Inhibition of leptin release by atrial natriuretic peptide (ANP) in human adipocytes

UCP1-dependent brown adipose activation accelerates cardiac metabolic remodeling and reduces initial hypertrophic and fibrotic responses to pathological stress

Brown adipose tissue (BAT) is correlated to cardiovascular health in rodents and humans, but the physiological role of BAT in the initial cardiac remodeling at the onset of stress is unknown. Activation of BAT via 48 h cold (16°C) in mice following transverse aortic constriction (TAC) reduced cardiac gene expression for LCFA uptake and oxidation in male mice and accelerated the onset of cardiac metabolic remodeling, with an early isoform shift of carnitine palmitoyltransferase 1 (CPT1) toward increased CPT1a, reduced entry of long chain fatty acid (LCFA) into oxidative metabolism (0.59 ± 0.02 vs. 0.72 ± 0.02 in RT TAC hearts, p < .05) and increased carbohydrate oxidation with altered glucose transporter content. BAT activation with TAC reduced early hypertrophic expression of β-MHC by 61% versus RT-TAC and reduced pro-fibrotic TGF-β1 and COL3α1 expression. While cardiac natriuretic peptide expression was yet to increase at only 3 days TAC, Nppa and Nppb expression were elevated in Cold TAC versus RT TAC hearts 2.7- and 2.4-fold, respectively. Eliminating BAT thermogenic activation with UCP1 KO mice eliminated differences between Cold TAC and RT TAC hearts, confirming effects of BAT activation rather than autonomous cardiac responses to cold. Female responses to BAT activation were blunted, with limited UCP1 changes with cold, partly due to already activated BAT in females at RT compared to thermoneutrality. These data reveal a previously unknown physiological mechanism of UCP1-dependent BAT activation in attenuating early cardiac hypertrophic and profibrotic signaling and accelerating remodeled metabolic activity in the heart at the onset of cardiac stress.

Atrial natriuretic peptide and leptin interactions in healthy men

Introduction

Atrial natriuretic peptide (ANP), a hormone secreted from the heart, controls cardiovascular and renal functions including arterial blood pressure and natriuresis. ANP also exerts metabolic effects in adipose tissue, liver and skeletal muscle, and interacts with the secretion of adipokines. We tested the hypothesis that ANP lowers concentrations of the anorexigenic adipokine leptin in healthy humans in vivo.

Methods

Human ANP or matching placebo was infused intravenously (iv) into healthy men in a controlled clinical trial.

Results

Within 135 minutes of iv ANP infusion, we observed an acute decrease in plasma leptin levels compared to controls. Free fatty acids markedly increased with ANP infusion in vivo, indicating activated lipolysis. In human SGBS adipocytes, ANP suppressed leptin release.

Discussion

The study shows that the cardiac hormone ANP reduces the levels of the anorexigenic adipokine leptin in healthy humans, providing further support for ANP as a cardiomyokine in a heart - adipose tissue axis. (registered in the German Clinical Trials Register and the WHO International Clinical Trials Registry Platform was granted under DRKS00024559).

Amelioration of the Abnormalities Associated with Metabolic Syndrome by L-Norvaline in Hyperlipidemic Diabetic Rats

The present study was designed to assess the treatment effect of arginase inhibitor, L-Norvaline in abnormalities associated with high fat diet (HFD) and fructose-induced metabolic syndrome. The HFD and fructose was fed to the rats for a period of 45 days. Animals having body weight of 350 g and fasting blood sugar level of more than 250 mg/dl were considered as hyperlipidemic diabetic rats (HDR) and selected for the study. The HDR were divided into three groups having six animals each. The HDR received L–Norvaline (10 mg/kg/day, i.p.) and standard drug, gemfibrozil (60 mg/kg/day, p.o.), for a period of 30 days. Various hormonal, biochemical and tissue parameters were evaluated at the end of the study. Both treatments significantly decreased body weight, BMI, fasting blood sugar and insulin level and improved insulin resistance in HDR as compared to the toxicant control group. A significant improvement was observed in the lipid profile, levels of nitrate, leptin, C-reactive protein and adiponectin in HDR. L-Norvaline also caused slight decrease in the malondialdehyde level, though, no prominent effect was observed on the level of superoxide dismutase and reduced glutathione in the pancreas of HDR, as compared to the toxicant control group. L-Norvaline treatment also improved the histo-architecture of pancreatic cells. Results of the present study concludes that L-Norvaline caused significant alleviation of the abnormalities of MetS indicating that it can be used as potential treatment strategy for managing the symptoms of metabolic syndrome.

