Recombinant ob protein reduces feeding and body weight in the ob/ob mouse

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

Hypothalamic regulatory pathways and potential obesity treatment targets

With an ever-growing population of obese people as well as comorbidities associated with obesity, finding effective weight loss strategies is more imperative than ever. One of the challenges in curbing the obesity crisis is designing successful strategies for long-term weight loss and weight-loss maintenance. Currently, weight-loss strategies include promotion of therapeutic lifestyle changes (diet and exercise), pharmacological therapy, and bariatric surgery. This review focuses on several pharmacological targets that activate central nervous system pathways that normally limit food intake and body weight. Though it is likely that no single therapy will prove effective for everyone, this review considers several recent pre-clinical targets, and several compounds that have been in human clinical trials.

Hunger-satiety signals in patients with Graves' thyrotoxicosis before, during, and after long-term pharmacological treatment

Patients with Graves' thyrotoxicosis lose weight despite increased appetite and food intake, thus suggesting a disturbed balance between energy intake and expenditure. Underlying mechanisms are not fully elucidated. The objective of this study was to investigate whether hormonal factors, known to affect hunger/satiety, change significantly over time as pharmacological treatment turns hyperthyroidism into euthyroidism. For that purpose 11 patients with Graves' thyrotoxicosis were given thiamazole and l-thyroxine for 18-20 mo. They were investigated on three occasions: Test 1: before pharmacological therapy; Test 2: during medication; Test 3: a few months after conclusion of the pharmacological treatment. Sixteen healthy subjects were also investigated for comparison. The participants were fasted overnight. Blood samples for determination of plasma glucose and serum concentrations of free T3 and T4, TSH, albumin, cortisol, insulin, GH, IGF-1, IGFBP-1, leptin, and ghrelin were drawn in the morning from an antecubital vein. Laboratory data obtained in test 1 were statistically compared with those in tests 2 and 3. The study showed that the free T3 level declined from 42.8 +/- 4.3 pmol/L in test 1 to 6.0 +/- 0.8 pmol/L in test 2 (85 +/- 2% decline), and 5.5 +/- 0.3 pmol/L in test 3 (86 +/- 2% decline). The free T4 level fell concomitantly from 65.2 +/- 4.8 to 16.6 +/- 1.7 and 14.4 +/- 1.2 pmol/L. The glucose level was significantly higher during hyperthyroidism (test 1) than during euthyroidism (tests 2 and 3), whereas cortisol, insulin, GH, IGF-1, and leptin levels were similar. The IGFBP-1 level was initially high (48.8 +/- 8.5 microg/L in test 1), but with a relative decline in free T3 of 85 +/- 2% (test 2) the IGFBP-1 level declined by 34 +/- 13%, and with a free T3 decline of 86 +/- 2% (test 3) the binding protein fell by 39 +/- 29%. This brought about increased IGF-1 bioavailability as reflected by a rising IGF-1/IGFBP-1 ratio from 5.1 +/- 1.1 to 13.8 +/- 2.9 (p < 0.01). The ghrelin level was low (2454 +/- 304 ng/L) in test 1. It increased to 3127 +/- 397 ng/L in test 2 (p < 0.05), and to 3348 +/- 279 ng/L in test 3 (p < 0.01).

CONCLUSION

Both ghrelin secretion and IGF-1 bioavailability are low in patients with untreated thyrotoxicosis, but increase markedly as pharmacotherapy makes them euthyroid. These metabolic changes may be caused by the transition of hyperthyroidism into euthyroidism rather than by the pharmacotherapy per se, since the metabolic changes prevailed also in the posttreatment period.

Deletion of the RIIβ-Subunit of Protein Kinase A Decreases Body Weight and Increases Energy Expenditure in the Obese, Leptin-Deficient ob/ob Mouse

Disruption of the RIIbeta regulatory subunit of protein kinase A (PKA) results in mice with a lean phenotype, nocturnal hyperactivity, and increased resting metabolic rate. In this report, we have examined whether deletion of RIIbeta would lead to increased metabolism and rescue the obese phenotype of the leptin-deficient ob/ob (ob) mouse. Body weight gain and food consumption were decreased, whereas basal oxygen consumption and nocturnal locomotor activity were increased in the double mutant animals compared with ob mice. The ob mice are unable to maintain body temperature when placed in a cold environment due to a loss of brown adipose tissue activation, and this cold sensitivity was partially rescued by concomitant disruption of RIIbeta. These findings indicate that PKA modifies the phenotype of the leptin-deficient mouse, leading to increases in both thermogenesis and energy expenditure.

Plasma leptin concentration increases early during highly active antiretroviral therapy for acquired immunodeficiency syndrome, independent of body weight

Leptin, the protein product of the obese gene (ob), is secreted by adipocytes. Circulating leptin levels correlate with fat mass in humans, including individuals infected with HIV. Leptin serves as an adipostatic hormone, a permissive factor for reproduction and a modulator of immune function. Leptin is a cytokine, and has been demonstrated to enhance CD4 cell proliferation and IL-2 secretion from CD4 cells in vitro. The role of leptin in HIV-positive patients treated with highly active antiretroviral therapy (HAART) has not been well defined. We haveevaluated leptin levels in HIV-infected individualsduringthe early phase of HAART. We measured plasma leptin levels in 15 antiretroviral-naive HIV positive patients at baseline and after 1 and 4 weeks of HAART. After the first week of therapy, mean leptin level and CD4 count were increased compared to baseline, 6.0 vs 7.2 ng/ml (p = 0.004) and 377 vs 432 cells/ul (p = 0.014), respectively. In contrast, mean body mass index (BMI) remained unchanged 27.0 vs 26.8 kg/m2 (p < 0.08). After four weeks of therapy, leptin and BMI values were unchanged compared to baseline, 6.0 vs 5.9 (p < 0.4) and 27.0 vs 26.9 (p < 0.5), respectively, whereas CD4 count continued to increase to 491 cells/ul (p < 0.012 compared to baseline). These data demonstrate an early transient increase in plasma leptin levels in HIV positive patients initiated on HAART, despite a lack of change in BMI. It is unclear if the transient increase in leptin is related to its role as a cytokine, a metabolic regulator, or reproductive factor.

Metabolic implications of obesity: before and after gastric bypass

Obesity is recognized as an inherited disease state, and attributes of modern civilization have enhanced its phenotypic penetrance greatly. Two thirds or more of obese persons suffer comorbidities,many of which are characteristic of (dys)metabolic syndrome of insulin resistance. Unfortunately patients with body mass index over 35 infrequently can achieve or maintain weight loss adequate to resolve these metabolic (and anatomic) issues by lifestyle or pharmacologic strategies and are served better by gastric bypass, in spite of its attendant risks, both surgical and nutritional. This article evaluates the metabolic consequences of obesity and highlights which of these are amenable to correction with weight reduction achieved by gastric bypass.

Relationship between plasma leptin, insulin and tumor necrosis factor alpha in obese children

OBJECTIVES

1. To evaluate the relationship between plasma leptin and TNFalpha concentrations in obese children and to assess the differences between hyperinsulinemic and normoinsulinemic groups. 2. To evaluate the relationship between plasma leptin and insulin levels in obese children. 3. To investigate the TNFalpha G308A mutation in obese children.

