Leptin levels in humans are acutely suppressed by isoproterenol despite acipimox-induced inhibition of lipolysis, but not by free fatty acids

Effect of ambrex (a herbal formulation) on oxidative stress in hyperlipidemic rats and differentiation of 3T3-L1 preadipocytes

Background:

Ambrex is a polyherbal formulation which consists of Withania somnifera, Orchis mascula, Cycas circirnalis, Shorea robusta with amber.

Objective:

The present study was designed to explore the potential effects of ambrex on the antioxidant status in high fat diet fed rats and to investigate the possible mechanisms focusing on the gene expression involved in adipogenesis and inflammation in 3T3-L1 cell line.

Materials and Methods:

Male Wistar rats were divided into four groups (n = 6); Group A received normal diet, Group B received high fat diet for 30 days, Group C and D received high fat diet for 30 days and treated with ambrex (40 mg/kg b.w) and atorvastatin (10 mg/kg b.w) for successive 15 days respectively. This study also assesses the effect of ambrex on adipogenesis in 3T3-L1 adipocytes.

Results:

The serum total cholesterol and triglycerides were significantly decreased in ambrex treated hyperlipidemic animals when compared to untreated animals. The activities of catalase, superoxide dismutase and reduced glutathione were significantly augmented in the serum, liver, and heart of hyperlipidemic rats treated with ambrex when compared to control. Ambrex treated rats had significant reductions in malondiadehyde levels in the serum, liver and heart compared to untreated rats. In addition, we observed that treatment with ambrex resulted in a major inhibition of pre-adipocyte differentiation of 3T3-L1 cells in vitro by suppression of peroxisome proliferator activated receptor gamma, sterol regulatory binding proteins, tumor necrosis factor-α, inducible nitricoxide synthase, leptin, and upregulation of thioredoxin 1 (TRX1) and TRX2 mRNA expression.

Conclusion:

Therefore, ambrex may be a potential drug for treatment of hyperlipidemia and related disorders.

Cessation of physical exercise changes metabolism and modifies the adipocyte cellularity of the periepididymal white adipose tissue in rats

All of the adaptations acquired through physical training are reversible with inactivity. Although significant reductions in maximal oxygen uptake (Vo2max) can be observed within 2 to 4 wk of detraining, the consequences of detraining on the physiology of adipose tissue are poorly known. Our aim was therefore to investigate the effects of discontinuing training (physical detraining) on the metabolism and adipocyte cellularity of rat periepididymal (PE) adipose tissue. Male Wistar rats, aged 6 wk, were divided into three groups and studied for 12 wk under the following conditions: 1) trained (T) throughout the period; 2) detrained (D), trained during the first 8 wk and detrained during the remaining 4 wk; and 3) age-matched sedentary (S). Training consisted of treadmill running sessions (1 h/day, 5 days/wk, 50-60% Vo2max). The PE adipocyte size analysis revealed significant differences between the groups. The adipocyte cross-sectional area (in μm(2)) was significantly larger in D than in the T and S groups (3,474 ± 68.8; 1,945.7 ± 45.6; 2,492.4 ± 49.08, respectively, P < 0.05). Compared with T, the isolated adipose cells (of the D rats) showed a 48% increase in the ability to perform lipogenesis (both basal and maximally insulin-stimulated) and isoproterenol-stimulated lipolysis. No changes were observed with respect to unstimulated lipolysis. A 15% reduction in the proportion of apoptotic adipocytes was observed in groups T and D compared with group S. The gene expression levels of adiponectin and PPAR-gamma were upregulated by factors of 3 and 2 in D vs. S, respectively. PREF-1 gene expression was 3-fold higher in T vs. S. From these results, we hypothesize that adipogenesis was stimulated in group D and accompanied by significant adipocyte hypertrophy and an increase in the lipogenic capacity of the adipocytes. The occurrence of apoptotic nuclei in PE fat cells was reduced in the D and T rats; these results raise the possibility that the adipose tissue changes after detraining are obesogenic.

