Effects of Chronic Nitric Oxide Inhibition on the Renal Excretory Response to Leptin

Functional Relationship between Leptin and Nitric Oxide in Metabolism

Leptin, the product of the ob gene, was originally described as a satiety factor, playing a crucial role in the control of body weight. Nevertheless, the wide distribution of leptin receptors in peripheral tissues supports that leptin exerts pleiotropic biological effects, consisting of the modulation of numerous processes including thermogenesis, reproduction, angiogenesis, hematopoiesis, osteogenesis, neuroendocrine, and immune functions as well as arterial pressure control. Nitric oxide (NO) is a free radical synthesized from L-arginine by the action of the NO synthase (NOS) enzyme. Three NOS isoforms have been identified: the neuronal NOS (nNOS) and endothelial NOS (eNOS) constitutive isoforms, and the inducible NOS (iNOS). NO mediates multiple biological effects in a variety of physiological systems such as energy balance, blood pressure, reproduction, immune response, or reproduction. Leptin and NO on their own participate in multiple common physiological processes, with a functional relationship between both factors having been identified. The present review describes the functional relationship between leptin and NO in different physiological processes.

Novel molecular aspects of ghrelin and leptin in the control of adipobiology and the cardiovascular system

Ghrelin and leptin show opposite effects on energy balance. Ghrelin constitutes a gut hormone that is secreted to the bloodstream in two major forms, acylated and desacyl ghrelin. The isoforms of ghrelin not only promote adiposity by the activation of hypothalamic orexigenic neurons but also directly stimulate the expression of several fat storage-related proteins in adipocytes, including ACC, FAS, LPL and perilipin, thereby stimulating intracytoplasmic lipid accumulation. Moreover, both acylated and desacyl ghrelin reduce TNF-α-induced apoptosis and autophagy in adipocytes, suggesting an anti-inflammatory role of ghrelin in human adipose tissue. On the other hand, leptin is an adipokine with lipolytic effects. In this sense, leptin modulates via PI3K/Akt/mTOR the expression of aquaglyceroporins such as AQP3 and AQP7 that facilitate glycerol efflux from adipocytes in response to the lipolytic stimuli via its translocation from the cytosolic fraction (AQP3) or lipid droplets (AQP7) to the plasma membrane. Ghrelin and leptin also participate in the homeostasis of the cardiovascular system. Ghrelin operates as a cardioprotective factor with increased circulating acylated ghrelin concentrations in patients with left ventricular hypertrophy (LVH) causally related to LV remodeling during the progression to LVH. Additionally, leptin induces vasodilation by inducible NO synthase expression (iNOS) in the vascular wall. In this sense, leptin inhibits the angiotensin II-induced Ca(2+) increase, contraction and proliferation of VSMC through NO-dependent mechanisms. Together, dysregulation of circulating ghrelin isoforms and leptin resistance associated to obesity, type 2 diabetes, or the metabolic syndrome contribute to cardiometabolic derangements observed in these pathologies.

Beyond salt: lifestyle modifications and blood pressure

Lifestyle changes have been shown to effect significant blood pressure (BP) reductions. Although there are several proposed neurohormonal links between weight loss and BP, body mass index itself appears to be the most powerful mediator of the weight-BP relationship. There appears to be a mostly linear relationship between weight and BP; as weight is regained, the BP benefit is mostly lost. Physical activity, but more so physical fitness (the physiological benefit obtained from physical activity), has a dose-dependent BP benefit but reaches a plateau at which there is no further benefit. However, even just a modest physical activity can have a meaningful BP effect. A diet rich in fruits and vegetables with low-fat dairy products and low in saturated and total fat (DASH) is independently effective in reducing BP. Of the dietary mineral nutrients, the strongest data exist for increased potassium intake, which reduces BP and stroke risk. Vitamin D is associated with BP benefit, but no causal relationship has been established. Flavonoids such as those found in cocoa and berries may have a modest BP benefit. Neither caffeine nor nicotine has any significant, lasting BP effect. Biofeedback therapies such as those obtained with device-guided breathing have a modest and safe BP benefit; more research is needed before such therapies move beyond those having an adjunctive treatment role. There is a strong, linear relationship between alcohol intake and BP; however, the alcohol effects on BP and coronary heart disease are divergent. The greatest BP benefit seems to be obtained with one drink per day for women and with two per day for men. This benefit is lost or attenuated if the drinking occurs in a binge form or without food. Overall, the greatest and most sustained BP benefit is obtained when multiple lifestyle interventions are incorporated simultaneously.