METHODS

Body mass index (BMI), fasting plasma glucose and insulin levels, oral glucose tolerance test results, homeostasis model assessment of insulin resistance (HOMA-IR) results, and plasma leptin and TNFalpha concentrations were evaluated in obese children (n = 45) and age- and gender-matched, lean healthy controls (n = 40).

RESULTS

In obese children the fasting insulin, HOMA-IR results, plasma leptin and TNFalpha concentrations were significantly higher than in controls (p <0.05). Furthermore, obese females showed higher plasma leptin and insulin resistance compared to obese males. While plasma leptin, TNFalpha levels and HOMA-IR results were similar in the prepubertal and pubertal groups, insulin levels were significantly higher in the pubertal group. Plasma leptin and TNFalpha concentrations were similar in hyperinsulinemic and normoinsulinemic obese children. In control children, plasma leptin concentrations showed a positive correlation with BMI, age, fasting insulin and HOMA-IR results. In obese children, plasma leptin levels did not correlate with BMI, fasting insulin or TNFalpha.

CONCLUSION

Plasma leptin concentrations did not show any correlation with TNFalpha levels in obese children. Furthermore, plasma leptin and TNFalpha concentrations were similar in hyperinsulinemic and normoinsulinemic obese children.

Longitudinal investigation of the relationship between breast milk leptin levels and growth in breast-fed infants

BACKGROUND

It has been shown that leptin is present in breast milk and human mammary epithelial cells are able to synthesize leptin. It has been suggested that leptin in human milk might be involved in the regulation of postnatal nutrition and growth.

AIMS

To investigate whether there is a relationship between leptin levels in human milk and weight gain in the postnatal period and to compare variations of milk-borne maternal leptin concentrations for small for gestational age (SGA), large for gestational age (LGA) and appropriate for gestational age (AGA) infants.

INFANTS AND METHODS

Forty-seven healthy lactating women aged from 17-38 years and their infants were included in the study. The infants were separated into three groups according to birth weight as SGA (n = 11), LGA (n = 14) and AGA (n = 22). All infants were fed with breast milk during the study period. Anthropometric measurements were performed on the 15th day of life and at 1, 2, and 3 months of age, and the body mass index (BMI) of the infants' mothers was calculated. Breast milk leptin levels were analyzed by radioimmunoassay.

RESULTS

Breast milk leptin levels were found reduced in the SGA group and increased in the LGA group compared to the AGA group at 15 days of life (13.4 +/- 2.2, 28.5 +/- 4.4 and 18.4 +/- 2 ng/ml, respectively; p <0.05). At 1 month of age, leptin levels in breast milk were significantly lower in the LGA group than in the AGA group (15.5 +/- 4.9, 19.4 +/- 1.7 ng/ml, respectively; p<0.05). There was no difference among the three groups at 2 and 3 months of age (p>0.05). There was a positive correlation between birth Weight and breast milk leptin levels on the 15th day (r = 0.47, p = 0.001). A negative correlation was found between weight gain during the first 15 days and 1 month of life and breast milk leptin levels on the 15th day (r = -0.44, p = 0.002; r = -0.40, p = 0.005, respectively). No relationship could be determined between breast milk leptin levels and BMI of the mothers.

CONCLUSION

Maternal milk of SGA, LGA and AGA infants had different leptin levels, especially during the first month of life. More rapid growth was shown in the SGA infants during the first postnatal 15 days compared to AGA and LGA infants, and human milk leptin levels were significantly reduced in the SGA group. However, LGA infants gained more weight during the second 15 days of life and breast milk leptin levels were dramatically decreased in LGA and increased in SGA infants at the end of first month of life. These findings suggest that the presence of leptin in breast milk might have a significant role in growth, appetite and regulation of nutrition in infancy, especially during the early lactation period, and the production of leptin in breast tissue by human mammary epithelial cells might be regulated physiologically according to necessity and state of the infant.

The relation of serum and follicular fluid leptin and ovarian steroid levels in response to induction of ovulation in in vitro fertilization cycles

OBJECTIVE

Leptin restores energy homeostasis and regulates appetite and body weight by communicating the energy status to the central nervous system. Although there is strong evidence that leptin affects reproduction, its role in the control of reproductive physiology is little understood.

STUDY DESIGN

We studied leptin concentrations in the serum and follicular fluid of 65 women undergoing ovarian hyperstimulation for in vitro fertilization (IVF). Fasting serum samples were collected (1) on the 3rd day of the cycle before IVF and (2) at the time of oocyte retrieval. Serum concentrations of leptin, estradiol (E2), progesterone, FSH, LH, prolactin, total testosterone, DHEA-SO4, and TSH and follicular fluid concentrations of leptin, E2, and progesterone were measured.

RESULTS

Serum leptin values increased on average by 66.4% over basal leptin levels on the day of oocyte pick-up (OPU). A positive correlation between leptin increase and body mass index was observed. The serum leptin level was similar to that in follicular fluid o the day of OPU. E2 levels increased 34.5-fold with controlled ovarian hyperstimulation. There was a negative correlation between the increase in leptin levels and in E2 levels (P <0.05) and in the number of oocytes harvested (P <0.05).

CONCLUSION

The significant increase in serum leptin levels during controlled ovarian hyperstimulation indicates a possible role of leptin in reproductive function. The increase in leptin levels is negatively correlated with ovarian response evaluated by E2 production and number of oocytes retrieved. This might be due to the reduced ovarian response through negative feedback of leptin to the ovaries at high levels.

Red yeast rice extracts suppress adipogenesis by down-regulating adipogenic transcription factors and gene expression in 3T3-L1 cells

The effects of red yeast rice extracts (RE) on adipocyte differentiation of 3T3-L1 cells were studied. RE were extracted from embryonic rice fermented with red yeast (Monascus ruber). These extracts significantly decreased glycerol-3-phosphate dehydrogenase (GPDH) activity and lipid accumulation, a marker of adipogenesis, in a dose-dependent manner. Moreover, mRNA expression levels of both CCAAT/enhancer-binding protein (C/EBP) alpha and peroxisome proliferator-activated receptor (PPAR) gamma, the key adipogenic transcription factors, were markedly decreased by RE. RE also inhibited the expression of PPARgamma at protein levels. RE decreased significantly gene expression of adipocyte fatty acid binding protein (aP2) and leptin, which are adipogenic marker proteins and C/EBPalpha and PPARgamma target genes. These results suggest that the inhibitory effect of RE on adipocyte differentiation might be mediated through the down-regulated expression of adipogenic transcription factors and other specific genes.

Chronic central administration of ghrelin reverses the effects of leptin

OBJECTIVE

To determine whether chronic central administration of ghrelin can block the effects of leptin on food intake, adiposity, and plasma concentrations of metabolic parameters and hormones.

DESIGN

Intracerebroventricular (ICV) infusions of leptin (5 microg/day) for 7 days, with or without ghrelin (1.2 microg/day), in rats. Rats administered leptin plus ghrelin were divided into ad lib-fed and food-restricted groups.

