Vascular smooth muscle-specific deletion of the leptin receptor attenuates leptin-induced alterations in vascular relaxation

Regulation of cardiovascular health and disease by visceral adipose tissue-derived metabolic hormones

Visceral adipose tissue (VAT) is a metabolic organ known to regulate fat mass, and glucose and nutrient homeostasis. VAT is an active endocrine gland that synthesizes and secretes numerous bioactive mediators called 'adipocytokines/adipokines' into systemic circulation. These adipocytokines act on organs of metabolic importance like the liver and skeletal muscle. Multiple preclinical and in vitro studies showed strong evidence of the roles of adipocytokines in the regulation of metabolic disorders like diabetes, obesity and insulin resistance. Adipocytokines, such as adiponectin and omentin, are anti-inflammatory and have been shown to prevent atherogenesis by increasing nitric oxide (NO) production by the endothelium, suppressing endothelium-derived inflammation and decreasing foam cell formation. By inhibiting differentiation of vascular smooth muscle cells (VSMC) into osteoblasts, adiponectin and omentin prevent vascular calcification. On the other hand, adipocytokines like leptin and resistin induce inflammation and endothelial dysfunction that leads to vasoconstriction. By promoting VSMC migration and proliferation, extracellular matrix degradation and inflammatory polarization of macrophages, leptin and resistin increase the risk of atherosclerotic plaque vulnerability and rupture. Additionally, the plasma concentrations of these adipocytokines alter in ageing, rendering older humans vulnerable to cardiovascular disease. The disturbances in the normal physiological concentrations of these adipocytokines secreted by VAT under pathological conditions impede the normal functions of various organs and affect cardiovascular health. These adipokines could be used for both diagnostic and therapeutic purposes in cardiovascular disease.

Smooth Muscle Cells from a Rat Model of Obesity and Hyperleptinemia Are Partially Resistant to Leptin-Induced Reactive Oxygen Species Generation

The aim of this study was to evaluate the effect of leptin on reactive oxygen species’ (ROS) generation of smooth muscle cells (SMCs) from a rat model of obesity and hyperleptinemia. Obesity and hyperleptinemia were induced in rats by a sucrose-based diet for 24 weeks. ROS generation was detected by using dichloro-dihydrofluorescein (DCF), a fluorescent ROS probe in primary SMCs culture. An increase in plasma leptin and oxidative stress markers was observed in sucrose-fed (SF) rats. At baseline SMCs from SF rats showed a more than twofold increase in fluorescence intensity (FI) compared to that obtained in control (C) cells. When the C cells were treated with 20 ng leptin, the FI increased by about 200%, whereas the leptin-induced FI in the SF cells increased only by 60%. In addition, sucrose feeding increased the levels of p22phox and gp91phox, subunits of Nox as an O2•− source in SMCs. Treatment of cells with leptin significantly increased p22phox and gp91phox levels in C cells and did not affect SF cells. Regarding STAT3 phosphorylation and the content of PTP1B and SOCS3 as protein markers of leptin resistance, they were found to be significantly increased in SF cells. These results suggest that SF aortic SMCs are partially resistant to leptin-induced ROS generation.

Mechanisms of leptin-induced endothelial dysfunction

PURPOSE OF REVIEW

Endothelial dysfunction is a major risk factor for many cardiovascular diseases, notably hypertension. Obesity increases the risk of endothelial dysfunction in association with increasing production of the adipokine leptin. Preclinical studies have begun to unravel the mechanisms whereby leptin leads to the development of endothelial dysfunction, which are sex-specific. This review will summarize recent findings of mechanisms of leptin-induced endothelial impairment in both male and females and in pregnancy.

RECENT FINDINGS

Leptin receptors are found in high concentrations in the central nervous system (CNS), via which leptin promotes appetite suppression and upregulates sympathetic nervous system activation. However, leptin receptors are expressed in many other tissues, including the vascular endothelial cells and smooth muscle cells. Recent studies in mice with vascular endothelial or smooth muscle-specific knockdown demonstrate that endothelial leptin receptor activation plays a protective role against endothelial dysfunction in male animals, but not necessarily in females. Clinical studies indicate that women may be more sensitive to obesity-associated vascular endothelial dysfunction. Emerging preclinical data indicates that leptin and progesterone increase aldosterone production and endothelial mineralocorticoid receptor activation, respectively. Furthermore, decades of clinical studies indicate that leptin levels increase in the hypertensive pregnancy disorder preeclampsia, which is characterized by systemic endothelial dysfunction. Leptin infusion in mice induces the clinical characteristics of preeclampsia, including endothelial dysfunction.