Obesity hypertension: the regulatory role of leptin

Leptin is a 16-kDa-peptide hormone that is primarily synthesized and secreted by adipose tissue. One of the major actions of this hormone is the control of energy balance by binding to receptors in the hypothalamus, leading to reduction in food intake and elevation in temperature and energy expenditure. In addition, increasing evidence suggests that leptin, through both direct and indirect mechanisms, may play an important role in cardiovascular and renal regulation. While the relevance of endogenous leptin needs further clarification, it appears to function as a pressure and volume-regulating factor under conditions of health. However, in abnormal situations characterized by chronic hyperleptinemia such as obesity, it may function pathophysiologically for the development of hypertension and possibly also for direct renal, vascular, and cardiac damage.

Leptin inhibits the proliferation of vascular smooth muscle cells induced by angiotensin II through nitric oxide-dependent mechanisms

Objective. This study was designed to investigate whether leptin modifies angiotensin (Ang) II-induced proliferation of aortic vascular smooth muscle cells (VSMCs) from 10-week-old male Wistar and spontaneously hypertensive rats (SHR), and the possible role of nitric oxide (NO). Methods. NO and NO synthase (NOS) activity were assessed by the Griess and ³H-arginine/citrulline conversion assays, respectively. Inducible NOS (iNOS) and NADPH oxidase subutnit Nox2 expression was determined by Western-blot. The proliferative responses to Ang II were evaluated through enzymatic methods. Results. Leptin inhibited the Ang II-induced proliferative response of VSMCs from control rats. This inhibitory effect of leptin was abolished by NOS inhibitor, NMMA, and iNOS selective inhibitor, L-NIL, and was not observed in leptin receptor-deficient fa/fa rats. SHR showed increased serum leptin concentrations and lipid peroxidation. Despite a similar leptin-induced iNOS up-regulation, VSMCs from SHR showed an impaired NOS activity and NO production induced by leptin, and an increased basal Nox2 expression. The inhibitory effect of leptin on Ang II-induced VSMC proliferation was attenuated. Conclusion. Leptin blocks the proliferative response to Ang II through NO-dependent mechanisms. The attenuation of this inhibitory effect of leptin in spontaneous hypertension appears to be due to a reduced NO bioavailability in VSMCs.

Leptin: linking obesity, the metabolic syndrome, and cardiovascular disease

The incidence and prevalence of obesity and the metabolic syndrome have risen markedly in the past decade, representing a serious cardiovascular health hazard with significant morbidity and mortality. The etiology of the metabolic syndrome and its various pathogenic mechanisms are incompletely defined and under intense investigation. Contemporary research suggests that the adipocyte-derived hormone leptin may be an important factor linking obesity, the metabolic syndrome, and cardiovascular disorders. Although recent evidence indicates that under normal conditions leptin may be an important factor in regulating pressure and volume, during situations of chronic hyperleptinemia and leptin resistance, this hormone may function pathophysiologically for the development of hypertension and cardiac and renal diseases. Future research will determine if reduction of circulating leptin and/or blockade of its peripheral actions can confer cardiovascular and renal protection in hyperleptinemic patients with obesity and the metabolic syndrome.

Leptin, a novel predictor of lung function in heart failure

BACKGROUND

Leptin is a protein hormone produced by adipose tissue. Leptin has proinflammatory properties and is usually elevated in patients with chronic heart failure. We assessed if serum leptin relates to the loss in lung function in noncachectic patients with chronic heart failure.

MATERIALS AND METHODS

One hundred thirty-five consecutively eligible non-Hispanic white subjects (age, 24 to 79 years; 85 men and 50 women) with a diagnosis of stable systolic heart failure were recruited prospectively, along with 106 matched control subjects. FVC, FEV(1), and single-breath diffusing capacity of the lung for carbon monoxide (DLco) were measured by spirometry. Plasma leptin was measured by radioimmunoassay. Multiple linear regression was applied.