MEASUREMENT

Body weight and food intake were monitored daily. Following killing on day 8, epididymal fat weight and fasting plasma concentrations of glucose, insulin, leptin, ghrelin, IGF-1, and adiponectin were determined.

RESULTS

ICV infusion of leptin decreased food intake by 39% and fat weight by 41%. Leptin decreased plasma concentrations of glucose, insulin, and leptin and increased plasma ghrelin levels. Central coadministration of ghrelin blocked the effects of leptin. Most of the effects of ghrelin were diminished by food restriction but ghrelin effect on adiposity and plasma insulin concentrations remained in food-restricted rats.

CONCLUSION

Chronic central administration of ghrelin reversed the effects of leptin, primarily by altering food intake, but ghrelin may have regulatory effects on adiposity and plasma insulin levels independent of feeding effect.

Adipokine levels in Cushing's syndrome; elevated resistin levels in female patients with Cushing's syndrome

BACKGROUND

Cushing's syndrome (CS) is associated with central adiposity, insulin resistance and impaired glucose homeostasis. Adipose tissue is thought to regulates glucose homeostasis via circulating adipokines, such as resistin, leptin and adiponectin, although their role in the insulin resistance associated with CS has not been established.

DESIGN

We examined the relationship between insulin resistance and adipokine levels in CS patients. We compared plasma levels of resistin, leptin and adiponectin in 10 women and four men patients with CS, with 14 health subjects matched for age, gender and body mass index. A subgroup of three women and four men with pituitary-dependent CS were re-examined at least 9 months after curative surgery.

RESULTS

CS patients had significantly more truncal fat and less lean body mass as assessed by DEXA compared to control subjects. Total cholesterol, triglycerides and insulin resistance, as calculated using the homeostasis model assessment of insulin resistance (HOMA-R), was significantly increased in CS patients. Of the adipokines measured, only resistin was significantly different between female CS patients and female control subjects (5.05 +/- 0.56 vs. 2.91 +/- 0.39 micro g/l, P = 0.015). Curative surgery significantly reduced total body fat and truncal fat, leptin, total and low-density lipoprotein (LDL) cholesterol, glucose and HOMA-R. A reduction in both resistin and adiponectin was also observed but the differences between pre- and post-treatment levels did not achieve statistical significance.

CONCLUSION

Here we report for the first time that resistin levels are significantly elevated in CS patients and may be important in the insulin resistance associated with glucocorticoid excess.

The role of craniofacial growth in leptin deficient (ob/ob) mice

OBJECTIVES

To elucidate the role of leptin on maxillo-facial morphological growth using hereditary obesity model ob/ob mice, and to examine the presence of the leptin receptor gene expression in the mouse condylar head cartilage.

DESIGN

Leptin was intraperitoneally administered once a day in 10 C57BL/6J (lean) and 10 C57BL/6J-ob (ob/ob) mice (leptin administration group), and phosphate-buffered saline (PBS) in 10 lean and 10 ob/ob mice (PBS administration group), between the fifth and 11th week after birth. The amount of fat, the body amount without fat, the rate of body fat, and the width of the condylar cervical area were measured during the 11th week, and roentgenographic cephalometric analysis was performed at the fifth, eighth, and 11th week. Furthermore, the condylar head cartilage in C57BL/6J mice was stereoscopically excised to extract total RNA, and RT-PCR method was performed regarding the leptin receptor gene.

RESULTS

The body fat amount in ob/ob mice with leptin production insufficiency was greater than that in lean mice, and significant differences were noted in every measurement item regarding maxillo-facial morphology. Recovery of bone length was noted in ob/ob mice by administering leptin. Furthermore, the expression of the leptin receptor gene in the condylar head cartilage was confirmed.

CONCLUSION

Exogenous leptin administration leads to significant increases in craniofacial dimensions; and leptin receptors are expressed in mandibular condylar cartilage. These results indicate an important role for leptin in craniofacial growth and morphology. We speculate that leptin's direct peripheral effect on bone and cartilage is closely involved in this role.

The role of leptin in female adolescence

Leptin, the ob gene product, is related to the onset of puberty in animal models, but its role in human puberty is still rather undefined. In prepubertal girls and boys, leptin concentrations increase slowly with age and body-fat mass. In boys, this increase is interrupted in early puberty, when testosterone and lean body mass increase. In girls, leptin, along with the body-fat mass, continue to increase during puberty. Plasma leptin concentrations are significantly correlated with fat mass at all Tanner stages in males and females. The diurnal variation of leptin concentrations seen in adults is apparent for all age groups with no significant changes in the pattern across puberty. Leptin is bound in blood by a high-affinity binding protein identical with the soluble leptin receptor (sOB-R). In the first year of life, the concentration of sOB-R is high, and then a continuous decline of sOB-R follows until midpuberty. The therapeutic response to leptin treatment in a child with leptin deficiency confirms the importance of leptin in the regulation of body weight in humans, and establishes an important role for this hormone in the regulation of appetite. Still no evidence is available that would indicate leptin is a primary signal that initiates the onset of human puberty. Instead, it may act in a permissive way as one of several metabolic factors to allow pubertal maturation to proceed and later reproduction to occur.

Alcoholism and obesity: overlapping neuropeptide pathways?

Ethanol is a caloric compound, and ethanol drinking and food intake are both appetitive and consummatory behaviors. Furthermore, both ethanol and food have rewarding properties. It is therefore possible that overlapping central pathways are involved with uncontrolled eating and excessive ethanol consumption. A growing list of peptides has been shown to regulate food intake and/or energy homeostasis. Peptides such as the melanocortins, corticotropin releasing factor, and cholecystokinin promote reductions of food intake while others such as galanin and neuropeptide Y stimulate feeding. The present review highlights research aimed at determining if ingestive peptides also regulate voluntary ethanol intake, with an emphasis on the melanocortins and neuropeptide Y. It is suggested that research directed at ingestive peptides may expand our understanding of the neurobiological mechanisms that drive ethanol self-administration, and may reveal new therapeutic candidates for treating alcohol abuse and alcoholism.

Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.

Study of the alteration of gene expression in adipose tissue of diet-induced obese mice by microarray and reverse transcription-polymerase chain reaction analyses

In the present study we developed a model of diet-induced obesity (DIO) in male C57 BL/6J mice using an 8-wk high fat diet. This model should better reflect the physiology of the majority of the human obese patients than mouse genetic models of obesity with defects in leptin or leptin signaling. At the end of the diet, DIO mice displayed an increased weight (20%) and higher leptin, insulin, glucose, and corticosterone plasma levels compared with mice fed a standard diet during the same period. Moreover, they became resistant to the central effect of peripheral administration of leptin. Oligonucleotide microarray studies were conducted in adipose tissue. They showed that a great number of genes are differentially expressed. The majority of these genes (69%) are down-regulated in DIO mice. Among those are genes encoding enzymes of the lipid metabolism or markers of adipocyte differentiation, enzymes involved in detoxification processes, as well as structural components of the cytoskeleton. Some other groups of genes displayed increased expression, such as those encoding inflammatory markers. The results of the microarray analysis were confirmed by semiquantitative RT-PCR studies run on a selected number of genes that were differentially expressed or not modified.