SUMMARY

Novel preclinical data indicate that the mechanisms whereby leptin promotes endothelial dysfunction are sex-specific. Leptin-induced endothelial dysfunction may also play a role in hypertensive pregnancy as well.

Effects of L-Citrulline Supplementation and Aerobic Training on Vascular Function in Individuals with Obesity across the Lifespan

Children with obesity are at higher risk for developing cardiometabolic diseases that once were considered health conditions of adults. Obesity is commonly associated with cardiometabolic risk factors such as dyslipidemia, hyperglycemia, hyperinsulinemia and hypertension that contribute to the development of endothelial dysfunction. Endothelial dysfunction, characterized by reduced nitric oxide (NO) production, precedes vascular abnormalities including atherosclerosis and arterial stiffness. Thus, early detection and treatment of cardiometabolic risk factors are necessary to prevent deleterious vascular consequences of obesity at an early age. Non-pharmacological interventions including L-Citrulline (L-Cit) supplementation and aerobic training stimulate endothelial NO mediated vasodilation, leading to improvements in organ perfusion, blood pressure, arterial stiffness, atherosclerosis and metabolic health (glucose control and lipid profile). Few studies suggest that the combination of L-Cit supplementation and exercise training can be an effective strategy to counteract the adverse effects of obesity on vascular function in older adults. Therefore, this review examined the efficacy of L-Cit supplementation and aerobic training interventions on vascular and metabolic parameters in obese individuals.

Vascular effects of disrupting endothelial mTORC1 signaling in obesity

The mechanistic target of rapamycin complex 1 (mTORC1) signaling complex is emerging as a critical regulator of cardiovascular function with alterations in this pathway implicated in cardiovascular diseases. In this study, we used animal models and human tissues to examine the role of vascular mTORC1 signaling in the endothelial dysfunction associated with obesity. In mice, obesity induced by high-fat/high-sucrose diet feeding for ∼2 mo resulted in aortic endothelial dysfunction without appreciable changes in vascular mTORC1 signaling. On the other hand, chronic high-fat diet feeding (45% or 60% kcal: ∼9 mo) in mice resulted in endothelial dysfunction associated with elevated vascular mTORC1 signaling. Endothelial cells and visceral adipose vessels isolated from obese humans display a trend toward elevated mTORC1 signaling. Surprisingly, genetic disruption of endothelial mTORC1 signaling through constitutive or tamoxifen inducible deletion of endothelial Raptor (critical subunit of mTORC1) did not prevent or rescue the endothelial dysfunction associated with high-fat diet feeding in mice. Endothelial mTORC1 deficiency also failed to reverse the endothelial dysfunction evoked by a high-fat/high-sucrose diet in mice. Taken together, these data show increased vascular mTORC1 signaling in obesity, but this vascular mTORC1 activation appears not to be required for the development of endothelial impairment in obesity.

Leptin in Atherosclerosis: Focus on Macrophages, Endothelial and Smooth Muscle Cells

Increasing adipose tissue mass in obesity directly correlates with elevated circulating leptin levels. Leptin is an adipokine known to play a role in numerous biological processes including regulation of energy homeostasis, inflammation, vascular function and angiogenesis. While physiological concentrations of leptin may exhibit multiple beneficial effects, chronically elevated pathophysiological levels or hyperleptinemia, characteristic of obesity and diabetes, is a major risk factor for development of atherosclerosis. Hyperleptinemia results in a state of selective leptin resistance such that while beneficial metabolic effects of leptin are dampened, deleterious vascular effects of leptin are conserved attributing to vascular dysfunction. Leptin exerts potent proatherogenic effects on multiple vascular cell types including macrophages, endothelial cells and smooth muscle cells; these effects are mediated via an interaction of leptin with the long form of leptin receptor, abundantly expressed in atherosclerotic plaques. This review provides a summary of recent in vivo and in vitro studies that highlight a role of leptin in the pathogenesis of atherosclerotic complications associated with obesity and diabetes.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Leptin receptor stimulation in late pregnant mouse uterine tissue inhibits spontaneous contractions by increasing NO and cGMP

