Acute changes in free-fatty acids (FFA) do not alter serum leptin levels

[Endocrine function in obesity]

Obesity is associated to significant disturbances in endocrine function. Hyper insulinemia and insulin resistance are the best known changes in obesity, but their mechanisms and clinical significance are not clearly established. Adipose tissue is considered to be a hormone-secreting endocrine organ; and increased leptin secretion from the adipocyte, a satiety signal, is a well-established endocrine change in obesity. In obesity there is a decreased GH secretion. Impairment of somatotropic function in obesity is functional and may be reversed in certain circumstances. The pathophysiological mechanism responsible for low GH secretion in obesity is probably multifactorial. There are many data suggesting that a chronic state of somatostatin hypersecretion results in inhibition of GH release. Increased FFA levels, as well as a deficient ghrelin secretion, probably contribute to the impaired GH secretion. In women, abdominal obesity is associated to hyperandrogenism and low sex hormone-binding globulin levels. Obese men, particularly those with morbid obesity, have decreased testosterone and gonadotropin levels. Obesity is associated to an increased cortisol production rate, which is compensated for by a higher cortisol clearance, resulting in plasma free cortisol levels that do not change when body weight increases. Ghrelin is the only known circulating orexigenic factor, and has been found to be decreased in obese people. In obesity there is also a trend to increased TSH and free T3 levels.

Integration of hormonal and nutrient signals that regulate leptin synthesis and secretion

This review summarizes recent advances in our understanding of the pre- and posttranscriptional mechanisms that regulate leptin production and secretion in adipocytes. Basal leptin production is proportional to the status of energy stores, i.e., fat cell size, and this is mainly regulated by alterations in leptin mRNA levels. Leptin mRNA levels are regulated by hormones, including glucocorticoids and catecholamines, but little is known about the transcriptional mechanisms involved. Leptin synthesis and secretion is also acutely modulated in response to hormones such as insulin and the availability of metabolic fuels. Acute variations in leptin production over a time course of minutes to hours are mediated at the levels of both translation and secretion. Increases in amino acids and insulin after a meal activate the mammalian target of rapamycin (mTOR) pathway, leading to an increase in specific rates of leptin biosynthesis. Cross-talk among mTOR, PKA, and AMP-activated protein kinase pathways appears to integrate hormonal and nutrient signals that regulate leptin mRNA translation, at least in part through mechanisms involving its 5'- and 3'-untranslated regions. In addition, the rate of leptin secretion from preformed stores in response to hormonal cues is also regulated. Insulin stimulates, and adrenergic agonists inhibit, leptin secretion, and this likely contributes to variations in the magnitude of nutrition-related leptin excursions and oscillations. Overall, the study of leptin production has contributed to a deepening understanding of leptin biology and, more broadly, to our understanding of the cellular and molecular mechanisms by which the adipocyte integrates hormonal and nutrient signals to regulate adipokine production.

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.

Reduction of plasma leptin concentrations by arginine but not lipid infusion in humans

OBJECTIVE

We examined short-term effects of arginine infusion on plasma leptin in diabetic and healthy subjects.

RESEARCH METHODS AND PROCEDURES

Arginine stimulation tests were performed in C-peptide negative type 1 [DM1; hemoglobin A(1c); 7.3 +/- 0.3%], hyperinsulinemic type 2 diabetic (DM2; 7.6 +/- 0.7%), and nondiabetic subjects (CON; 5.4 +/- 0.1%).

RESULTS

Fasting plasma leptin correlated linearly with body mass index among all groups (r = 0.61, p = 0.001). During arginine infusion, peak plasma insulin was lower in DM1 than in DM2 (p < 0.05) and CON (p < 0.01). Plasma leptin decreased within 30 minutes by approximately 11% in DM1 (p < 0.001), DM2 (p < 0.01), and CON (p < 0.005), slowly returning to baseline thereafter. Plasma free fatty acids (FFAs) were higher in DM1 (0.6 +/- 0.1 mM) and DM2 (0.6 +/- 0.1 mM) than in CON (0.4 +/- 0.1 mM, p < 0.05) and transiently declined by approximately 50% (p < 0.05) at 45 minutes in all groups before rebounding toward baseline. To examine the direct effects of FFAs on plasma leptin, we infused healthy subjects with lipid/heparin and glycerol during fasting, and somatostatin-insulin ( approximately 35 pM) -glucagon ( approximately 90 ng/mL) clamps were performed. In both protocols, plasma leptin continuously declined by approximately 25% (p < 0.05) during 540 minutes without any difference between the high and low FFA conditions.