Metabolic regulation of fertility through presynaptic and postsynaptic signaling to gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction and are inhibited by negative energy balance. In normal adults, these neurons maintain elevated intracellular chloride so that GABA(A) receptor activation is excitatory. We hypothesized that fasting alters homeostatic mechanisms to eliminate excitatory responses to GABA but rejected this hypothesis when brief, local GABA application elicited action currents in GnRH neurons from fed and fasted mice. This response was specific to GABA(A) receptors, because glycine elicited no response. We next found that fasting reduced the frequency of spontaneous GABAergic postsynaptic currents (PSCs) and that this was reversed by in vivo treatment with leptin during the fast. In the presence of tetrodotoxin to minimize presynaptic actions, leptin also potentiated the postsynaptic response of these cells to GABA(A) receptor activation. Postsynaptic effects of leptin on GABAergic miniature PSCs were eliminated by inhibiting JAK2/3 (Janus kinase), the tyrosine kinase through which leptin receptors signal. In all experiments, elimination of PSCs at ECl or by treatment with the GABAA receptor antagonist bicuculline confirmed that PSCs were specifically mediated by GABA(A) receptor chloride channels. These data demonstrate that fasting and leptin act presynaptically and postsynaptically to alter GABAergic drive to GnRH neurons, providing evidence for GABAergic communication of metabolic cues to GnRH neurons, and suggest the possibility for functional leptin receptors on GnRH neurons. They further demonstrate cytokine modulation of the postsynaptic response to GABA in mammals, which may be important to central neural regulation in both healthy and diseased states.

Quantitative trait loci for insulin-like growth factor I, leptin, thyroxine, and corticosterone in genetically heterogeneous mice

Genotype information was collected at 87 loci in a group of 1,108 UM-HET3 mice bred as the progeny of [BALB/cJ x C57BL/6J]F1 mothers and [C3H/HeJ x DBA/2J]F1 fathers, for which thyroxine (T4), insulin-like growth factor I (IGF-I), and leptin levels had been measured at 4 and 15 mo of age. The data provided significant evidence for quantitative trait loci (QTL) modulating IGF-I levels on chromosomes 1, 3, 8, 10, and 17; for loci affecting T4 on chromosomes 4, 15, and 17; and for leptin on chromosome 3. Fecal levels of corticosterone at 17 mo of age were influenced by a QTL on chromosome 1. Nine other gene/hormone associations reached a nominal P < 0.01, providing suggestive but not statistical evidence for additional QTL. QTL with an influence on a given hormone were in nearly all cases additive, with little or no evidence for epistasis. Of the 12 strongest QTL, 5 had effects that were age dependent, having more effect in 15-mo-old than in 4-mo-old mice in all but one case; the other QTL had effects that were apparently age-independent. These results show that the genetic controls over late-life hormone levels are complex and dependent on effects of genes that act both early and late in the life course.

Metabolic regulation of fertility through presynaptic and postsynaptic signaling to gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction and are inhibited by negative energy balance. In normal adults, these neurons maintain elevated intracellular chloride so that GABA(A) receptor activation is excitatory. We hypothesized that fasting alters homeostatic mechanisms to eliminate excitatory responses to GABA but rejected this hypothesis when brief, local GABA application elicited action currents in GnRH neurons from fed and fasted mice. This response was specific to GABA(A) receptors, because glycine elicited no response. We next found that fasting reduced the frequency of spontaneous GABAergic postsynaptic currents (PSCs) and that this was reversed by in vivo treatment with leptin during the fast. In the presence of tetrodotoxin to minimize presynaptic actions, leptin also potentiated the postsynaptic response of these cells to GABA(A) receptor activation. Postsynaptic effects of leptin on GABAergic miniature PSCs were eliminated by inhibiting JAK2/3 (Janus kinase), the tyrosine kinase through which leptin receptors signal. In all experiments, elimination of PSCs at ECl or by treatment with the GABAA receptor antagonist bicuculline confirmed that PSCs were specifically mediated by GABA(A) receptor chloride channels. These data demonstrate that fasting and leptin act presynaptically and postsynaptically to alter GABAergic drive to GnRH neurons, providing evidence for GABAergic communication of metabolic cues to GnRH neurons, and suggest the possibility for functional leptin receptors on GnRH neurons. They further demonstrate cytokine modulation of the postsynaptic response to GABA in mammals, which may be important to central neural regulation in both healthy and diseased states.

Effects of leptin gene expression in mice in vivo by electroporation and hydrodynamics-based gene delivery

In vivo electroporation and hydrodynamics-based gene delivery were utilized to test the effect of leptin gene transfer on food intake, and body and fat weights of mice. Gene transfer of pVRmob by electroporation caused a significant reduction in body weight compared with the control counterpart (p<0.05), although a lesser effect was found in food intake, and the weights of interscapular brown and epididymal fat by electroporation. As might be expected, the hydrodynamics-based transfection method significantly reduced body weight over 1 week post-transfection (p<0.05). Furthermore, epididymal fat was decreased by 50% at 1 week after gene transfer (p<0.001). These results suggest that both electroporation and hydrodynamics-based gene delivery may be effective approaches for systemic delivery of recombinant leptin to the central nervous system, and that the efficiency of gene transfer in hydrodynamics-based gene delivery was markedly higher than that in electroporation at least within the first week after transfection.

Hyperleptinemia precipitates diet-induced obesity in transgenic mice overexpressing leptin

Transgenic mice overexpressing leptin backcrossed to the C57BL/6J genetic background (LepTg) have a lean phenotype, characterized by a 95% reduction in adipose mass; reduced plasma levels of glucose, triglycerides, insulin, and IGF-1; and a 75% decrease in adipocyte size. High-fat diet treatment for 20 wk revealed that, compared with normal mice, the LepTg mice had an increased susceptibility to diet-induced obesity, as demonstrated by their rate of weight gain, higher accumulation of sc white adipose tissue mass, hypertrophy of adipocytes, and normalization of their reduced metabolic parameters. The stromal vascular fraction of white adipose tissue from the LepTg mice was highly cellular and contained cells capable of rapid lipid accumulation in primary cultures. The precipitous diet-induced obesity of the LepTg mice was accompanied with 10-fold and 1.6-fold elevations in insulin and IGF-1, respectively, suggesting that the trophic action of insulin and IGF-1 on the preadipocytes and small adipocytes may have caused them to rapidly differentiate and accumulate triacylglycerol stores. Other contributing factors may involve a shift in insulin sensitivity triggered by hyperleptinemia and a decrease in energy expenditure. These studies demonstrate that a chronic response to hyperleptinemia as in the LepTg mice is a predisposing factor to diet-induced obesity and suggest that individuals who are particularly lean because of increased leptin secretion may develop rapid obesity under conditions of a high-fat diet.