A higher response of plasma neuropeptide Y, growth hormone, leptin levels and extracellular glycerol levels in subcutaneous abdominal adipose tissue to Acipimox during exercise in patients with bulimia nervosa: single-blind, randomized, microdialysis study

Background

Neuropeptide Y (NPY) is an important central orexigenic hormone predominantly produced by the hypothalamus, and recently found to be secreted in adipose tissue (AT). Acipimox (Aci) inhibits lipolysis in AT and reduces plasma glycerol and free fatty acid (FFA) levels. Exercise and Aci are enhancers of growth hormone (GH) and NPY secretion and exercise may alter leptin levels. We expect to find abnormal neuropeptidergic response in plasma and AT in patients with bulimia nervosa (BN). We hypothesize that Aci influences these peptides via a FFA-independent mechanism and that Aci inhibits lipolysis through a cyclic adenosine monophosphate (cAMP)-dependent pathway. Dysregulations of the AT-brain axis peptides might be involved in binge eating as is the case in BN.

Methods

The objective of this study was to determine the responses of plasma NPY, GH, leptin, FFA and glycerol levels to exercise in BN patients and healthy women (C) given the anti-lipolytic drug Aci or placebo. The secondary objective of this study was to compare the responses of extracellular glycerol levels and plasma glycerol levels to exercise alone or together with Aci administration in BN patients and C women. Extracellular glycerol was measured in vivo in subcutaneous (sc) abdominal AT using microdialysis. Eight BN and eight C women were recruited for this single-blind, randomized study. Aci or placebo was given 1 hour before the exercise (45 min, 2 W/kg of lean body mass [LBM]). NPY, GH, leptin, FFA, glycerol plasma and AT glycerol levels were measured using commercial kits.

Results

The primary outcome of this study was that the exercise with Aci administration resulted in plasma NPY and GH increase (after a 45-minute exercise) and leptin (after a 90-minute post-exercise recovering phase) increased more in BN patients. The secondary outcomes of this study were that the exercise with Aci administration induced a higher decrease of extracellular glycerol in BN patients compared to the C group, while the exercise induced a higher increase of glycerol concentrations in sc abdominal AT of BN patients. Plasma glycerol levels decreased more in BN patients and plasma FFA levels were depressed in both groups after the exercise with Aci administration. The exercise induced similar increases in plasma NPY, GH, FFA and glycerol levels, and a similar decrease in the plasma leptin level in both groups.

Conclusions

We confirm the results of a single-blind, randomized, microdialysis study, i.e. that the Aci-induced elevation in plasma NPY and GH levels during the exercise is higher in BN patients and that Aci increased plasma leptin levels in the post-exercise recovering phase (90-minute) more in BN patients. The post-exercise rise (45-minute) in AT glycerol is much more attenuated by acute Aci treatment in BN patients. Simultaneously, we found facilitated turnover of plasma glycerol after the exercise together with Aci administration in BN. Our results support the hypotheses that Aci exerts an effect on the FFA-independent and cAMP-dependent mechanism.

Trial Registration

Australia and New Zealand Clinical Trials Register (ANZCTR): ACTRN12611000955910

CD36-facilitated fatty acid uptake inhibits leptin production and signaling in adipose tissue