The adipokine, leptin exerts inhibitory effect on both spontaneous and oxytocin-induced contractions in myometrium. However, the mechanisms involved in leptin-induced effect are not clear. In the present study, we studied the altered characteristics of uterine contractions in the presence of leptin and the possible mechanisms of its effect in late pregnant (18.5 day) mouse uterus. We conducted functional, biochemical and molecular biology studies to demonstrate the mechanism of leptin-induced response. Leptin exerted an inhibitory response (E_max 40.5 ± 3.99%) on basal uterine contractions. The extent of inhibition was less than that obtained with known uterine relaxants, salbutamol (E_max103 ± 8.66%) and BRL-37344 (E_max 84.79 ± 8.12%). Leptin-induced uterine response was inhibited by leptin receptor antagonist SHLA and JAK-STAT pathway inhibitor, AG-490. The relaxant response was also subdued by NO-cGMP-PK-G pathway blockers L-NAME, 1400W, ODQ and KT-5823. Further, leptin enhanced the levels of NO and cGMP in uterine tissues. Also, SHLA, AG-490 and a combination of 1400 W and L-NAME prevented leptin-induced increase in NO. Similar effect was observed on cGMP levels in presence of leptin and SHLA. However, leptin did not influence CaCl₂-induced response in potassium-depolarized tissues. We also detected leptin receptor protein in late pregnant mouse uterus located in endometrial luminal epithelium and myometrial layers. Real-time PCR studies revealed significantly higher expression of short forms of the receptor (ObRa and ObRc) in comparison to the long form (ObRb). In conclusion, the results of the present study suggest that leptin inhibits mouse uterine contraction by stimulating short forms of the leptin receptors and activating NO pathway in a JAK-STAT-dependent manner.

Role of endothelium-independent vasodilation in the relationship between visceral adiposity and reduced cognitive performance in older adults with type 2 diabetes

OBJECTIVE

In this cross-sectional analysis, we sought to assess the relationship of adiposity and forearm microvascular reactivity with cognitive dysfunction among older Asians with type 2 diabetes (T2D).

METHODS

Subjects with T2D aged ≥ 55 years were analyzed (N = 907). Cognitive performance was assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and Mini-Mental State Exam (MMSE). Visceral fat area (VFA) was estimated by tetrapolar multi-frequency bioimpedance. Forearm microvascular endothelium-dependent vasodilation (EDV) and endothelium-independent vasodilation (EIV) were assessed by laser Doppler imaging with iontophoresis.

RESULTS

RBANS total score was correlated with VFA, EDV, and EIV (all P < .05). However, VFA was correlated with EIV, but not with EDV. Multivariable linear regression showed significant association between VFA and RBANS total score (B = -0.02, 95% CI= -0.03 to -0.01) or memory (immediate and delayed) index scores. These associations were attenuated after adjustment for EIV. Mediation analysis showed that EIV partially mediated the relationship between visceral adiposity and RBANS scores (all Sobel tests P < .05). EIV also mediated the relationship between VFA and MMSE score.

CONCLUSIONS

Impaired endothelium-independent vascular smooth muscle reactivity may exert a mediatory effect on the association between increased visceral adiposity and decreased cognitive performance in older adults with T2D.

Oxidative and inflammatory signals in obesity-associated vascular abnormalities

Obesity is associated with increased cardiovascular morbidity and mortality in part due to vascular abnormalities such as endothelial dysfunction and arterial stiffening. The hypertension and other health complications that arise from these vascular defects increase the risk of heart diseases and stroke. Prooxidant and proinflammatory signaling pathways as well as adipocyte-derived factors have emerged as critical mediators of obesity-associated vascular abnormalities. Designing treatments aimed specifically at improving the vascular dysfunction caused by obesity may provide an effective therapeutic approach to prevent the cardiovascular sequelae associated with excessive adiposity. In this review, we discuss the recent evidence supporting the role of oxidative stress and cytokines and inflammatory signals within the vasculature as well as the impact of the surrounding perivascular adipose tissue (PVAT) on the regulation of vascular function and arterial stiffening in obesity. In particular, we focus on the highly plastic nature of the vasculature in response to altered oxidant and inflammatory signaling and highlight how weight management can be an effective therapeutic approach to reduce the oxidative stress and inflammatory signaling and improve vascular function.