Obese Patients Experience More Severe CSA than Non-Obese Patients

Objective: To investigate whether central sleep apnea (CSA) is associated with an increased risk of obesity. Materials and methods: From 1 January 2000 to 31 December 2015, we screened 24,363 obese patients from the 2005 longitudinal health insurance database, which is part of the Taiwan National Health Insurance Research Database. From the same database, 97,452 non-obese patients were also screened out. Age, gender, and index dates were matched. Multiple logistic regression was used to analyze the previous exposure risk of obese and CSA patients. A p-value of <0.05 was considered significant. Results: Obese patients were more likely to be exposed to CSA than non-obese patients would (AOR = 2.234, 95% CI = 1.483–4.380, p < 0.001). In addition, the closeness of the exposure time to the index time is positively correlated with the severity of obesity and has a dose–response effect (CSA exposure < 1 year, AOR = 2.386; CSA exposure ≥ 1 year and <5 years, AOR = 1.725; CSA exposure time ≥ 5 years, AOR = 1.422). The CSA exposure time of obese patients was 1.693 times that of non-obese patients. Longer exposure time is associated with more severe obesity and has a dose-response effect (CSA exposure < 1 year, AOR = 1.420; CSA exposure ≥ 1 year and <5 years, AOR = 2.240; CSA ≥ 5 years, AOR = 2.863). Conclusions: In this case-control study, patients with CSA had a significantly increased risk of obesity. Long-term exposure to CSA and obesity is more likely and has a dose-response effect.

Role of natriuretic peptides in the cardiovascular-adipose communication: a tale of two organs

Natriuretic peptides have long been known for their cardiovascular function. However, a growing body of evidence emphasizes the role of natriuretic peptides in the energy metabolism of several substrates in humans and animals, thus interrelating the heart, as an endocrine organ, with various insulin-sensitive tissues and organs such as adipose tissue, muscle skeletal, and liver. Adipose tissue dysfunction is associated with altered regulation of the natriuretic peptide system, also indicated as a natriuretic disability. Evidence points to a contribution of this natriuretic disability to the development of obesity, type 2 diabetes mellitus, and cardiometabolic complications; although the causal relationship is not fully understood at present. However, targeting the natriuretic peptide pathway may improve metabolic health in obesity and type 2 diabetes mellitus. This review will focus on the current literature on the metabolic functions of natriuretic peptides with emphasis on lipid metabolism and insulin sensitivity. Natriuretic peptide system alterations could be proposed as one of the linking mechanisms between adipose tissue dysfunction and cardiovascular disease.

Characterization of the inflammatory-metabolic phenotype of heart failure with a preserved ejection fraction: a hypothesis to explain influence of sex on the evolution and potential treatment of the disease

Accumulating evidence points to the existence of an inflammatory-metabolic phenotype of heart failure with a preserved ejection fraction (HFpEF), which is characterized by biomarkers of inflammation, an expanded epicardial adipose tissue mass, microvascular endothelial dysfunction, normal-to-mildly increased left ventricular volumes and systolic blood pressures, and possibly, altered activity of adipocyte-associated inflammatory mediators. A broad range of adipogenic metabolic and systemic inflammatory disorders - e.g. obesity, diabetes and metabolic syndrome as well as rheumatoid arthritis and psoriasis - can cause this phenotype, independent of the presence of large vessel coronary artery disease. Interestingly, when compared with men, women are both at greater risk of and may suffer greater cardiac consequences from these systemic inflammatory and metabolic disorders. Women show disproportionate increases in left ventricular filling pressures following increases in central blood volume and have greater arterial stiffness than men. Additionally, they are particularly predisposed to epicardial and intramyocardial fat expansion and imbalances in adipocyte-associated proinflammatory mediators. The hormonal interrelationships seen in inflammatory-metabolic phenotype may explain why mineralocorticoid receptor antagonists and neprilysin inhibitors may be more effective in women than in men with HFpEF. Recognition of the inflammatory-metabolic phenotype may improve an understanding of the pathogenesis of HFpEF and enhance the ability to design clinical trials of interventions in this heterogeneous syndrome.

Heart Failure With Preserved Ejection Fraction and Adipose Tissue: A Story of Two Tales

Heart failure with preserved ejection fraction (HFpEF) is characterized by signs and symptoms of heart failure in the presence of a normal left ventricular ejection fraction. Although it accounts for up to 50% of all clinical presentations of heart failure, there are no evidence-based therapies for HFpEF to reduce morbidity and mortality. Additionally there is a lack of mechanistic understanding about the pathogenesis of HFpEF. HFpEF is associated with many comorbidities (such as obesity, hypertension, type 2 diabetes, atrial fibrillation, etc.) and is coupled with both cardiac and extra-cardiac abnormalities. Large outcome trials and registries reveal that being obese is a major risk factor for HFpEF. There is increasing focus on investigating the link between obesity and HFpEF, and the role that the adipose tissue and the heart, and the circulating milieu play in development and pathogenesis of HFpEF. This review discusses features of the obese-HFpEF phenotype and highlights proposed mechanisms implicated in the inter-tissue communication between adipose tissue and the heart in obesity-associated HFpEF.

Do single or sequential measurements of leptin and adiponectin in plasma have prognostic value in pulmonary arterial hypertension?