Leptin plays an important role in regulating energy expenditure in response to food intake, but nutrient regulation of leptin is incompletely understood. In this study using in vivo and in vitro approaches, we examined the role of fatty acid uptake in modulating leptin expression and production. Leptin levels are doubled in the CD36-null mouse, which has impaired cellular fatty acid uptake despite a 40% decrease in fat mass. The CD36-null mouse is protected from diet-induced weight gain but not from that consequent to leptin deficiency. Leptin secretion in the CD36-null mouse is strongly responsive to glucose intake, whereas a blunted response is observed in the wild-type mouse. This indicates that leptin regulation integrates opposing influences from glucose and fatty acid and loss of fatty acid inhibition allows unsuppressed stimulation by glucose/insulin. Fatty acid inhibition of basal and insulin-stimulated leptin release is linked to CD36-facilitated fatty acid flux, which is important for fatty acid activation of peroxisome proliferator-activated receptor gamma and likely contributes to the nutrient sensing function of adipocytes. Fatty acid uptake also may modulate adipocyte leptin signaling. The ratio of phosphorylated to unphosphorylated signal transducer and activator of transcription 3, an index of leptin activity, is increased in CD36-null fat tissue disproportionately to leptin levels. In addition, expression of leptin-sensitive fatty acid oxidative enzymes is enhanced. Targeting adipocyte CD36 may offer a way to uncouple leptin production and adiposity.

Genistein restricts leptin secretion from rat adipocytes

The isoflavones--genistein and daidzein -- compounds found in high concentrations in soy play an important role in prevention of many diseases and affect some metabolic pathways. In the performed experiment it was demonstrated that genistein (5mg/kg b.w.) administered intragastrically for three days to male Wistar rats substantially diminished blood leptin level. Studies with isolated rat adipocytes revealed that this phytoestrogen strongly restricted leptin secretion from these cells. These effects were not accompanied by any changes in leptin gene expression in adipocytes. Daidzein-- an analogue of genistein -- used at similar concentrations did not affect blood leptin concentration, leptin secretion and expression of its gene. To determine the influence of genistein and daidzein on leptin release, adipocytes isolated from the epididymal fat tissue were incubated for 2h in Krebs--Ringer buffer. Leptin secretion stimulated by glucose with insulin was significantly diminished by genistein (0.25--1mM). This effect of genistein may arise from several aspects of its action in adipocytes documented in the literature such as the inhibition of glucose transport and metabolism, the attenuation of insulin signalling, the inhibition of cAMP phosphodiesterase and the stimulation of lipolysis. However, the bypassing of the restrictive action of genistein on glucose transport and glycolysis (by the use of alanine instead of glucose) and on insulin action (by the use of nicotinic acid) was not sufficient to restore leptin secretion from isolated adipocytes. It was also demonstrated that the restriction of the stimulatory influence of genistein on cAMP/protein kinase A (PKA) pathway (by the inhibition of PKA activity) did not improve leptin release. Results obtained in our experiments point at the restriction of glucose metabolism following formation of pyruvate as the pivotal reason of the inhibitory action of genistein on leptin release.

In situ lipolytic regulation in subjects born small for gestational age

OBJECTIVE

Subjects born small for gestational age (SGA) who are prone to develop insulin resistance in adulthood display an abnormal development pattern of the adipose tissue during fetal and postnatal life. Since the lipolytic activity of the adipose tissue is critical in the development of insulin resistance, the purpose of this study was to investigate whether SGA itself might affect lipolysis regulation.

STUDY DESIGN

We studied the effect of catecholamines, by local injection of isoproterenol, and the effect of insulin, using two-step infusion at 8 and 40 mU/m2/min, on the in situ lipolysis of the subcutaneous abdominal adipose tissue of 23 subjects born SGA and 23 born appropriate for gestational age (AGA), using the microdialysis technique.

RESULTS

Under isoproterenol infusion, the increase in dialysate glycerol concentration was significantly 1.5-fold higher in the SGA than in the AGA group (P=0.02) and induced a 20% increase in the plasma FFA concentration (P=0.04), whereas no significant increase was observed in the AGA group. The antilipolytic action of insulin on dialysate glycerol concentration was similar in both groups throughout the insulin infusion.

CONCLUSION

Subjects born SGA demonstrated a hyperlipolytic reactivity to catecholamines, which might be regarded as an additional deleterious component of the insulin resistance associated with SGA. In contrast, being born SGA does not directly affect the antilipolytic action of insulin, showing that it does not play a major role in causing the long-term metabolic complications associated with reduced fetal growth.

