma glucose concentration, body weight, water intake, insulin contents and insulin signal-related proteins were monitored. STZ-induced hyperglycemia (fasting and non-fasting), body weight loss and polyposia were significantly depressed by NA dose-dependently. In STZ/NA120 and STZ/NA240 mice, pancreatic insulin content was retained by 28 and 43% of normal control (10.5+/-0.93 microU/ml), respectively, and histological damage of pancreatic beta cells was also less severe than that observed in STZ mice. When given the calorie-controlled high fat diet, the STZ/NA mice caused hyperlipidemia, and significantly increased insulin resistance. These observations suggest that the combined administration of STZ and NA causes partial depletion of pancreatic insulin and that the high fat constituents lead to insulin resistance in this model. The present mouse model, therefore, well exhibits the recent diabetic pathophysiological characteristics of a majority of Asian patients.

Fast Food, Central Nervous System Insulin Resistance, and Obesity

Rates of obesity and insulin resistance have climbed sharply over the past 30 years. These epidemics are temporally related to a dramatic rise in consumption of fast food; until recently, it was not known whether the fast food was driving the obesity, or vice versa. We review the unique properties of fast food that make it the ideal obesigenic foodstuff, and elucidate the mechanisms by which fast food intake contributes to obesity, emphasizing its effects on energy metabolism and on the central regulation of appetite. After examining the epidemiology of fast food consumption, obesity, and insulin resistance, we review insulin’s role in the central nervous system’s (CNS) regulation of energy balance, and demonstrate the role of CNS insulin resistance as a cause of leptin resistance and in the promotion of the pleasurable or “hedonic” responses to food. Finally, we analyze the characteristics of fast food, including high-energy density, high fat, high fructose, low fiber, and low dairy intake, which favor the development of CNS insulin resistance and obesity.

Leptin action is modified by an interaction between dietary fat content and ambient temperature

Mice adapted to a high-fat diet are reported to be leptin resistant; however, we previously reported that mice fed a high-fat (HF) diet and housed at 23 degrees C remained sensitive to peripheral leptin and specifically lost body fat. This study tested whether leptin action was impaired by a combination of elevated environmental temperature and a HF diet. Male C57BL/6 mice were adapted to low-fat (LF) or HF diet from 10 days of age and were housed at 27 degrees C from 28 days of age. From 35 days of age, baseline food intake and body weight were recorded for 1 wk and then mice on each diet were infused with 10 microg leptin/day or PBS from an intraperitoneal miniosmotic pump for 13 days. HF-fed mice had a higher energy intake than LF-fed mice and were heavier but not fatter. Serum leptin was lower in PBS-infused HF- than LF-fed mice. Leptin significantly inhibited energy intake of both LF-fed and HF-fed mice, and this was associated with a significant increase in hypothalamic long-form leptin receptors with no change in short-form leptin receptor or brown fat uncoupling protein-1 mRNA expression. Leptin significantly inhibited weight gain in both LF- and HF-fed mice but reduced the percentage of body fat mass only in LF-fed mice. The percentage of lean and fat tissue in HF-fed mice did not change, implying that overall growth had been inhibited. These results suggest that dietary fat modifies the mechanisms responsible for leptin-induced changes in body fat content and that those in HF-fed mice are sensitive to environmental temperature.

Resistance to IP leptin-induced adipose apoptosis caused by high-fat diet in mice

The objectives of this experiment were to determine whether leptin causes adipocyte apoptosis in mice, whether peripheral administration is an effective means of studying leptin-induced adipocyte apoptosis, and whether high-fat feeding results in reduced responsiveness to leptin-induced adipocyte apoptosis. Continuous 13-day intraperitoneal infusion of 10 microg/day leptin significantly increased adipocyte apoptosis in the epididymal/parametrial fat pads of male and female mice, but only male mice developed reduced responsiveness to leptin-induced adipocyte apoptosis after high-fat (45% fat) feeding for 5 or 15 weeks. There was a positive correlation between serum leptin concentration and percent apoptotic adipocytes. These findings demonstrate that leptin administered peripherally is effective in inducing adipocyte apoptosis in mice, thus extending the possibility of studying this effect of leptin in a wider variety of animal models. In addition, high-fat feeding has a gender-specific effect on development of reduced responsiveness to leptin-induced adipocyte apoptosis.