Hormone levels and cataract scores as sex-specific, mid-life predictors of longevity in genetically heterogeneous mice

Serum levels of thyroxine (T4), leptin, and insulin-like growth factor-I (IGF-I), as well as cataract severity, were evaluated as predictors of life span in a population of genetically heterogeneous mice (UM-HET3). Long life span was predicted by low levels of leptin at age 4 months in females, and by low levels of IGF-I at age 15 months and high levels of T4 at age 4 months, in males. Cataract severity at either 18 or 24 months was also a significant predictor of life span in females only, but in contrast to what has been reported in human studies, relatively severe cataract was correlated with longer life span. Additional work is needed to evaluate the role of these hormones as potential modulators of the aging process, and to resolve the conflicting data obtained for cataract severity as a predictor of life span.

The role of leptin in the etiopathogenesis of anorexia nervosa and bulimia

Leptin is an adipocyte-derived signal factor (167 amino acid protein) encoded by the ob gene in chromosome 7q31 that regulates eating behaviour via central neuroendocrine mechanisms. It has been shown that serum leptin levels correlate with weight and percentage body fat in normal and obese individuals, but the exact correlation between leptin and body weight in anorexic and bulimic patients has not yet been clarified. We investigated leptin levels in the serum of 58 female subjects aged 15-36 years: 10 with bulimia nervosa (BN); 12 with anorexia nervosa (AN); 12 overweight controls (not BN); 12 weight-reduced controls (not AN); and 12 normal weight controls. The aim of the study was to evaluate the possible correlations between leptin levels and the body mass index (BMI) in all five groups. Our results showed that the serum leptin levels of the bulimic patients were similar to those of the healthy controls, with a positive correlation between leptin and BMI. Although bulimic patients have very bad nutritional behaviour, their leptin levels do not appear altered. Serum leptin was significantly (p<0.001) reduced in the anorexic patients because of the dramatic decrease in adipose mass caused by the nutritional defect, as: is further supported by the significantly (p<0.001) low level of transferrinemia. Our data suggest that, although significantly reduced, serum leptin levels in fasting anorexic patients are non-linearly related to body weight (BMI).

Obesity-associated activation of angiotensin and endothelin in the cardiovascular system

The renin-angiotensin system (RAS) and the endothelin system have been implicated in the pathogenesis of human cardiovascular and renal diseases, and inhibition of the RAS markedly improves morbidity and survival. Obesity in humans is associated with an increased risk for the development of hypertension, atherosclerosis and focal-segmental glomerulosclerosis, however the exact mechanisms underlying these pathologies in obese individuals are not known. This article discusses the clinical importance of obesity and the current evidence for local activation of the renin-angiotensin system and its interactions with the endothelin system in obesity and the cardiovascular pathologies associated with it.

Arcuate nucleus-specific leptin receptor gene therapy attenuates the obesity phenotype of Koletsky (fa(k)/fa(k)) rats

Leptin signaling in the hypothalamic arcuate nucleus (ARC) is hypothesized to play an important role in energy homeostasis. To investigate whether leptin signaling limited to this brain area is sufficient to reduce food intake and body weight, we used adenoviral gene therapy to express the signaling isoform of the leptin receptor, lepr(b), in the ARC of leptin receptor-deficient Koletsky (fa(k)/fa(k)) rats. Successful expression of adenovirus containing lepr(b) (Ad-lepr(b)) selectively in the ARC was documented by in situ hybridization. Using real-time PCR, we further demonstrated that bilateral microinjection of Ad-lepr(b) into the ARC restored low hypothalamic levels of lepr(b) mRNA to values approximating those of wild-type (Fa(k)/Fa(k)) controls. Restored leptin receptor expression in the ARC reduced both mean daily food intake (by 13%) and body weight gain (by 33%) and increased hypothalamic proopiomelanocortin mRNA by 65% while decreasing neuropeptide Y mRNA levels by 30%, relative to fa(k)/fa(k) rats injected with a control adenovirus (Ad-lacZ) (P < 0.05 for each comparison). In contrast, Ad-lepr(b) delivery to either the lateral hypothalamic area of fa(k)/fa(k) rats or to the ARC of wild-type Fa(k)/Fa(k) rats had no effect on any of these parameters. These findings collectively support the hypothesis that leptin receptor signaling in the ARC is sufficient to mediate major effects of leptin on long-term energy homeostasis. Adenoviral gene therapy is thus a viable strategy with which to study the physiological importance of specific molecules acting in discrete brain areas.

Chemical sympathectomy alters regulation of body weight during prolonged ICV leptin infusion

To assess the importance of the sympathetic nervous system in regulating body weight during prolonged leptin infusion, we evaluated food intake, body weight, and physical activity in conscious, unrestrained rats. Initial studies illustrated that prolonged intracerebroventricular (ICV) infusion of leptin enhanced substrate oxidation so that adipose tissue lipid stores were completely ablated, and muscle triglyceride and liver glycogen stores were depleted. After neonatal chemical sympathectomy, changes in weight and food intake were compared in groups of sympathectomized (SYM) and control (CON) adult animals during ICV infusion of leptin. CON animals lost 60 +/- 9 g over 10 days vs. 25 +/- 3 g in the SYM animals when food intake was matched between the two groups. Greater weight loss despite similar energy intake points to an important role of the sympathetic nervous system in stimulating energy expenditure during ICV leptin infusion by increasing the resting metabolic rate, since no differences in physical activity were observed between CON and SYM groups. In conclusion, activation of the SNS by leptin increases energy expenditure by augmenting the resting metabolic rate.

Leptin depresses food intake in great tits (Parus major)

Food availability for wild organisms typically varies both in time and space, requiring a mechanism that regulates the storage of excess energy and makes it possible to use stores during energy shortfall. Leptin, a protein hormone encoded by an obesity gene, has been suggested to be the signal mediator for this flux of energy. In a controlled laboratory experiment on caged great tits (Parus major) we evaluated the effect of leptin on food intake and behaviour. Experimental birds were given an intramuscular injection of 10 microg leptin dissolved in phosphate buffered saline (PBS), while the control birds were injected with PBS only at 09:00 h after a night's fasting. Within the first 20 min after injections we observed a significant difference in food intake between groups: control birds initially fed at higher rates compared to leptin treated birds. The cumulative food intake suggested that the effect of leptin disappeared after approximately 40-50 min post-injections. Similar results have previously been found in domesticated chickens. To our knowledge, this is the first study to show that leptin depresses food intake in wild birds.

Insulin and leptin revisited: adiposity signals with overlapping physiological and intracellular signaling capabilities

The adipocyte-derived hormone leptin and the pancreatic beta cell-derived hormone insulin each function as afferent signals to the hypothalamus in an endocrine feedback loop that regulates body adiposity. Although these two hormones, and the receptors on which they act, are unrelated and structurally distinct, they exert overlapping effects in the arcuate nucleus, a key hypothalamic area involved in energy homeostasis. Defects in either insulin or leptin signaling in the brain result in hyperphagia, disordered glucose homeostasis, and reproductive dysfunction. To explain this striking physiological overlap, we hypothesize that hypothalamic insulin and leptin signaling converge upon a single intracellular signal transduction pathway, known as the insulin-receptor-substrate phosphatidylinositol 3-kinase pathway. Here we synthesize data from a variety of model systems in which such "cross-talk" between insulin and leptin signal transduction has either been observed or can be inferred, discuss our own data demonstrating that insulin and leptin both activate hypothalamic phosphatidylinositol 3-kinase signaling, and discuss the significance of such convergence with respect to neuronal function in normal individuals and in pathological states such as obesity. Identification of the key early molecular events mediating the action of both insulin and leptin in hypothalamic neurons promises new insight into the regulation of these neurons in health and disease.

