No abstract

1 Possible nitric oxide (NO)-mediated eects on lipolysis were investigated in vivo in human subcutaneous adipose tissue using microdialysis, as well as in vitro on isolated fat cells of non-obese, healthy volunteers. NO donors were added to the ingoing dialysate solvents. 2 Changes in lipolysis and local blood¯ow were investigated by measuring glycerol levels and ethanol ratios, respectively, in the microdialysates. 4 Nitric oxide gas as well as the NO donor, nitroglycerine, reduced glycerol release from isolated adipocytes in vitro. 5 Expression of inducible nitric oxide synthase (iNOS) in human adipose tissue was shown by Western blot analysis. Biologically active NOS was demonstrated by measuring total enzymatic activity. 6 In conclusion, the data demonstrate that inhibition of NO release in subcutaneous adipose tissue results in an increased lipolysis in vivo. These eects, which were also observed in vitro, are independent of local blood¯ow changes. Furthermore, the demonstration of enzymatic NOS activity and the expression of inducible nitric oxide synthase (iNOS) in adipose tissue indicate that locally synthesized NO may play a role in the physiological control of lipolysis in human adipose tissue.

In isolated adipocytes, the nitrosothiols S-nitroso-Nacetyl-penicillamine (SNAP) and S-nitrosoglutathione stimulate basal lipolysis, whereas the nitric oxide (NO ⅐ ) donor 1-propamine, 3-(2-hydroxy-2-nitroso-1-propylhydrazine) (PAPA-NONOate) or NO gas have no effect. The increase in basal lipolysis due to nitrosothiols was prevented by dithiothreitol but not by a guanylate cyclase inhibitor. In addition the cyclic GMP-inhibited low K m , cyclic AMP phosphodiesterase activity was inhibited by SNAP suggesting that SNAP acting as NO ؉ donor increases basal lipolysis through a S-nitrosylation mediated inhibition of phosphodiesterase. Contrasting with these findings, SNAP reduced both isoproterenol-stimulated lipolysis and cyclic AMP production, whereas it failed to modify forskolin-, dibutyryl cyclic AMP-, or isobutylmethylxanthine-stimulated lipolysis, suggesting that SNAP interferes with the ␤-adrenergic signal transduction pathway upstream the adenylate cyclase. In contrast with SNAP, PAPA-NONOate or NO gas inhibited stimulated lipolysis whatever the stimulating agents used without altering cyclic AMP production. Moreover PAPA-NONOate slightly reduces (30%) the hormone-sensitive lipase (HSL) activity indicating that stimulated lipolysis inhibition by NO ⅐ is linked to both inhibition of the HSL activity and the cyclic AMP-dependent activation of HSL. These data suggest that NO ⅐ or related redox species like NO ؉ /NO ؊ are potential regulators of lipolysis through distinct mechanisms.Nitric oxide (NO) 1 has emerged as a chemical messenger in several biological systems. This molecule, which is the smallest biological signal known in mammalian cells, can control vital functions such as neurotransmission and blood vessel tone as well as host defense and immunity (1, 2). Some of the effects due to NO are elicited through the activation of soluble guanylate cyclase, leading to an increase in intracellular cyclic GMP content (3). In addition NO and NO-related species interact with redox metal-containing proteins and/or with thiol groups of proteins (4). It has been suggested that S-nitrosylation of proteins could mediate signaling functions and that thiols may be involved in the stabilization and metabolism of NO (5,6).NO is synthesized via L-arginine oxidation by a family of nitric oxide synthase isoforms (NOS). NOS are either constitutively expressed and calcium/calmodulin-dependent (NOS I and NOS III were originally described in neuronal tissue and endothelial cells, respectively) or inducible and almost calcium/ calmodulin-independent (NOS II was originally identified in macrophages) (7). We have recently shown that white adipose tissue expresses the NOS II and NOS III isoforms (8). The constitutive expression of NOS II in this tissue can be related to the fact that tumor necrosis factor ␣ is expressed and secreted by adipose tissue, which is also an important target of this cytokine (9). Indeed tumor necrosis factor ␣ was reported to decrease lipoprotein lipase activity and expression (10,11) and to stimulate lipo...