DISCUSSION

Arginine infusion transiently decreased plasma leptin concentrations both in insulin-deficient and hyperinsulinemic diabetic patients, indicating a direct inhibitory effect of the amino acid but not of insulin or FFAs.

A pulse of insulin and dexamethasone stimulates serum leptin in fasting human subjects

OBJECTIVES

We have previously shown that dexamethasone increases serum leptin in fed but not in fasted human subjects. We hypothesized that insulin and/or glucose mediated the effect of food intake. The primary aim of this study was to determine whether the administration of a pulse of insulin with dexamethasone was sufficient to increase serum leptin in vivo in fasted human subjects. Whether the presence of transient hyperglycemia and the dose of insulin were important was tested as a secondary aim.

METHODS

Twenty-nine normal subjects were studied. In experiment 1 (meal-like), a pulse of insulin (0.03 U/kg s.c.) and of dexamethasone (2 mg i.v.) was given, and the blood glucose transiently elevated to 50 mg/dl above baseline for the first 2 h. In experiments 2 and 3 (dose-response), the effect of two doses of insulin (0.03 U/kg in experiment 2 and 0.06 U/kg in experiment 3) was tested in combination with dexamethasone, this time without transient hyperglycemia. Nine subjects were studied under fasting conditions, with or without dexamethasone, as a control experiment.

RESULTS

A meal-like transient hyperinsulinemia and hyperglycemia, with a pulse of dexamethasone, increased serum leptin levels from baseline by 54+/-21% at 9 h (P=0.038). In the absence of transient hyperglycemia, leptin increased significantly after doses of both insulin and dexamethasone. The effect of insulin was dose-dependent, with a larger increment of serum leptin at 9 h after the highest dose of insulin (75.2+/-15.7% vs 21.3+/-8.5%, P=0.013). Fasting, with or without dexamethasone, resulted in a significant 20% decrease in leptin from morning basal levels. Conversely, the administration of a pulse of insulin and glucose, in the absence of dexamethasone, prevented the drop in serum leptin observed during fasting, regardless of the insulin dose or the serum glucose elevation.

CONCLUSIONS

With the permissive effect of dexamethasone, a single pulse of insulin triggered a rise in serum leptin in humans, even in the absence of transient hyperglycemia. A single pulse of insulin with glucose can prevent the drop in serum leptin normally observed during fasting.

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

Relative hypoleptinemia in patients with type 1 and type 2 diabetes mellitus

OBJECTIVE

To determine the relation between plasma leptin concentrations and metabolic control in human diabetes mellitus.

DESIGN AND SUBJECTS

Cross sectional study consisting of 156 patients with diabetes mellitus type 1 (n=42), type 2 (n=114), and non-diabetic subjects (n=74).

RESULTS

Plasma leptin concentrations were lower (P<0.05) in type 1 (8.3+/-1.7 ng/ml) and type 2 diabetic (14.9+/-1.8 ng/ml) than in non-diabetic humans (18.3+/-1.9 ng/ml). Only female type 1 and type 2 diabetic subjects also had decreased leptin/BMI ratios (P<0.05 vs non-diabetic females). The log rank test identified age-adjusted correlation of plasma leptin concentration with sex (P<0.0004) and body mass index (P<0.0218), but not with glycosylated haemoglobin A1c (P>0.5) in all groups. Plasma leptin was correlated with age (P<0.0058) and serum triglycerides (P<0.0199) in type 1 diabetic patients, and with serum cholesterol (P<0.0059) and LDL (P<0.0013) in type 2 diabetic patients.

CONCLUSIONS

Defective leptin production and/or secretion might be present independently of metabolic control in female patients with type 1 or type 2 diabetes mellitus.

Downregulation of leptin by free fatty acids in rat adipocytes: effects of triacsin C, palmitate, and 2-bromopalmitate

Free fatty acid (FFA) has been reported to decrease leptin mRNA levels in 3T3-L1 adipocytes. When using this cell line, it is difficult to determine the protein levels because a very small amount of leptin is secreted into the medium. The effect of FFA on leptin secretion from adipocytes has not yet been determined. In addition, in vivo studies have failed to demonstrate a FFA-induced decrease in plasma leptin levels. To clarify the effect of FFA on leptin production, we investigated the leptin protein level in the medium and the mRNA level in primary cultured rat adipocytes treated with triacsin C, which is a potent inhibitor of acyl-coenzyme A (CoA) synthetase, palmitate, and 2-bromopalmitate. Triacsin C (0 to 5 x 10(-5) mol/L) decreased leptin concentrations in the culture medium in a dose-dependent manner. Leptin mRNA levels were decreased to 10% of the control in the presence of triacsin C. The concentration of triacsin C needed to suppress leptin production was similar to the Ki value (approximately 10(-5) mol/L) for inhibition of acyl-CoA synthetase. Both palmitate and 2-bromopalmitate decreased leptin concentra-tions but did not affect the triacsin C-induced decrease in leptin additively. In conclusion, both protein and mRNA levels of leptin were decreased by triacsin C and FFA in primary cultured rat adipocytes. Our findings suggest that FFA is involved in the regulation of leptin production in adipocytes.