Does body mass play a role in the regulation of food intake?

It is widely believed that body fatness (and hence total body mass) is regulated by a lipostatic feedback system. This system is suggested to involve at least one peripheral signalling compound, which signals to the brain the current size of body fat stores. In the brain the level of the signal is compared with a desirable target level, and food intake and energy expenditure are then regulated to effect changes in the size of body fat stores. There is considerable support for this theory at several different levels of investigation. Patterns of body-mass change in subjects forced into energy imbalance seem to demonstrate homeostasis, and long-term changes in body mass are minor compared with the potential changes that might result from energy imbalance. Molecular studies of signalling compounds have suggested a putative lipostatic signal (leptin) and a complex network of downstream processing events in the brain, polymorphisms of which lead to disruption of body-mass regulation. This network of neuropeptides provides a rich seam of potential pharmaceutical targets for the control of obesity. Despite this consistent explanation for the observed phenomena at several different levels of enquiry, there are alternative explanations. In the present paper we explore the possibility that the existence of lipostatic regulation of body fatness is an illusion generated by the links between body mass and energy expenditure and responses to energy imbalance that are independent of body mass. Using computer-based models of temporal patterns in energy balance we show that common patterns of change in body mass following perturbation can be adequately explained by this 'non-lipostatic' model. This model has some important implications for the interpretations that we place on the molecular events in the brain, and ultimately in the search for pharmaceutical agents for alleviation of obesity.

An adipocentric view of signaling and intracellular trafficking

Adipocytes have traditionally been considered to be the primary site for whole body energy storage mainly in the form of triglycerides and fatty acids. This occurs through the ability of insulin to markedly stimulate both glucose uptake and lipogenesis. Conventional wisdom held that defects in fuel partitioning into adipocytes either because of increased adipose tissue mass and/or increased lipolysis and circulating free fatty acids resulted in dyslipidemia, obesity, insulin resistance and perhaps diabetes. However, it has become increasingly apparent that loss of adipose tissue (lipodystrophies) in both animal models and humans also leads to metabolic disorders that result in severe states of insulin resistance and potential diabetes. These apparently opposite functions can be resolved by the establishment of adipocytes not only as a fuel storage depot but also as a critical endocrine organ that secretes a variety of signaling molecules into the circulation. Although the molecular function of these adipocyte-derived signals are poorly understood, they play a central role in the maintenance of energy homeostasis by regulating insulin secretion, insulin action, glucose and lipid metabolism, energy balance, host defense and reproduction. The diversity of these secretory factors include enzymes (lipoprotein lipase (LPL) and adipsin), growth factors [vascular endothelial growth factor (VEGF)], cytokines (tumor necrosis factor-alpha, interleukin 6) and several other hormones involved in fatty acid and glucose metabolism (leptin, Acrp30, resistin and acylation stimulation protein). Despite the large number of molecules secreted by adipocytes, our understanding of the pathways and mechanisms controlling intracellular trafficking and exocytosis in adipocytes is poorly understood. In this article, we will review the current knowledge of the trafficking and secretion processes that take place in adipocytes, focusing our attention on two of the best characterized adipokine molecules (leptin and adiponectin) and on one of the most intensively studied regulated membrane proteins, the GLUT4 glucose transporter.

Effects of continuous lumbar intrathecal infusion of leptin in rats on weight regulation

Intracranial leptin alters food consumption and body weight. To systematically characterize the effects of extended continuous spinal intrathecal delivery on such regulation, female rats received continuous lumbar spinal infusion (14 days) through catheters connected to osmotic minipumps of a vehicle or one of several doses of recombinant murine leptin (0.03-10 microg/day). The following observations were made. (1) Leptin resulted in a dose-dependent suppression in body weight and food consumption at doses above 0.3 microg/day. (2) Food consumption was initially reduced. Weight fell for 7 days and then plateaued at a level proportional to dose. (3) The ratio of food consumed to body weight was constant for control animals across the study. Leptin-infused rats slowed the initial fall in weight by increasing food consumption, such that the food to body weight ratio returned to that of control values. Rats were thus regulating food consumption to sustain body weight as defined by leptin dose. (4) On day 14, cisternal cerebrospinal fluid was obtained and leptin measured. Concentrations covaried in a log linear fashion with infusion dose. Body weight and food consumption covaried similarly with cisternal leptin concentrations across dose groups. Our data suggest that steady state infusions of leptin induce a degree of appetite suppression that leads to a steady state level of body weight loss and not simply to a simple block of consumatory behavior. The unexpected potency of the observed effects of intrathecal leptin relative to doses that are required after i.c.v. delivery suggests that at least a portion of the effects of intrathecal leptin may reflect a medullary action. The observed correlation of cisternal leptin levels with the behavioral effects is also consistent with a reliable distribution of the infused leptin to target supraspinal sites.

Mutations in the human leptin and leptin receptor genes as models of serum leptin receptor regulation

A part of serum Ob leptin, an adipocyte-secreted peptide, is bound to a soluble Ob receptor (sObR). Immunoreactive sObR was measured in 125 lean or obese control subjects (group 1), 18 individuals with a mutation in the leptin gene impairing leptin secretion (group 2), and 10 individuals with a mutation in the ObR gene, leading to production of a truncated ObR not anchored to cell membranes (group 3). In group 1, sObR levels were negatively correlated with age and BMI in children and with BMI in adults. sObR levels were also negatively correlated with leptin levels. Leptin binding activity and sObR levels coeluted in gel-filtration chromatography. In group 2, sObR levels did not differ from those in lean control subjects and were not correlated with BMI. A single peak was detected in chromatographic fractions. In group 3, sObR levels were high and positively correlated with BMI. Immunoreactive sObR coeluted with leptin binding activity. These data demonstrate that leptin is not needed for ObR gene expression, and they suggest that leptin plays a role in receptor downregulation because sObR levels are negatively correlated with leptin levels and BMI in control subjects, whereas sObR levels are not depressed in obese leptin-deficient or leptin receptor-deficient individuals.

Seasonal changes in adiposity: the roles of the photoperiod, melatonin and other hormones, and sympathetic nervous system

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity-Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Daily CCK injection enhances reduction of body weight by chronic intracerebroventricular leptin infusion

In the present study, we tested the hypothesis that a single daily injection of the gut peptide CCK, together with continuous leptin infusion, would produce significantly greater loss of body weight than leptin alone. We found that a single daily intraperitoneal injection of CCK-8 (0.5 microg/kg) significantly enhanced the weight-reducing effects of 0.5 microg/day leptin infused continuously into the lateral ventricle of male Sprague-Dawley rats by osmotic minipump. However, CCK and leptin together did not enhance reduction of daily chow intake. Furthermore, there was no synergistic reduction of 30-min sucrose intake, although a significant main effect of both leptin and CCK was observed on sucrose intake. These results 1) confirm our previous reports of synergy between leptin and CCK on body weight, 2) demonstrate that enhancement of leptin-induced weight loss does not require bolus administration of leptin, and 3) suggest that enhanced body weight loss following leptin and CCK does not require synergistic reduction of food intake by leptin and CCK.