Leptin (a neuroendocrine peptide that enhances metabolism and acts on the hypothalamus to suppress appetite) and adiponectin (a protein that has insulin-sensitizing, anti-inflammatory, and antiproliferative properties) are involved in the pathobiology of pulmonary arterial hypertension (PAH). We hypothesized that plasma leptin and adiponectin as well as the leptin/adiponectin ratio are abnormal in PAH patients and their levels track with disease severity and functional changes during follow-up. We tested this hypothesis in a cohort of patients included in the 16-week, international, multicenter, double-blind, placebo-controlled FREEDOM-C2 study. Blood was collected at baseline and week 16 in 178 out of 310 randomized patients with PAH. Baseline plasma leptin and adiponectin concentrations were 25 ± 31 ng/mL and 7.8 ± 6.1 ug/mL, respectively. Leptin, adiponectin, and leptin/adiponectin (mean ± SD) changes at 16 week were of small magnitude. Leptin at baseline was significantly associated with older age, higher BMI, higher Borg dyspnea index, and lower NT-pro BNP. Women had higher levels of leptin than men (30.5 ± 33.2 versus 7.2 ± 6.4 ng/mL), even when adjusting for background therapy and etiology (linear regression: β = 21.8, P < 0.001). Adiponectin was negatively associated with BMI and positively associated with NT-pro BNP. Changes in leptin, adiponectin, and leptin/adiponectin ratio adjusted for weight at 16 weeks did not predict functional class, distance walk in 6 min or survival at one, two, three, or four years. Plasma leptin and adiponectin at baseline and their change at 16-week do not appear to significantly impact prognosis in PAH.

Low leptin concentration may identify heart failure patients with central sleep apnea

Low leptin concentration has been shown to be associated with central sleep apnea in heart failure patients. We hypothesized that low leptin concentration predicts central sleep apnea. Consecutive ambulatory New York Heart Association (NYHA) classes I-IV heart failure patients were studied prospectively, including measurement of serum leptin, echocardiography and polysomnography. Sleep apnea was defined by type (central/mixed/obstructive) and by apnea-hypopnea index ≥5 by polysomnography. Subjects were divided into four groups by polysomnography: (1) central sleep apnea, (2) mixed apnea, (3) no apnea and (4) obstructive sleep apnea. Fifty-six subjects were included. Eighteen subjects were diagnosed with central sleep apnea, 15 with mixed apnea, 12 with obstructive apnea and 11 with no sleep apnea. Leptin concentration was significantly lower in central sleep apnea compared to obstructive apnea (8 ± 10.7 ng mL^-1 versus 19.7 ± 14.7 ng mL^-1 , P ˂ 0.01) or no sleep apnea (8 ± 10.7 ng mL^-1 versus 17.1 ± 8.4 ng mL^-1 , P ˂ 0.01). Logistic regression showed leptin to be associated independently with central sleep apnea [odds ratio (OR): 0.19; 95% confidence interval (CI): 0.06-0.62; area under the curve (AUC): 0.80, P < 0.01]. For the detection of central sleep apnea, a cut-off value for leptin concentration 5 ng mL^-1 yielded a sensitivity of 50% and specificity of 89%. In conclusion, a low leptin concentration may have utility for the screening of heart failure patients for central sleep apnea.

An Emerging Role of Natriuretic Peptides: Igniting the Fat Furnace to Fuel and Warm the Heart

Natriuretic peptides are produced in the heart and have been well characterized for their actions in the cardiovascular system to promote diuresis and natriuresis, thereby contributing to maintenance of extracellular fluid volume and vascular tone. For this review, we scanned the literature using PubMed and MEDLINE using the following search terms: beiging, adipose tissue, natriuretic peptides, obesity, and metabolic syndrome. Articles were selected for inclusion if they represented primary data or review articles published from 1980 to 2015 from high-impact journals. With the advent of the newly approved class of drugs that inhibit the breakdown of natriuretic peptides, thereby increasing their circulation, we highlight additional functions for natriuretic peptides that have recently become appreciated, including their ability to drive lipolysis, facilitate beiging of adipose tissues, and promote lipid oxidation and mitochondrial respiration in skeletal muscle. We provide evidence for new roles for natriuretic peptides, emphasizing their ability to participate in body weight regulation and energy homeostasis and discuss how they may lead to novel strategies to treat obesity and the metabolic syndrome.

Paradoxical effects of partial leptin deficiency on bone in growing female mice

Morbidly obese, leptin-deficient ob/ob mice display low bone mass, mild osteoclast-rich osteopetrosis, and increased bone marrow adiposity. While partial leptin deficiency results in increased weight, the skeletal manifestations of partial leptin deficiency are less well defined. We therefore analyzed femora and lumbar vertebrae in growing (7-week-old) female C57BL/6 wildtype (WT) mice, partial leptin-deficient ob/+ mice, and leptin-deficient ob/ob mice. The bones were evaluated by dual energy absorptiometry, microcomputed tomography and histomorphometry. As expected, ob/+ mice were heavier, had more white adipose tissue, and lower serum leptin than WT mice, but were lighter and had less white adipose tissue than ob/ob mice. With a few exceptions, cancellous bone architecture, cell (osteoblast, osteoclast, and adipocyte), and dynamic measurements did not differ between WT and ob/+ mice. In contrast, compared to WT and ob/+ mice, ob/ob mice had lower cancellous bone volume fraction, and higher bone marrow adiposity in the femur metaphysis, and higher cancellous bone volume fraction in lumbar vertebra. Paradoxically, ob/+ mice had greater femoral bone volume than either WT or ob/ob mice. There was a positive correlation between body weight and femur volume in all three genotypes. However, the positive effect of weight on bone occurred with lower body weight in leptin-producing mice. The paradoxical differences in bone size among WT, ob/+, and ob/ob mice may be explained if leptin, in addition to stimulating bone growth and cancellous bone turnover, acts to lower the set-point at which increased body weight leads to a commensurate increase in bone size.

