Contribution of sphingolipids to the pathogenesis of obesity

Enhancement of Sphingomyelinase-Induced Endothelial Nitric Oxide Synthase-Mediated Vasorelaxation in a Murine Model of Type 2 Diabetes

Sphingolipids are important biological mediators both in health and disease. We investigated the vascular effects of enhanced sphingomyelinase (SMase) activity in a mouse model of type 2 diabetes mellitus (T2DM) to gain an understanding of the signaling pathways involved. Myography was used to measure changes in the tone of the thoracic aorta after administration of 0.2 U/mL neutral SMase in the presence or absence of the thromboxane prostanoid (TP) receptor antagonist SQ 29,548 and the nitric oxide synthase (NOS) inhibitor L-NAME. In precontracted aortic segments of non-diabetic mice, SMase induced transient contraction and subsequent weak relaxation, whereas vessels of diabetic (Lepr^db/Lepr^db, referred to as db/db) mice showed marked relaxation. In the presence of the TP receptor antagonist, SMase induced enhanced relaxation in both groups, which was 3-fold stronger in the vessels of db/db mice as compared to controls and could not be abolished by ceramidase or sphingosine-kinase inhibitors. Co-administration of the NOS inhibitor L-NAME abolished vasorelaxation in both groups. Our results indicate dual vasoactive effects of SMase: TP-mediated vasoconstriction and NO-mediated vasorelaxation. Surprisingly, in spite of the general endothelial dysfunction in T2DM, the endothelial NOS-mediated vasorelaxant effect of SMase was markedly enhanced.

Ceramide modulates electrophysiological characteristics and oxidative stress of pulmonary vein cardiomyocytes

BACKGROUND

Ceramide is involved in regulating metabolism and energy expenditure, and its abnormal myocardial accumulation may contribute to heart injury or lipotoxic cardiomyopathy. Whether ceramide can modulate the electrophysiology of pulmonary veins (PVs) remains unknown.

MATERIALS AND METHODS

We used conventional microelectrodes to measure the electrical activity of isolated rabbit PV tissue preparations before and after treatment with various concentrations of ceramide with or without H₂ O₂ (2 mM), MitoQ, wortmannin or 740 YP. A whole-cell patch clamp and fluorescence imaging were used to record the ionic currents, calcium (Ca²⁺ ) transients, and intracellular reactive oxygen species (ROS) and sodium (Na⁺ ) in isolated single PV cardiomyocytes before and after ceramide (1 μM) treatment.

RESULTS

Ceramide (0.1, 0.3, 1 and 3 μM) reduced the beating rate of PV tissues. Furthermore, ceramide (1 μM) suppressed the 2 mM H₂ O₂ -induced faster PV beating rate, triggered activities and burst firings, which were further reduced by MitoQ. In the presence of wortmannin, ceramide did not change the PV beating rate. The H₂ O₂ -induced faster PV beating rate could be counteracted by MitoQ or wortmannin with no additive effect from the ceramide. Ceramide inhibited pPI3K. Ceramide reduced Ca²⁺ transients, sarcoplasmic reticulum Ca²⁺ contents, L-type Ca²⁺ currents, Na⁺ currents, late Na⁺ currents, Na⁺ -hydrogen exchange currents, and intracellular ROS and Na⁺ in PV cardiomyocytes, but did not change Na⁺ -Ca²⁺ exchange currents.

CONCLUSION

C2 ceramide may exert the distinctive electrophysiological effect of modulating PV activities, which may be affected by PI3K pathway-mediated oxidative stress, and might play a role in the pathogenesis of PV arrhythmogenesis.

Central role of ceramide biosynthesis in body weight regulation, energy metabolism, and the metabolic syndrome

Although obesity is associated with multiple features of the metabolic syndrome (insulin resistance, leptin resistance, hepatic steatosis, chronic inflammation, etc.), the molecular changes that promote these conditions are not completely understood. Here, we tested the hypothesis that elevated ceramide biosynthesis contributes to the pathogenesis of obesity and the metabolic syndrome. Chronic treatment for 8 wk of genetically obese (ob/ob), and, high-fat diet-induced obese (DIO) mice with myriocin, an inhibitor of de novo ceramide synthesis, decreased circulating ceramides. Decreased ceramide was associated with reduced weight, enhanced metabolism and energy expenditure, decreased hepatic steatosis, and improved glucose hemostasis via enhancement of insulin signaling in the liver and muscle. Inhibition of de novo ceramide biosynthesis decreased adipose expression of suppressor of cytokine signaling-3 (SOCS-3) and induced adipose uncoupling protein-3 (UCP3). Moreover, ceramide directly induced SOCS-3 and inhibited UCP3 mRNA in cultured adipocytes suggesting a direct role for ceramide in regulation of metabolism and energy expenditure. Inhibition of de novo ceramide synthesis had no effect on adipose tumor necrosis factor-alpha (TNF-alpha) expression but dramatically reduced adipose plasminogen activator inhibitor-1 (PAI-1) and monocyte chemoattactant protein-1 (MCP-1). This study highlights a novel role for ceramide biosynthesis in body weight regulation, energy expenditure, and the metabolic syndrome.

Sphingolipids: players in the pathology of metabolic disease

The contribution of aberrant production of bioactive lipids to pathophysiological changes associated with obesity has risen to the forefront of lipid research. Increased diacylglycerol has been appreciated as a cause of insulin resistance, but emerging data support a role for sphingolipids in other metabolic diseases including obesity, diabetes, atherosclerosis and metabolic syndrome. Recent data demonstrate that elevation of plasma free fatty acids promotes aberrant sphingolipid production and composition in various tissues including skeletal muscle, pancreas and adipocytes. Moreover, rectifying these aberrant sphingolipid profiles often attenuates pathologies associated with their production. Although data thus far generate more questions than they answer, they indicate a major contribution of sphingolipids to pathologies associated with obesity. This review summarizes recent work in these areas.