Leptin, gut, and food intake

Hyperphagia (overeating) is often associated with energy over-storage and obesity, which may lead to a myriad of serious health problems, including heart disease, hypertension, and type 2 diabetes. Thus, understanding the complex pathological mechanisms underlying hyperphagia and obesity has an important clinical significance. Leptin, or ob protein, is a key element in the long-term regulation of food intake and body weight homeostasis. It circulates in the blood at levels correlated with body fat mass. Leptin binds to specific receptors in the hypothalamus to mediate events that regulate feeding behavior. In light of new evidence, the initial view that leptin is an adipocyte-derived signal, which acts centrally to decrease body weight, has been modified. It has been shown that leptin may also have specific functions in the gastrointestinal tract, suggesting that feeding and energy homeostasis is regulated by both central and peripheral signals. Evidence supports the view that leptin integrates short-term, meal-related signals from the gut into long-term regulation of energy balance. In addition, the gastric leptin level is altered by the nutritional state and the administration of cholecystokinin. This commentary aims to review the evidence of the role of leptin as a peripherally acting signal in the gut in the regulation of nutrient intake, adiposity, and body weight. Based on currently available data, some potential future studies are suggested.

Leptin and the treatment of obesity: its current status

Leptin, the protein product of the ob gene, is primarily an adipocyte-secreted hormone, whose functional significance is rapidly expanding. Although early research efforts were focused on defining leptin's role in reversing obesity in rodents, there is now substantial evidence indicating that its influence extends to several hypothalamic-pituitary-endocrine axes, including gonadal, adrenal, thyroid, growth hormone, and pancreatic islets. A role for leptin in hematopoiesis, angiogenesis, immune function, osteogenesis, and wound healing has also been documented. The results of recent clinical trials with recombinant human leptin indicated that its effectiveness in restoring energy balance and correcting obesity-related endocrinopathies in genetically obese rodent models extended only partially to the management of human obesity. New efforts in drug development have focused on leptin-related synthetic peptide agonists as potential anti-obesity pharmacophores.

Hyperleptinemia in A(y)/a mice upregulates arcuate cocaine- and amphetamine-regulated transcript expression

The effects of leptin on cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AGRP) expression in the hypothalamic arcuate nucleus of obese A(y)/a mice were investigated. CART mRNA expression was upregulated by 41% and AGRP mRNA downregulated by 78% in hyperleptinemic A(y)/a mice relative to levels in lean a/a mice. The mRNA expression of these neuropeptides in either young nonobese A(y)/a mice or rats treated with SHU-9119, a synthetic melanocortin-4 receptor (MC4R) antagonist, did not differ significantly from that in the corresponding controls. After a 72-h fast, which decreased the concentration of serum leptin, CART and AGRP mRNA expression decreased and increased, respectively, in A(y)/a mice. The expression levels of these neuropeptides in leptin-deficient A(y)/a ob/ob double mutants were comparable to those in a/a ob/ob mice. Leptin thus modulates both CART and AGRP mRNA expression in obese A(y)/a mice, whereas leptin signals are blocked at the MCR4R level. Taken together, the present findings indicate that differential expression of these neuropeptides in A(y)/a and ob/ob mice results in dissimilar progression toward obesity.

Higher serum leptin level in women than in men with type 1 diabetes

Leptin is the protein product of the obese (ob) gene, a lipostatic hormone that contributes to body weight regulation through suppressing appetite and/or stimulating energy expenditure in humans and/or rodents. In humans, serum leptin concentrations are increased in relation to increased body fat content. Studies have shown a higher leptin level in women compared with men. However, the gender influence on serum leptin concentrations has never been evaluated in patients with type 1 diabetes. In this study, serum leptin levels and percentage body fat mass were measured in men and women with type 1 diabetes. Fasting serum leptin levels were higher in women (16.7 + 11.6 ng/mL) than in men (3.0 +/- 1.5 ng/mL; P < 0.05) and were independent of exogenous insulin intake and of glucose control. Percentage body fat and fat mass were significant determinants of leptin concentration, whereas age and duration of diabetes were not related to leptin concentration. Subgroups of men (n = 12) and women (n = 11) with total body fat between 20 and 30% were compared. Leptin levels were also higher in women compared with men (13.5 +/- 8.3 ng/mL versus 3.2 +/- 1.7 ng/mL; P < 0.05, respectively). In conclusion, our findings indicate that gender is an important determinant of serum leptin concentration in type 1 diabetics, this gender difference is partly explained by body fat distribution and that type 1 diabetic women may be more resistant than type 1 diabetic men to leptin's alleged lipostatic actions.

Leptin and reproduction

Since the cloning of leptin by Friedman's laboratory in 1994, over 3000 papers have been published on leptin, making it one of the most active research areas in all of science. Leptin appears to be a pleiotrophic hormone affecting many different tissues in the body. This review focuses on the role of leptin in reproduction. Evidence is accumulating that leptin potentially has roles in the regulation of GnRH and LH secretion, puberty, pregnancy, and lactation. Reciprocal regulation of leptin and its receptors by gonadal hormones and the implications and controversies thereof are also discussed in the review. Signaling pathways utilized by leptin are starting to become more clear, particularly JAK/STAT, MAPK, and SOCS3 have been implicated as mediators/modulators of leptin effects at the cellular level. At the hypothalamic level, there is also evidence that CART (cocaine and amphetamine-related transcript) is involved as a downstream mediator of leptin effects, especially with regards to control of GnRH secretion. While leptin clearly has many effects upon the reproductive axis, defining its precise roles is not without controversies. This review presents both pro and con findings, thereby demarking controversial areas that undoubtedly will be fertile ground for future investigation.

Correlation between plasma leptin concentration and body fat content in dogs

OBJECTIVE

To evaluate the relationship between plasma leptin concentration and body fat content in dogs.

ANIMALS

20 spayed female Beagles that were 10 months old at the start of the experiment.

PROCEDURE

Dogs were kept under regulated feeding and exercise conditions for 21 weeks, resulting in a wide range of body weights, body condition scores (BCS), and subcutaneous thicknesses. Plasma leptin concentration was measured by use of a canine leptin-specific ELISA test to evaluate its correlation to body fat content estimated by the deuterium oxide dilution method. Plasma concentrations of glucose, cholesterol, triglycerides (TG), and nonesterified fatty acids (NEFA) were also measured.