Metabolic actions of natriuretic peptides and therapeutic potential in the metabolic syndrome

Natriuretic peptides (NPs) are a group of peptide-hormones mainly secreted from the heart, signaling via c-GMP coupled receptors. NP are well known for their renal and cardiovascular actions, reducing arterial blood pressure as well as sodium reabsorption. Novel physiological functions have been discovered in recent years, including activation of lipolysis, lipid oxidation, and mitochondrial respiration. Together, these responses promote white adipose tissue browning, increase muscular oxidative capacity, particularly during physical exercise, and protect against diet-induced obesity and insulin resistance. Exaggerated NP release is a common finding in congestive heart failure. In contrast, NP deficiency is observed in obesity and in type-2 diabetes, pointing to an involvement of NP in the pathophysiology of metabolic disease. Based upon these findings, the NP system holds the potential to be amenable to therapeutical intervention against pandemic diseases such as obesity, insulin resistance, and arterial hypertension. Various therapeutic approaches are currently under development. This paper reviews the current knowledge on the metabolic effects of the NP system and discusses potential therapeutic applications.

Leptin deficiency promotes central sleep apnea in patients with heart failure

BACKGROUND

Leptin-deficient animals hyperventilate. Leptin expression by adipocytes is attenuated by atrial natriuretic peptide (ANP). Increased circulating natriuretic peptides (NPs) are associated with an increased risk of central sleep apnea (CSA). This study tested whether serum leptin concentration is inversely correlated to NP concentration and decreased in patients with heart failure (HF) and CSA.

METHODS

Subjects with HF (N = 29) were studied by measuring leptin, NPs, CO2 chemosensitivity (Δminute ventilation [V.e]/Δpartial pressure of end-tidal CO2 [Petco2]), and ventilatory efficiency (V.e/CO2 output [V.co2]) and were classified as CSA or no sleep-disordered breathing by polysomnography. CSA was defined as a central apnea-hypopnea index ≥ 15. The Student t test, Mann-Whitney U test, and logistic regression were used for analysis, and data were summarized as mean ± SD; P &lt; .05 was considered significant.

RESULTS

Subjects with CSA had higher ANP and brain natriuretic peptide (BNP) concentrations (P &lt; .05), ΔV.e/ΔPetco2 (2.39 ± 1.03 L/min/mm Hg vs 1.54 ± 0.35 L/min/mm Hg, P = .01), and V.e/V.co2 (43 ± 9 vs 34 ± 7, P &lt; .01) and lower leptin concentrations (8 ± 10.7 ng/mL vs 17.1 ± 8.8 ng/mL, P &lt; .01). Logistic regression analysis (adjusted for age, sex, and BMI) demonstrated leptin (OR = 0.07; 95% CI, 0.01-0.71; P = .04) and BNP (OR = 4.45; 95% CI, 1.1-17.9; P = .05) to be independently associated with CSA.

CONCLUSIONS

In patients with HF and CSA, leptin concentration is low and is inversely related to NP concentration. Counterregulatory interactions of leptin and NP may be important in ventilatory control in HF.

The relationship between leptin and ventilatory control in heart failure

BACKGROUND

Increased serum leptin concentration has been linked to increased ventilation in patients with mild heart failure (HF). However, in animal models the absence of leptin has also been associated with increased ventilation. This study evaluated the relationship of circulating leptin concentration with exercise ventilation in HF patients.

METHODS AND RESULTS

Fifty-eight consecutive ambulatory HF patients were stratified by quintiles of leptin concentration, with a lowest quintile of mean leptin concentration of 1.8 ± 8.9 ng/mL and a highest of 33.3 ± 30.3 ng/mL. Peak exercise ventilatory efficiency (VE/VCO2) was significantly elevated in the lowest (46 ± 6 vs 34 ± 4; P < .01) as well as in the highest (38 ± 8 vs 34 ± 4; P < .05) leptin concentration quintiles compared with the reference middle quintile. Multiple regression analysis adjusted for confounders such as age, sex, and body mass index showed leptin concentration to be independently inversely correlated to VE/VCO2 in the low-to-normal quintiles (β = -0.64; P < .01), positively in the normal-to-high quintiles (β = 0.52; P = .02), and positively correlated to PETCO2 in the low-to-normal quintiles (β = 0.59; P = .01) and inversely in the normal-to-high quintiles (β = -0.53; P = .02).

CONCLUSIONS

In HF patients, both high and low leptin concentrations are associated with increased VE/VCO2 and decreased PETCO2 with a nonlinear U-shaped relationship, suggesting that either leptin deficiency or leptin resistance may modulate ventilatory control in HF patients.