Regulation of leptin secretion from white adipocytes by free fatty acids

Norepinephrine stimulates lipolysis and concurrently inhibits insulin-stimulated leptin secretion from white adipocytes. To assess whether there is a cause-effect relationship between these two metabolic events, the effects of fatty acids were investigated in isolated rat adipocytes incubated in buffer containing low (0.1%) and high (4%) albumin concentrations. Palmitic acid (1 mM) mimicked the inhibitory effects of norepinephrine (1 microM) on insulin (10 nM)-stimulated leptin secretion, but only at low albumin concentrations. Studies investigating the effects of the chain length of saturated fatty acids [from butyric (C4) to stearic (C18) acids] revealed that only fatty acids with a chain length superior or equal to eight carbons effectively inhibited insulin-stimulated leptin secretion. Long-chain mono- and polyunsaturated fatty acids constitutively present in adipocyte triglyceride stores (oleic, linoleic, gamma-linolenic, palmitoleic, eicosapentanoic, and docosahexanoic acids) also completely suppressed leptin secretion. Saturated and unsaturated fatty acids inhibited insulin-stimulated leptin secretion with the same potency and without any significant effect on basal secretion. On the other hand, inhibitors of mitochondrial fatty acid oxidation (palmoxirate, 2-bromopalmitate, 2-bromocaproate) attenuated the stimulatory effects of insulin on leptin release without reversing the effects of fatty acids or norepinephrine, suggesting that fatty acids do not need to be oxidized by the mitochondria to inhibit leptin release. These results demonstrate that long-chain fatty acids mimic the effects of norepinephrine on leptin secretion and suggest that they may play a regulatory role as messengers between stimulation of lipolysis by norepinephrine and inhibition of leptin secretion.

Leptin: a review of its peripheral actions and interactions

Following the discovery of leptin in 1994, the scientific and clinical communities have held great hope that manipulation of the leptin axis may lead to the successful treatment of obesity. This hope is not yet dashed; however the role of the leptin axis is now being shown to be ever more complex than was first envisaged. It is now well established that leptin interacts with pathways in the central nervous system and through direct peripheral mechanisms. In this review, we consider the tissues in which leptin is synthesized and the mechanisms which mediate leptin synthesis, the structure of leptin and the knowledge gained from cloning leptin genes in aiding our understanding of the role of leptin in the periphery. The discoveries of expression of leptin receptor isotypes in a wide range of tissues in the body have encouraged investigation of leptin interactions in the periphery. Many of these interactions appear to be direct, however many are also centrally mediated. Discovery of the relative importance of the centrally mediated and peripheral interactions of leptin under different physiological states and the variations between species is beginning to show the complexity of the leptin axis. Leptin appears to have a range of roles as a growth factor in a range of cell types: as be a mediator of energy expenditure; as a permissive factor for puberty; as a signal of metabolic status and modulation between the foetus and the maternal metabolism; and perhaps importantly in all of these interactions, to also interact with other hormonal mediators and regulators of energy status and metabolism such as insulin, glucagon, the insulin-like growth factors, growth hormone and glucocorticoids. Surely, more interactions are yet to be discovered. Leptin appears to act as an endocrine and a paracrine factor and perhaps also as an autocrine factor. Although the complexity of the leptin axis indicates that it is unlikely that effective treatments for obesity will be simply derived, our improving knowledge and understanding of these complex interactions may point the way to the underlying physiology which predisposes some individuals to apparently unregulated weight gain.

Intracellular fatty acid downregulates ob gene expression in 3T3-L1 adipocytes

The effect of intracellular free fatty acid (FFA) accumulation on ob gene expression in adipocytes was examined. In fully differentiated 3T3-L1 adipocytes, triacsin C, a specific acyl CoA synthetase inhibitor with a K(i) of 8.97 microM, inhibited ob gene expression by 20% at 5 x 10(-5)M. At this concentration, triacsin C induced accumulation of intracellular FFA. Treatment with both chylomicron and triacsin C reduced ob gene expression more than treatment with triacsin C alone. Treatment with 2-bromopalmitate, a poorly metabolizable palmitate analog, reduced ob gene expression by 50% at 10(-4)M, but palmitate at the same concentration had no effect. This is the first demonstration that the ob gene is downregulated by intracellular FFA accumulation, thereby raising the possibility that ob product is regulated in response to lipolysis.