RESULTS

Body fat content (9 to 60% of body weight) was positively and closely correlated (r = 0.920; n = 20; P < 0.001) to plasma leptin concentration (0.67 to 8.06 ng/ml), compared with other variables (ie, glucose, cholesterol, TG, and NEFA; r = 0.142, 0.412, 0.074, and 0.182, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

The positive relationship between plasma leptin concentration and body fat content in dogs was similar to correlations reported for humans and rodents, suggesting that plasma leptin is a quantitative marker of adiposity in dogs.

Plasma leptin response to an epinephrine infusion in lean and obese women

OBJECTIVE

Because leptin production by adipose tissue is under hormonal control, we examined the impact of epinephrine administration on plasma leptin concentrations.

RESEARCH METHODS AND PROCEDURES

We measured plasma leptin, insulin, and free fatty acid (FFA) responses after a 60-minute epinephrine infusion (0.010 microg/kg fat free mass/min) followed by a 30-minute recovery period (no infusion) in a group of 11 lean (mean body mass index +/- SD: 22.6 +/- 1.1 kg/m(2)) and 15 obese (30.0 +/- 1.3 kg/m(2)) premenopausal women. Leptin, insulin, and FFA levels were measured in plasma before (-15 and 0 minutes) and at every 30 minutes over the 90-minute period.

RESULTS

In both lean and obese individuals, plasma leptin was significantly reduced by epinephrine (p < 0.0001). Body fat mass was associated with fasting leptin levels (r = 0.64, p < 0.0005) as well as with the decrease in leptinemia (r = -0.51, p < 0.01) produced by epinephrine administration. Furthermore, we noted a large range of leptin response to epinephrine among our subjects, especially in obese women (from -12 to -570 ng/mL per 60 minutes). However, there was no association between postepinephrine leptin and FFA levels (r = -0.14, p = 0.55).

DISCUSSION

Results of this study indicate that leptin levels decrease after epinephrine administration in both lean and obese premenopausal women. However, the heterogeneity in the response of leptin to catecholamines suggests potential alterations of the leptin axis that may contribute to generate a positive energy balance and, thus, may favor weight gain in some obese individuals.

Overfeeding rapidly induces leptin and insulin resistance

In common forms of obesity, hyperphagia, hyperinsulinemia, and hyperleptinemia coexist. Here, we demonstrate rapid induction of insulin and leptin resistance by short-term overfeeding. After 3 and 7 days on the assigned diet regimen, rats were tested for their biological responses to acute elevations in plasma insulin and leptin concentrations. Severe resistance to the metabolic effects of both leptin and insulin ensued after just 3 days of overfeeding. During the insulin clamp studies, glucose production was decreased by approximately 70% in control rats and 28-53% in overfed rats. Similarly, leptin infusion doubled the contribution of gluconeogenesis to glucose output in control rats but failed to modify gluconeogenesis in overfed animals. These findings demonstrate a paradoxical and rapid collapse of the leptin system in response to nutrient excess. This partial failure is tightly coupled with the onset of insulin resistance.

Increased leptin concentrations correlate with increased concentrations of inflammatory markers in morbidly obese individuals

OBJECTIVE

To study whether an increase of plasma leptin concentrations, as observed in the case of increased body weight, is associated with an inflammatory state.

SUBJECTS

Sixty-three healthy subjects with body mass index (BMI) ranging from 20 to 61 kg/m2.

MEASUREMENTS

Plasma concentrations of leptin, the inflammatory parameter soluble TNF-alpha receptors (TNFR55 and TNFR75), the acute phase proteins lipopolysaccharide binding protein (LBP), serum amyloid A (SAA), alpha-acid glycoprotein (AGP), C-reactive protein (CRP), plasminogen activator inhibitor-1 (PAI-1) and the anti-inflammatory soluble Interleukin-1 decoy receptor (sIL-1RII) were measured.

RESULTS

As expected, BMI correlated significantly with leptin (r=0.823, P<0.001), but also with all acute phase proteins, both soluble TNF receptors and PAI concentrations. After correction for BMI and sex, no significant correlation between leptin and the acute phase proteins was seen. Interestingly, however, leptin strongly correlated with both TNF receptors (r=0.523, P<0.001 for TNFR55 and r=0.438, P<0.001 for TNFR75).

CONCLUSIONS

This study shows the development of a pro-inflammatory state with increasing body weight. The BMI independent relationship between leptin and both soluble TNF-receptors is consistent with a regulatory role for leptin in the inflammatory state in morbidly obese subjects.

Mouse feeding behavior: ethology, regulatory mechanisms and utility for mutant phenotyping

Ingestive behaviors, feeding and drinking, constitute unconditioned, obligatory functions that are tightly regulated in the rodent according to demands of the external and internal milieu. Dependent measures of food intake have been used extensively in rats to infer the identity and function of neurochemical pathways, which mediate energy balance. A recent interest in application of appetitive measures in mice can be attributed jointly to the discovery of novel markers of energy balance in genetically obese mice as well as systematic targeting of known feeding regulatory pathways in bioengineered mutant mice. Accordingly, this review will attempt to provide the reader interested in behavioral phenotyping of knockout or transgenic mice with information regarding the ethology of mouse eating behavior, known mechanisms of appetitive regulation and examples of successes and pitfalls encountered when studying food intake in mutant mice.

Leptin attenuates follicular apoptosis and accelerates the onset of puberty in immature rats

Human and rat granulosa cells express receptors to leptin which synergies with glucocorticoid hormones in stimulation of ovarian steroidogenesis. To examine whether leptin affects follicular development and maturation, we injected recombinant ovine leptin (300 ng-10 microg/animal) daily to immature 21 day-old female rats. Non-treated rats reached puberty at 44.5+/-1.6 (n=9) days. In contrast, in leptin treated animals, puberty was reached at 34.5+/-1.6 (n=9) days. Ovarian sections revealed hypertrophy of granulosa cells in leptin treated animals. Moreover, the number of ovulations was 2-fold higher in the treated animals compared to controls (3-4 ovulations versus 7-8 on the first three estrous cycles, P<0.001). Leptin dramatically reduced incidence of follicular apoptosis measured by TUNEL, and was already evident after 7 days of leptin injection (12% of apoptosis in leptin treated group compared to 52% in controls, P<0.001). Maximal protection against apoptosis was achieved at 1-3 microg leptin/animal. The levels of FSH, LH, progesterone and the steroidogenic factors ADX and STAR were elevated earlier in development in the leptin treated animals compared to control animals which is in line with the achievement of early puberty in the leptin treated animals compared to non treated ones. To reveal whether modulation of death and survival genes is involved in leptin attenuation of follicular apoptosis, we examined the expression of the survival gene Bcl-2 and the death gene Bax in Western blots of ovarian homogenates. There was a pronounced elevation in Bcl-2 expression during 7-14 days of leptin injections up to 16.3-fold (P<0.001) compared to Bcl-2 expression in controls. Bax expression was elevated only 3.4 fold (P<0.001), leading to an increase in the Bcl-2/Bax ratio of 4.7 fold (P<0.001). Expression of the tumor suppressor gene p 53 and the oncogene Mdm2 did not change significantly. Our data suggests that leptin may be involved in accelerating follicular maturation by attenuating follicular atresia and increasing the ratio of Bcl-2/Bax.