Reduction of leptin levels by four cardiac hormones: Implications for hypertension in obesity

Circulating levels of leptin are increased in obesity and have been proposed to contribute to the development of hypertension in obese individuals. Four cardiac hormones, specifically, vessel dilator, long-acting natriuretic peptide (LANP), kaliuretic peptide and atrial natriuretic peptide (ANP), have blood pressure-lowering properties and correlate with the presence of hypertension in obesity. The objective of this study was to determine whether one or more of these cardiac hormones was able to decrease the levels of leptin in the hypothalamus, an area of the brain that has been demonstrated to synthesize more than 40% of leptin in the circulation. The effects of these four cardiac hormones on leptin were examined using dose-response curves in the rat hypothalamus, which synthesizes leptin. Vessel dilator, LANP, kaliuretic peptide and ANP maximally decreased the levels of leptin in hypothalamic cells by 79, 76, 80 and 62%, respectively (P<0.0001 for each). The cardiac hormones decreased leptin levels over a concentration range of 100 pM to 10 μM, with the most significant reductions in leptin levels occurring when the concentrations of the hormones were at micromolar levels. The results of the study suggest that the four cardiac hormones lead to significant reductions in hypothalamic leptin levels, which may be an important mechanism for alleviating leptin-induced hypertension in obesity.

Natriuretic peptides and cGMP signaling control of energy homeostasis

Since the discovery of natriuretic peptides (NPs) by de Bold et al. in 1981, the cardiovascular community has been well aware that they exert potent effects on vessels, heart remodeling, kidney function, and the regulation of sodium and water balance. Who would have thought that NPs are also able to exert metabolic effects and contribute to an original cross talk between heart, adipose tissues, and skeletal muscle? The attention on the metabolic role of NPs was awakened in the year 2000 with the discovery that NPs exert potent lipolytic effects mediated by the NP receptor type A/cGMP pathway in human fat cells and that they contribute to lipid mobilization in vivo. In this review, we will discuss the biological effects of NPs on the main tissues involved in the regulation of energy metabolism (i.e., white and brown adipose tissues, skeletal muscle, liver, and pancreas). These recent results on NPs are opening a new chapter into the physiological properties and therapeutic usefulness of this family of hormones.

Thirty years of the heart as an endocrine organ: physiological role and clinical utility of cardiac natriuretic hormones

Thirty years ago, De Bold et al. ( 20 ) reported that atrial extracts contain some biologically active peptides, which promote a rapid and massive diuresis and natriuresis when injected in rats. It is now clear that the heart also exerts an endocrine function and in this way plays a key role in the regulation of cardiovascular and renal systems. The aim of this review is to discuss some recent insights and still-debated findings regarding the cardiac natriuretic hormones (CNHs) produced and secreted by cardiomyocytes (i.e., atrial natriuretic peptide and B-type natriuretic peptide). The functional status of the CNH system depends not only on the production/secretion of CNHs by cardiomyocytes but also on both the peripheral activation of circulating inactive precursor of natriuretic hormones and the transduction of the hormone signal by specific receptors. In this review, we will discuss the data supporting the hypothesis that the production and secretion of CNHs is the result of a complex integration among mechanical, chemical, hemodynamic, humoral, ischemic, and inflammatory inputs. The cross talk among endocrine function, adipose tissue, and sex steroid hormones will be discussed more in detail, considering the clinically relevant relationships linking together cardiovascular risk, sex, and body fat development and distribution. Finally, we will review the pathophysiological role and the clinical relevance of both peripheral maturation of the precursor of B-type natriuretic peptides and hormone signal transduction .

Leptin, but not adiponectin, is a predictor of recurrent cardiovascular events in men: results from the LIPID study

OBJECTIVE

To investigate the relationships between plasma leptin and adiponectin levels and recurrent cardiovascular events (cardiovascular death, nonfatal myocardial infarction and stroke) in men with earlier acute coronary syndromes.

DESIGN, SUBJECTS AND MEASUREMENTS

A nested case-control study examined circulating leptin and adiponectin levels in plasma obtained 4-6 years after entry into the Long-Term Intervention with Pravastatin in Ischaemic Disease (LIPID) trial. Plasma was assayed from 184 men who suffered recurrent events within 4.4 years after blood collection and 184 matched controls who remained free of further events. The association between cardiovascular events and the explanatory variables was examined by conditional logistic regression analysis.

RESULTS

Relative risk (RR) increased across increasing leptin quartiles; the highest quartile compared with the lowest quartile was related to the highest risk (P for trend=0.002); the increased risk remained after adjustment for risk factors (P=0.018) or for obesity (P=0.038), but in the final model (adjusted for randomized treatment, other drugs, LIPID risk score, age and body mass index), the risk was attenuated (RR=1.61, 95% CI: 0.72-3.57, P for trend=0.34). Adiponectin did not predict cardiovascular events. Subjects randomly allocated to pravastatin had 6% lower leptin levels (P=0.04) than those allocated to placebo.

CONCLUSION

Plasma leptin was a significant and independent predictor of recurrent cardiovascular events (cardiovascular death, nonfatal myocardial infarction and stroke) in men with earlier acute coronary syndromes.