Decrease in serum leptin after prolonged physical activity in men

PURPOSE

This study was designed to determine whether serum leptin levels were affected by a 5-d military course after 3 wk of combat training.

METHODS

26 male soldiers (mean age = 21 +/- 2 yr) were examined at the beginning of the training program and just at the end of the 5-d course. The combination of continuous heavy physical activity and sleep deprivation led to energy deficiency. Blood samples were analyzed for serum leptin, insulin, cortisol, adrenocorticotropin (ACTH), and testosterone; plasma was analyzed for free fatty acids (FFA), glycerol, glucose, and catecholamines.

RESULTS

At the end of the 5-d course, there was a significant reduction in serum leptin (0.40 +/- 0.04 ng x mL(-1) versus 1.47 +/- 0.14 ng x mL(-1), < 0.001), i.e., a mean decrease of 67.00 +/- 3.75%. Plasma norepinephrine and dopamine rose significantly from 296 +/- 17 ng x L(-1) to 672 +/- 48 ng x L(-1) and 23 +/- 3 ng x L(-1) to 40 +/- 5 ng x L(-1) ( < 0.001 and < 0.01, respectively), whereas epinephrine remained unchanged. Serum concentrations of the anabolic hormone, insulin, fell from 31.17 +/- 3.03 microU x mL(-1) to 17.79 +/- 1.58 microU x mL(-1) ( < 0.001), whereas plasma FFA and glycerol were increased ( < 0.001, < 0.05, respectively). A statistically significant correlation appeared between the changes in leptin and insulin (r = 0.5306, < 0.01). Serum testosterone decreased significantly ( < 0.001), whereas serum cortisol, ACTH, and plasma glucose were unchanged at the end of the course. The training program had no significant effect on mean body mass index.

CONCLUSION

A 4-wk strenuous military training program, which induced an energy deficiency, reduced serum leptin to a third of normal levels. The decrease in serum leptin was attributed to the exercise-induced elevation in catecholamines and hypoinsulinemia.

Hormonal regulation of interleukin-6 production in human adipocytes

OBJECTIVE

To elucidate the hormonal regulation of interleukin-6 (IL-6) production by human adipose tissue and its relation to leptin.

DESIGN

In vitro study. Human adipocytes were incubated with dexamethasone (with or without RU486), norepinephrine and epinephrine (with or without propranolol), or insulin.

MEASUREMENTS

IL-6 and leptin secretion by human adipocytes.

RESULTS

A gradual increase in IL-6 secretion by adipocytes during differentiation was observed. A positive correlation was found between basal IL-6 release and both glycerol 3-phosphate dehydrogenase activity--a marker of adipocyte differentiation-and leptin release. Dexamethasone decreased IL-6 secretion and increased leptin secretion in a dose-dependent manner. Both catecholamines increased IL-6 and leptin secretion. The effects of dexamethasone and catecholamines on IL-6 and leptin were abrogated by RU486 and propranolol, respectively. Incubation with insulin resulted in a dose-dependent stimulation of IL-6 and leptin secretion.

CONCLUSIONS

IL-6 is produced by human adipocytes and is a potential marker of adipocyte differentiation. Furthermore it is a hormonally regulated cytokine, suppressed by glucocorticoids, and stimulated by catecholamines and insulin in physiological concentrations.