RESULTS

The relationships of FEV(1), FVC, and DLco with leptin differed significantly between heart failure and control subjects after controlling for age, sex, percentage of body fat, and ejection fraction. In heart failure, leptin was as an independent predictor of FVC values (additional R(2) = 0.05, p < 0.0001), FEV(1) values (additional R(2) = 0.05, p < 0.0001), and DLco values (additional R(2) = 0.14, p < 0.0001). In a final multiple regression model predicting lung function in heart failure, the independent effect of leptin was significant after further adjustments.

CONCLUSIONS

The predictive information provided by leptin is additive to that provided by measures of body fat in heart failure patients, especially for DLco. Leptin may play a role in the impairment of lung function in subjects with heart failure.

Obesity-hypertension: emerging concepts in pathophysiology and treatment

The incidence and prevalence of obesity has risen markedly in the last decade, and this epidemic represents a serious health hazard with significant morbidity and mortality. Although hypertension is recognized as one of the most serious consequences of obesity, its pathophysiology remains incompletely understood. Contemporary research suggests that the recently discovered hormone leptin may represent a common link between these 2 pathologic conditions. Leptin is primarily synthesized and secreted by adipocytes. One of the major functions of this hormone is the control of energy balance. By binding to receptors in the hypothalamus, it reduces food intake and promotes elevation in temperature and energy expenditure. In addition, increasing evidence suggests that leptin, through both direct and indirect actions, may play an important role in cardiovascular and renal functions. Although the relevance of endogenous leptin needs further clarification for the control of renal sodium excretion and vascular tone, it appears to be a potential pressure and volume-regulating factor in normal situations. However, in conditions of chronic hyperleptinemia, such as obesity, leptin may function pathophysiologically for the development of hypertension as well as cardiac and renal disease. Thus, in addition to weight control, reduction of circulating leptin may confer cardiovascular and renal protective effects in patients with obesity-hypertension.

Regulation of renal ouabain-resistant Na+-ATPase by leptin, nitric oxide, reactive oxygen species, and cyclic nucleotides: implications for obesity-associated hypertension

This study examined the effect of leptin on renal ouabain-resistant Na(+)-ATPase, which drives the reabsorption of about 10% of sodium transported in the proximal tubule. Chronic leptin administration (0.25 mg/kg s.c. twice daily for seven days) increased Na(+)-ATPase activity by 62.9%. This effect was prevented by the coadministration of superoxide dismutase mimetic, tempol, or the NADPH oxidase inhibitor, apocynin (2 mM in the drinking water). Acutely administered NO donors decreased Na(+)-ATPase activity. This effect was abolished by soluble guanylate cyclase inhibitor, ODQ, but not by protein kinase G inhibitors. Exogenous cGMP reduced Na(+)-ATPase activity, but its synthetic analogues, 8-bromo-cGMP and 8-pCPT-cGMP, were ineffective. The inhibitory effect of NO donors and cGMP was abolished by EHNA, an inhibitor of cGMP-stimulated phosphodiesterase (PDE2). Exogenous cAMP analogue and dibutyryl-cAMP increased Na(+)-ATPase activity and abolished the inhibitory effect of cGMP. Finally, the administration of superoxide-generating mixture (xanthine oxidase+hypoxanthine) increased Na(+)-ATPase activity. The results suggest that nitric oxide decreases renal Na(+)-ATPase activity by stimulating cGMP, which in turn activates PDE2 and decreases cAMP concentration. Increased production of reactive oxygen species may lead to the elevation of Na(+)-ATPase activity by scavenging NO and limiting its inhibitory effect. Chronic hyperleptinemia is associated with increased Na(+)-ATPase activity due to excessive oxidative stress.

Leptin and hypertension in obesity

Leptin, a peptide discovered more than 10 years ago, decreases food intake and increases sympathetic nerve activity to both thermogenic and non-thermogenic tissue. Leptin was initially believed to be an anti-obesity hormone, owing to its metabolic effects. However, obese individuals, for unknown reasons, become resistant to the satiety and weight-reducing effect of the hormone, but preserve leptin-mediated sympathetic activation to non-thermogenic tissue such as kidney, heart, and adrenal glands. Leptin has been shown to influence nitric oxide production and natriuresis, and along with chronic sympathetic activation, especially to the kidney, it may lead to sodium retention, systemic vasoconstriction, and blood pressure elevation. Consequently, leptin is currently considered to play an important role in the development of hypertension in obesity.