Control of lipolysis by natriuretic peptides and cyclic GMP

Human fat cell lipolysis was, until recently, thought to be mediated exclusively by a cAMP-dependent protein kinase (PKA)-regulated pathway under the control of catecholamines and insulin. We have shown that atrial- and B-type natriuretic peptides (ANP and BNP respectively) stimulate lipolysis in human fat cells through a cGMP-dependent protein kinase (PKG) signaling pathway independent of cAMP production and PKA activity. Pharmacological or physiological (exercise) increases in plasma ANP levels stimulate lipid mobilization in humans. This pathway becomes important during chronic treatment with beta-adrenoceptor antagonists, which inhibit catecholamine-induced lipolysis but enhance cardiac ANP release. These findings have metabolic implications and point to potential problems when natriuretic peptide secretion is altered or during therapeutic use of recombinant BNP.

Metabolic regulation: effects of natriuretic peptide interactions

In addition to their well-established effects on blood pressure and volume homeostasis, natriuretic peptides have complex effects on carbohydrate and lipid metabolism. In vivo, pharmacological and physiological concentrations of atrial natriuretic peptides induce lipolysis in a concentration-dependent manner and increase the lipid oxidation rate. The response appears to be mediated through the stimulation of natriuretic peptide receptor-A. More recent studies suggest that natriuretic peptides also affect the production of several adipokines. These mechanisms may be relevant, as natriuretic peptide availability is altered in numerous physiological and pathological conditions, including physical exercise, congestive heart failure and obesity.

Atrial natriuretic peptide inhibits the production of adipokines and cytokines linked to inflammation and insulin resistance in human subcutaneous adipose tissue

AIMS/HYPOTHESIS

Increased adipose tissue secretion of adipokines and cytokines has been implicated in the chronic low-grade inflammation state and insulin resistance associated with obesity. We tested here whether the cardiovascular and metabolic hormone atrial natriuretic peptide (ANP) was able to modulate adipose tissue secretion of several adipokines (derived from adipocytes) and cytokines (derived from adipose tissue macrophages).

SUBJECTS AND METHODS

We used protein array to measure the secretion of adipokines and cytokines after a 24-h culture of human subcutaneous adipose tissue pieces treated or not with a physiological concentration of ANP. The effect of ANP on protein secretion was also directly studied on isolated adipocytes and macrophages. Gene expression was measured by real-time RT-quantitative PCR.

RESULTS

ANP decreased the secretion of the pro-inflammatory cytokines IL-6 and TNF-alpha, of several chemokines, and of the adipokines leptin and retinol-binding protein-4 (RBP-4). The secretion of the anti-inflammatory molecules IL-10 and adiponectin remained unaffected. The cytokines were mainly expressed in macrophages that expressed all components of the ANP-dependent signalling pathway. The adipokines, leptin, adiponectin and RBP-4 were specifically expressed in mature adipocytes. ANP directly inhibited the secretion of IL-6 and monocyte chemoattractant protein-1 by macrophages. The inhibitory effects of ANP on leptin and growth-related oncogene-alpha secretions were not seen under selective hormone-sensitive lipase inhibition.

CONCLUSIONS/INTERPRETATION

We suggest that ANP, either by direct action on adipocytes and macrophages or through activation of adipocyte hormone-sensitive lipase, inhibits the secretion of factors involved in inflammation and insulin resistance.

Regulatory role for the arginine–nitric oxide pathway in metabolism of energy substrates

Nitric oxide (NO) is synthesized from L-arginine by NO synthase in virtually all cell types. Emerging evidence shows that NO regulates the metabolism of glucose, fatty acids and amino acids in mammals. As an oxidant, pathological levels of NO inhibit nearly all enzyme-catalyzed reactions through protein oxidation. However, as a signaling molecule, physiological levels of NO stimulate glucose uptake as well as glucose and fatty acid oxidation in skeletal muscle, heart, liver and adipose tissue; inhibit the synthesis of glucose, glycogen, and fat in target tissues (e.g., liver and adipose); and enhance lipolysis in adipocytes. Thus, an inhibition of NO synthesis causes hyperlipidemia and fat accretion in rats, whereas dietary arginine supplementation reduces fat mass in diabetic fatty rats. The putative underlying mechanisms may involve multiple cyclic guanosine-3',5'-monophosphate-dependent pathways. First, NO stimulates the phosphorylation of adenosine-3',5'-monophosphate-activated protein kinase, resulting in (1) a decreased level of malonyl-CoA via inhibition of acetyl-CoA carboxylase and activation of malonyl-CoA decarboxylase and (2) a decreased expression of genes related to lipogenesis and gluconeogenesis (glycerol-3-phosphate acyltransferase, sterol regulatory element binding protein-1c and phosphoenolpyruvate carboxykinase). Second, NO increases the phosphorylation of hormone-sensitive lipase and perilipins, leading to the translocation of the lipase to the neutral lipid droplets and, hence, the stimulation of lipolysis. Third, NO activates expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, thereby enhancing mitochondrial biogenesis and oxidative phosphorylation. Fourth, NO increases blood flow to insulin-sensitive tissues, promoting substrate uptake and product removal via the circulation. Modulation of the arginine-NO pathway through dietary supplementation with L-arginine or L-citrulline may aid in the prevention and treatment of the metabolic syndrome in obese humans and companion animals, and in reducing unfavorable fat mass in animals of agricultural importance.