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

Leptin secretion is complexly regulated in humans. Insulin has been shown to stimulate leptin secretion, whereas in vitro data suggest that catecholamines and free fatty acids (FFAs) inhibit leptin secretion. To dissect differential effects on leptin secretion, we performed two experimental protocols in 11 lean healthy subjects in addition to a saline infusion plus oral acipimox to suppress lipolysis (SAL + ACX) as a control experiment: (1) isoproterenol (approximately 30 ng/kg x min, to increase the heart rate by approximately 50 bpm) plus oral acipimox (ISO + ACX, 240 minutes) and (2) Intralipid (Pharmacia & Upjohn, Erlangen, Germany) plus heparin (LIP, 420 minutes). During SAL + ACX, FFAs decreased from 0.44 +/- 0.04 to 0.06 +/- 0.02 mmol/L (P = .001), while serum insulin and leptin remained unchanged. During ISO + ACX, FFAs decreased similarly from 0.41 +/- 0.13 to 0.09 +/- 0.02 mmol/L (P= .001), while insulin increased from 47 +/- 8 to a maximum of 116 +/- 15 pmol/L (P= .001) and serum leptin decreased acutely from 6.4 +/- 2.1 to a minimum of 5.4 +/- 1.8 ng/mL after 90 minutes (P = .003 vSAL + ACX). After 150 minutes, leptin returned to control levels. During LIP, the elevation of FFAs from 0.34 +/- 0.04 to 1.71 +/- 0.19 mmol/L (P = .001) had no effect on serum insulin or leptin concentrations (both P = nonsignificant). In conclusion, our results show that in humans, isoproterenol acutely suppresses leptin levels independently of increased FFAs, and elevated FFAs have no acute effect on leptin levels. The fact that an inhibition of leptin secretion occurred despite conditions that are known to suppress intracellular cyclic adenosine monophosphate (cAMP) levels, as demonstrated by suppressed lipolysis, suggests that signaling mechanisms other than those mediated by cAMP must be involved in modulating leptin secretion.

Changes in plasma leptin during the treatment of diabetic ketoacidosis

To test the hypothesis that insulin regulates leptin, we measured the plasma leptin concentration before and during treatment of diabetic ketoacidosis (DKA), a condition characterized by extreme insulin deficiency. The study included 17 patients with type 1 diabetes (7 males and 10 females), aged 10+/-1 yr (mean +/- SE), with a body mass index of 17.6+/-1.9 kg/m2. Patients were treated with continuous insulin infusion and fluid and electrolyte replacement. Plasma leptin was measured every 6 h in the first 24 h, during which patients received a total insulin dose of 0.6-2.0 U/kg. Plasma leptin concentrations were also measured in a control group of 29 stable type 1 diabetic children (12 males and 17 females) and 25 healthy children (11 males and 14 females), aged 11+/-1 yr, with a body mass index of 18.5+/-1.1 kg/m2. Before treatment, plasma leptin concentrations were significantly lower in patients with DKA than those in diabetic and healthy controls (4.9+/-1.2 vs. 9.0+/-1.8 and 11.2+/-2.1 ng/mL, respectively; P < 0.05). In the DKA patients, plasma leptin increased to 6.4+/-1.5, 7.5+/-1.9, 9.1+/-2.7, and 8.9+/-2.5 at 6, 12, 18, and 24 h, respectively, after starting treatment (P = 0.001). Thus, leptin levels increased by 38+/-10% and 92+/-38% within 6 and 24 h of starting treatment. There was no difference in the change in plasma leptin by 24 h between subjects who could eat (n = 7) and those who could not (n = 10). The plasma leptin increase was paralleled by a rise in insulin level and a decline in glucose and cortisol levels at 6 and 24 h. In conclusion, DKA was associated with decreased plasma leptin concentrations. Treatment resulted in a significant increase in plasma leptin, which may be due to the effect of insulin on leptin production. Our data lend support to the hypothesis that insulin is the link between caloric intake and plasma leptin.