Mechanisms of leptin secretion from white adipocytes

The mechanisms regulating leptin secretion were investigated in isolated rat white adipocytes. Insulin (1-100 nM) linearly stimulated leptin secretion from incubated adipocytes for at least 2 h. The adrenergic agonists norepinephrine, isoproterenol (two nonselective beta-agonists), or CL-316243 (potent beta3) all inhibited insulin (10 nM)-stimulated leptin release. The inhibitory effects of norepinephrine and isoproterenol could be reversed not only by the nonselective antagonist propranolol but also by the selective antagonists ICI-89406 (beta1) or ICI-118551 (beta2), the beta2-antagonist being less effective than the beta1. Insulin-stimulated leptin secretion could also be inhibited by a series of agents increasing intracellular cAMP levels, such as lipolytic hormones (ACTH and thyrotropin-stimulating hormone), various nonhydrolyzable cAMP analogs, pertussis toxin, forskolin, methylxanthines (caffeine, theophylline, IBMX), and specific inhibitors of phosphodiesterase III (imazodan, milrinone, and amrinone). Significantly, antilipolytic agents other than insulin (adenosine, nicotinic acid, acipimox, and orthovanadate) did not mimic the acute stimulatory effects of insulin on leptin secretion under these conditions. We conclude that norepinephrine specifically inhibits insulin-stimulated leptin secretion not only via the low-affinity beta3-adrenoceptors but also via the high-affinity beta1/beta2-adrenoceptors. Moreover, it is suggested that 1) activation of phosphodiesterase III by insulin represents an important metabolic step in stimulation of leptin secretion, and 2) lipolytic hormones competitively counterregulate the stimulatory effects of insulin by activating the adenylate cyclase system.

Octanoate attenuates adipogenesis in 3T3-L1 preadipocytes

Preadipocytes exposed to octanoate accumulate less lipid than cells exposed to long-chain fatty acids. This effect of octanoate involves significant attenuation of expression of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma, steroid regulatory binding element protein (SREBP)-1c and CCAAT element binding protein (C/EBPalpha) at both the mRNA and protein levels. Expression of differentiation markers, including adipocyte fatty acid binding protein (ALBP), glycerol-3-phosphate dehydrogenase (GPDH) and leptin, was also significantly diminished by octanoate. However, octanoate did not prevent the decrease in preadipocyte factor-1 (Pref-1) expression that occurs during adipogenesis, nor did it inhibit the early induction of C/EBPbeta,delta. Treatment with synthetic PPARgamma ligands partially offset the inhibitory effect of octanoate on differentiation. Ectopic expression of PPARgamma2 in 3T3-L1 cells partially restored lipid accretion and GPDH activity in octanoate-treated cells. Adding octanoate together with troglitazone attenuated the effects of troglitazone on adipocyte differentiation in both normal 3T3-L1 cells and engineered 3T3-L1 cells that expressed ectopic PPARgamma2, implying that octanoate might compete against troglitazone for its binding to PPARgamma. These results suggest that octanoate may block adipogenesis at least in part by its influence on the expression/activation of PPARgamma.

Regulation of leptin production

Fat mass is the primary determinant of serum leptin in humans with energy intake and gender also having significant effects. Gender influences leptin production through the reproductive hormones. Glucose metabolism links food intake to leptin production and hexosamine biosynthesis appears to play a significant role in this process. Catecholamines inhibit leptin production and the sympathetic nervous system has been proposed to be the efferent arm of the leptin signal transduction pathway between adipose tissue and the central nervous system. Additional regulators of leptin production include glucocorticoids, cytokines and agonists of PPAR gamma. In addition to adipose tissue, leptin is produced in several other places including placenta, bone marrow, stomach, muscle and perhaps brain, thus increasing the number of potential regulatory roles for this hormone. Future work will be needed to fully elucidate the mechanisms regulating leptin synthesis/release in each tissue as well as its regulatory functions.

Effects of acipimox on the lipolysis rate in subcutaneous adipose tissue of obese subjects

BACKGROUND

Acipimox is a hypolipidaemic agent reducing serum concentrations of triglycerides and non-esterified fatty acids. Acipimox may reduce triglyceride synthesis by decreasing non-esterified fatty acid availability from adipocytes, but this effect has yet to be demonstrated in vivo. Lipolysis after acipimox treatment was examined in subcutaneous adipose tissue of severely obese subjects with associated metabolic disorders.