Nitric oxide-induced downregulation of leptin production by 3T3-L1 adipocytes

Leptin secreted mainly by adipocytes plays an important role in insulin sensitivity in metabolic syndrome. Inducible nitric oxide synthase (iNOS) in 3T3-L1 adipocytes is induced by lipopolysaccharide (LPS) and several proinflammatory cytokines such as tumor necrosis factor-alpha and interferon-gamma (IFN-gamma). Because the role of iNOS-derived nitric oxide (NO) in adipocyte function has not been fully clarified, the question that we addressed in the present study was whether iNOS-derived NO is involved in regulation of leptin secretion by adipocytes. Incubation of 3T3-L1 adipocytes for 12h with a mixture of IFN-gamma and LPS caused not only a 55% reduction in leptin secretion and a 52% reduction in leptin mRNA, but also significant induction of iNOS at both protein and mRNA levels. Inhibition of leptin secretion that had been induced by the IFN-gamma-LPS mixture was completely nullified by NOS inhibitors such as Nomega-monomethyl-L-arginine and aminoguanidine. Treatment of adipocytes with NO donors such as an NONOate and S-nitrosoglutathione produced an effect on leptin secretion similar to that of the IFN-gamma-LPS mixture. It is likely therefore that NO mediates downregulation of leptin caused by the IFN-gamma-LPS mixture in 3T3-L1 adipocytes, which suggests an important role for NO in adipocyte functions.

Relationship among nitric oxide, leptin, ACTH, corticosterone, and IL-1beta, in the early and late phases of adjuvant arthritis in male Long Evans rats

Leptin, a hormone regulating body weight, food intake, and metabolism, is associated with activation of immune cells and inflammation. In this study we analyzed levels of leptin, adrenocorticotropic hormone (ACTH), corticosterone, interleukin 1beta (IL-1beta), and nitric oxide (NO) production on days 10 and 22 of adjuvant arthritis (AA) in male Long Evans rats to ascertain possible relationship of leptin with its modulators during the early and late phases of chronic inflammation. The circulating leptin levels were significantly reduced already on day 10 of AA compared to controls (1.97+/-0.22 ng/ml vs. 3.08+/-0.25 ng/ml, p<0.05); on day 22 no significant further drop was observed (1.06+/-0.21 ng/ml). Leptin mRNA in epididymal fat tissue was reduced in arthritic animals compared to controls on day 22 (0.61+/-0.09 vs. 1.30+/-0.1 arbU/GAPDH (p<0.01). IL-1beta concentration in spleen was enhanced on day 10 of AA (24.55+/-4.67 pg/100 microg protein vs. 14.33+/-1.71 pg/100 microg protein; p<0.05); on day 22 it did not differ from controls. ACTH and corticosterone levels were significantly elevated only on day 22 of AA (ACTH: 306.17+/-42.22 pg/ml vs. 157.61+/-23.94 pg/ml; p<0.05; corticosterone: 5.24+/-1.38 microg/100 ml vs. 1.05+/-0.23 microg/100 ml; p<0.01). Nitrate levels were enhanced similarly on days 10 (49.86+/-1.83 microM) and 22 of AA (43.58+/-2.17 microM), compared to controls (23.42+/-1.39 microM, p<0.001). These results show that corticosterone does not stimulate leptin production during AA. The suppression of leptin may be a consequence of permanent activation of NO, IL-1beta, and of lower weight gain. Circulating leptin does not seem to play a key role in the progression of chronic arthritis.

The broad homeostatic role of natriuretic peptides

A brief non-inclusive review on natriuretic peptides (NP), their receptors, and their main functional properties is presented. The three main NP, atrial (ANP), brain (BNP) and C-type (CNP) are considered. Guanylyl cyclase receptors modulate all the known systemic effects of NP. Clearance receptors determine the metabolic disposal of NP and in this manner regulate their plasma levels and/or local tissue concentrations. Structure-function properties, and homeostatic properties of NP receptors are presented. ANP, which plays a major role in pressure-volume homeostasis, is discussed in relationship to its effects on renal hemodynamic and excretory functions, inhibition of the renin-angiotensin-aldosterone system, vasorelaxant, and third-spacing action. For BNP special attention is directed to its role as a negative modulator of ventricular remodeling, in view of its anti-hypertrophic, anti-fibrotic and anti-inflammatory effects in the heart. The major effect of CNP in promoting vertebral and longitudinal bone growth is briefly addressed. Finally, emphasis is placed on the recent discovery that ANP affects fat metabolism in humans due to its powerful lipolytic action.