Role of PI3K and PKB/Akt in acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, a peptide hormone secreted by white adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have documented that leptin stimulates natriuresis and nitric oxide (NO) production, but the mechanism of these effects is incompletely elucidated. We examined whether phosphoinositide 3-kinase (PI3K) and its downstream effector, protein kinase B/Akt are involved in acute natriuretic and NO-mimetic effects of leptin in anaesthetized rats. Leptin (1 mg/kg i.v.) induced a marked increase in natriuresis and this effect was abolished by pretreatment with either wortmannin (15 microg/kg) or LY294002 (0.6 mg/kg), two structurally different PI3K inhibitors. Moreover, leptin increased plasma concentration and urinary excretion of NO metabolites, nitrites+nitrates (NO(x)), and of NO second messenger, cyclic GMP. In addition, leptin increased NO(x) and cGMP in aortic tissue. The stimulatory effect of leptin on NO(x) and cGMP was prevented by PKB/Akt inhibitor, triciribine, but not by either wortmannin or LY294002. Triciribine had no effect on leptin-induced natriuresis. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney. These results suggest that leptin-induced natriuresis is mediated by PI3K but not Akt, whereas NO-mimetic effect of leptin results from PI3K-independent stimulation of Akt.

The inhibitory effect of leptin on angiotensin II-induced vasoconstriction in vascular smooth muscle cells is mediated via a nitric oxide-dependent mechanism

Leptin inhibits the contractile response induced by angiotensin (Ang) II in vascular smooth muscle cells (VSMCs) of the aorta. We studied in vitro and ex vivo the role of nitric oxide (NO) in the effect of leptin on the Ang II-induced vasoconstriction of the aorta of 10-wk-old Wistar rats. NO and nitric oxide synthase (NOS) activity were assessed by the Griess and (3)H-arginine/citrulline conversion assays, respectively. Stimulation of inducible NOS (iNOS) as well as Janus kinases/signal transducers and activators of transcription (JAK/STAT) and phosphoinositide 3-kinase (PI3K)/Akt signaling pathways were determined by Western blot. The contractile responses to Ang II were evaluated in endothelium-denuded aortic rings using the organ bath system. Changes in intracellular Ca(2+) were measured in VSMCs using fura-2 fluorescence. Leptin significantly (P < or = 0.01) stimulated NO release and NOS activity in VSMCs. Leptin's effect on NO was abolished by the NOS inhibitor, N(G)-monomethyl l-arginine, or the iNOS selective inhibitor L-N(6)-(1-iminoethyl)-lysine. Accordingly, leptin increased iNOS protein expression, with a comparable time course with that of NO production and NOS activity. Leptin also significantly increased STAT3 (P < or = 0.01) and Akt (P < or = 0.001) phosphorylation. Moreover, either the JAK2 inhibitor, AG490, or the PI3K inhibitor, wortmannin, significantly (P < or = 0.05) abrogated the leptin-induced increase in iNOS protein. Finally, both N(G)-monomethyl L-arginine and L-N(6)-(1-iminoethyl)-lysine inhibitors completely blunted (P < or = 0.001) the leptin-mediated inhibition of the Ang II-induced VSMC activation and vasoconstriction. These findings suggest that the endothelium-independent depressor action of leptin is mediated by an increase of NO bioavailability in VSMCs. This process requires the up-regulation of iNOS through mechanisms involving JAK2/STAT3 and PI3K/Akt pathways.

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.

Leptin blockade attenuates sodium excretion in saline-loaded normotensive rats

Previous investigations in normotensive animals have demonstrated a marked natriuretic and diuretic response following the acute administration of supraphysiologic doses of synthetic leptin. However, the importance of endogenous leptin in the regulation of renal sodium and water balance is not yet defined. This study examined the hemodynamic and renal excretory effects of circulating leptin blockade with a specific polyclonal antibody in groups of normotensive, chronically saline-loaded Sprague-Dawley rats. In the experimental group (n = 10), leptin antibody significantly decreased urinary sodium excretion and urinary flow by approximately 30% compared to the control rats (n = 10). Mean arterial pressure remained unchanged. Collectively, these results are interpreted to suggest that leptin is an important renal sodium-regulating factor under conditions of mild sodium and volume expansion.