METHODS

The microdialysis technique was performed in abdominal subcutaneous adipose tissue of eight hyperinsulinaemic subjects. After oral treatment with acipimox, glycerol concentration was determined as an index of lipolysis rate. Blood flow was assessed by the ethanol escape technique. The rates of release of glycerol from human adipose tissue maximally stimulated by norepinephrine were also investigated in the presence of acipimox. Eight weight- and age-matched subjects served as a control group.

RESULTS

Under acipimox treatment, basal glycerol release decreased in subcutaneous adipose tissue, whereas no effect was observed on blood flow. In stimulated adipose tissue acipimox showed no effect.

CONCLUSION

In the present study basal glycerol outflow from adipose tissue was inhibited by acipimox. The anti-lipolytic action of the agent may diminish elevated plasma concentrations of free fatty acids in subjects with severe obesity.

Acute stimulation of leptin concentrations in humans during hyperglycemic hyperinsulinemia. Influence of free fatty acids and fasting

OBJECTIVE

To assess the acute regulation of leptin concentrations by insulin, glucose and free fatty acids (FFAs).

DESIGN

Four protocols: saline control experiment (CON); hyperglycemic clamps (approximately 8.3 mmol/l, 120 min) after an overnight fast (12 FAST); after a 36 h fast (36 FAST); and after a 36 h fast during which Intralipid/heparin was given over the last 24 h (36 FAST+FFA).

SUBJECTS

Lean, young, healthy volunteers; control group (n=6), experimental group (n=6).

MEASUREMENTS

Serum leptin concentrations.

RESULTS

Glucose and insulin concentrations were similar during the three clamp protocols. Average FFAs during the last 60 min of the clamp were 671+/-68 microM (CON),109+/-15 microM (12 FAST), 484+/-97 microM (36 FAST) and 1762+/-213 microM (36 FAST+FFA). Leptin concentrations decreased similarly during 36 FAST and 36 FAST+FFA. Leptin concentrations at 120 min (expressed as percentage of mean basal value) were 0.82+/-0.02 (CON), 0.93+/-0.08 (12 FAST) (P=0.29), 1.19+/-0.06 (36 FAST) (P<0.01) and 1.44+/-0.12 (36 FAST+FFA) (P<0.01).

CONCLUSION

During a one-day fast leptin concentrations decrease regardless of maintainance of an isocaloric balance. During acute hyperinsulinemic hyperglycemia leptin concentrations increase only after a preceding fast. This increase was most pronounced during simultaneous elevation of FFAs. Overall, our findings are compatible with the hypothesis that leptin secretion may be coupled to triglyceride synthesis rather than to the absolute lipid content of the adipocyte. International Journal of Obesity (2001) 25, 138-142

Regulation of leptin production in humans

Serum levels of the adipocyte hormone leptin are increased in proportion to body fat stores as a result of increased production in enlarged fat cells from obese subjects. In vitro studies indicate that insulin and glucocorticoids work directly on adipose tissue to upregulate in a synergistic manner leptin mRNA levels and rates of leptin secretion in human adipose tissue over the long term. Thus, the increased leptin expression observed in obesity could result from the chronic hyperinsulinemia and increased cortisol turnover. Superimposed upon the long-term regulation, nutritional status can influence serum leptin over the short term, independent of adiposity. Fasting leads to a gradual decline in serum leptin that is probably attributable to the decline in insulin and the ability of catecholamines to decrease leptin expression, as observed in both in vivo and in vitro studies. In addition, increases in serum leptin occur approximately 4-7 h after meals. Increasing evidence indicates that insulin, in concert with permissive effects of cortisol, can increase serum leptin over this time frame and likely contributes to meal-induced increases in serum leptin. Further research is required to elucidate the cellular and molecular mechanisms underlying short- and long-term nutritional and hormonal regulation of leptin production and secretion.