Role of leptin in blood pressure regulation and arterial hypertension

Leptin is a 16-kDa protein secreted by white adipose tissue that is primarily involved in the regulation of food intake and energy expenditure. Plasma leptin concentration is proportional to the amount of adipose tissue and is markedly increased in obese individuals. Recent studies suggest that leptin is involved in cardiovascular complications of obesity, including arterial hypertension. Acutely administered leptin has no effect on blood pressure, probably because it concomitantly stimulates the sympathetic nervous system and counteracting depressor mechanisms such as natriuresis and nitric oxide (NO)-dependent vasorelaxation. By contrast, chronic hyperleptinemia increases blood pressure because these acute depressor effects are impaired and/or additional sympathetic nervous system-independent pressor effects appear, such as oxidative stress, NO deficiency, enhanced renal Na reabsorption and overproduction of endothelin. Although the cause-effect relationship between leptin and high blood pressure in humans has not been demonstrated directly, many clinical studies have shown elevated plasma leptin in patients with essential hypertension and a significant positive correlation between leptin and blood pressure independent of body adiposity both in normotensive and in hypertensive individuals. In addition, leptin may contribute to end-organ damage in hypertensive individuals such as left ventricular hypertrophy, retinopathy and nephropathy, independent of regulating blood pressure. Here, current knowledge about the role of leptin in the regulation of blood pressure and in the pathogenesis of arterial hypertension is presented.

An Unsuspected Metabolic Role for Atrial Natriuretic Peptides

In normal and obese humans, lipid mobilization and systemic nonesterified fatty acid levels are thought to be acutely controlled by catecholamines (ie, epinephrine and norepinephrine) and insulin. Natriuretic peptides (NPs) are known to play a key role in the regulation of salt and water balance and blood pressure homeostasis. They are involved in the pathophysiology of hypertension and heart failure. NPs have recently been found to exert potent lipolytic effects (ie, activating the breakdown of stored triacylglycerols) in isolated human fat cells and to promote lipid mobilization in vivo. Atrial natriuretic peptide increases the intracellular 3', 5'-cyclic guanosine monophosphate (cGMP) concentration which activates cGMP-dependent protein kinase leading to perilipin and hormone-sensitive lipase phosphorylation and lipolysis. NPs promote lipid mobilization when administered intravenously. NPs are also responsible for the residual lipid-mobilizing action observed under oral beta-blockade in subjects performing physical exercise. NPs are therefore novel factors which may open promising research pathways to explain the control of lipid mobilization in physiological and pathological conditions. The metabolic impact of altered production and circulation of NPs remains to be established. The potential influence of NPs on the development of lipid disorders, obesity-related cardiovascular events, and cardiac cachexia will be discussed in this review.

Surface protein expression between human adipose tissue-derived stromal cells and mature adipocytes

Adipose tissue contains a stroma that can be easily isolated. Thus, human adipose tissue presents an source of multipotent stromal cells. In order to determine the implication of hematopoietic markers in adipocyte biology, we have defined part of the phenotype of the human adipose tissue-derived stromal cells, and compared this to fully differentiated adipocytes. Flow cytometry demonstrates that the protein expression phenotype of both cell types are similar and includes the expression of CD10, CD13, CD34, CD36, CD55, CD59 and CD65. No significant difference between subcutaneous and omental adipose tissue could be demonstrated concerning the expression of these markers. However, the expression of CD34, CD36 and CD65 is cell-dependent. While the expression of CD36 and CD65 doubled between stromal cells and mature adipocytes, the expression of CD34 decreased, despite this protein being present on the mature adipocyte. As CD34 is described as a stem cell marker and it being unlikely to be expressed on differentiated cells, this result was confirmed by immunostaining and western blot. The clear function of this protein on the adipocyte membrane remains to be determined. The characterization of new proteins on mature adipocytes could have broad implications for the comprehension of the biology of this tissue.

Differential regulation of atrial natriuretic peptide- and adrenergic receptor-dependent lipolytic pathways in human adipose tissue

The aim of the study was to investigate the regulation affecting the recently described atrial natriuretic peptide (ANP)-dependent lipolytic pathway in comparison with the adrenergic lipolytic cascade. We studied in vivo the effect of a euglycemic-hyperinsulinemic clamp on the changes occurring in the extracellular glycerol concentration (EGC) of subcutaneous adipose tissue (SCAT) during ANP or epinephrine perfusion in a microdialysis probe. Homologous desensitization and the incidence of hyperinsulinemia on the ANP- and catecholaminergic-dependent control of lipolysis were also investigated in vitro on fat cells from SCAT. When perfused in SCAT, epinephrine and ANP promoted an increase in EGC; the EGC increase was significantly lower during the clamp. The reduction of epinephrine-induced lipolysis was limited (18%) when phentolamine (an alpha(2)-adrenergic receptor [AR] antagonist) was perfused together with epinephrine. Unlike the effect of epinephrine, the response to ANP observed during the second perfusion was reduced by 32%. The increase in extracellular guanosine 3',5' -cyclic monophosphate concentration, which reflects ANP activity, was also reduced during the second perfusion. Desensitization of the lipolytic effects of ANP was observed in vitro after a 2-hour period of recovery, while the effects of alpha(2)-AR agonist or of epinephrine were unchanged. Insulin was without any effect on ANP-induced lipolysis and alpha(2)-AR-mediated antilipolysis, while it reduced beta-AR-induced lipolysis. The ANP-dependent lipolytic pathway undergoes desensitization in vitro and in situ. Insulin had no inhibitory effect on either ANP- or alpha(2)-AR-dependent pathways, while it counteracted the beta-AR pathway.