Nonadrenergic imidazoline binding sites and amine oxidase activities in fat cells

The imidazoline I2-site ligands BU 224 and 2-BFI inhibit MAO-A and MAO-B activities, hydrogen peroxide production, and lipolysis in rodent and human adipocytes

Numerous imidazolinic agents exhibit antihyperglycaemic properties and have been described to promote insulin secretion, however their effects on adipose tissue development have been poorly investigated. Since white adipose tissue (WAT) plays an important role in glucose homeostasis and expresses imidazoline (I(2)) binding sites abundantly, this work aimed at studying extrapancreatic actions of two I(2)-site ligands, BU 224 and 2-BFI in adipocytes. Interaction with monoamine oxidase (MAO) was investigated by measuring the ability to modulate [(14)C]tyramine oxidation and hydrogen peroxide production. Direct influence on glucose uptake or on lipolytic activity was tested on mouse, rat, rabbit and human adipocytes. BU 224 and 2-BFI behaved as reversible inhibitors of both MAO-A and -B, as demonstrated by total inhibition of tyramine oxidation in human adipocytes and platelets or in liver from rats previously treated with selective MAO-inhibitors. Moreover, they weakly inhibited semicarbazide-sensitive amine oxidase. Like classical MAO-inhibitors, they were unable to produce hydrogen peroxide and to activate glucose uptake but prevented tyramine to do so in rodent or human adipocytes. BU 224 and 2-BFI also differed from MAO-inhibitors since they inhibited lipolysis at millimolar concentrations via a still undefined pathway independent of alpha(2)-adrenoceptor stimulation, beta-adrenergic antagonism and MAO activation. However, chronic treatment of obese Zucker rats with 2-BFI did not modify the maximal lipolytic capacity or the mild insulin resistance status of their adipocytes. Taken together, our observations demonstrate on WAT novel effects of BU 224 and 2-BFI different from their already reported actions on brain or endocrine pancreas.

Lipid-lowering actions of imidazoline antihypertensive agents in metabolic syndrome X

Agonists active at I1-imidazoline receptors (I1R) not only lower blood pressure but also ameliorate glucose intolerance, insulin resistance, and hyperlipidemia with long-term treatment. We sought to determine the possible mechanism for the lipid-lowering actions of imidazolines in a model of metabolic Syndrome X, the spontaneously-hypertensive obese (SHROB) rat. The acute actions of moxonidine and rilmenidine, selective I1R agonists, were compared to a specific alpha2-adrenergic receptor agonist, guanabenz, with and without selective receptor blockers. Moxonidine and rilmenidine rapidly reduced plasma triglyceride (20+/-4% and 21+/-5%, respectively) and cholesterol (29+/-9% and 27+/-9%). In contrast, the specific alpha2-adrenergic receptor agonist guanabenz failed to reduce plasma lipids. Blocking experiments showed that moxonidine's actions were mediated by I1R and not alpha2-adrenergic receptors. To evaluate a hepatic site of action, radioligand binding studies with liver plasma membranes confirmed the presence of I1R. Intraportal moxonidine reduced plasma triglycerides by 23+/-3% within 10 min. Moxonidine inhibited hepatic triglyceride secretion by 75% compared to vehicle treatment. Tracer studies with 2H2O suggested that moxonidine inhibits de novo fatty acid synthesis. Thus, activation of I1R lowers plasma lipids, with the main site of action probably within the liver to reduce synthesis and secretion of triglycerides. More selective I1R agonists might provide monotherapy for hyperlipidemic hypertension.

Effects of octopamine on lipolysis, glucose transport and amine oxidation in mammalian fat cells

Octopamine is known to exert adrenergic effects in mammals although specific octopamine receptors have been cloned only in invertebrates. It has been shown that octopamine can stimulate alpha(2)-adrenoceptors (ARs) in Chinese hamster ovary cells transfected with human alpha(2)-ARs. More recently, we reported that octopamine stimulates lipolysis through beta(3)-rather than beta(1)-or beta(2)-AR activation in white adipocytes from different mammalian species. The present study was thus undertaken to further characterize the adrenergic properties of octopamine. For this purpose, several biological processes known to be regulated by adrenergic stimulation were studied in response to octopamine, noradrenaline, adrenaline and tyramine in white adipocytes from different mammals. First, octopamine was fully lipolytic in garden dormouse and Siberian hamster while tyramine was ineffective. Although being around one hundred-fold less potent that noradrenaline, octopamine was slightly more potent in these hibernators known for their high sensitivity to beta(3)-AR agonists than in rat and chiefly more active than in human adipocytes known for their limited responses to beta(3)-AR agonists. Second, octopamine reduced insulin-dependent glucose transport in rat fat cells, a response also observed with noradrenaline and selective beta(3)-AR agonists but not with beta(1)-or beta(2)-agonists. Third, human adipocytes, which endogenously express a high level of alpha(2)-ARs, exhibited a clear alpha(2)-adrenergic antilipolytic response to adrenaline but not to octopamine. Moreover, octopamine exhibited only a very weak affinity for the alpha(2A)-ARs labeled by [3H]RX821002 in human adipocyte membranes. In Syrian hamster adipocytes, which also possess alpha(2)-ARs, octopamine induced only a weak antilipolysis. Finally, octopamine was a substrate of fat cell amine oxidases, with an apparent affinity similar to that of noradrenaline. All these results demonstrate that octopamine, tyramine noradrenaline and adrenaline can be degraded by adipocyte amine oxidases. However these biogenic amines interact differently with adipocyte adrenoceptors: tyramine is inactive, adrenaline and noradrenaline activate both beta- and alpha(2)-ARs while octopamine activates only beta(3)-ARs and is devoid of alpha(2)-adrenergic agonism. Thus, octopamine could be considered as an endogenous selective beta(3)-AR agonist.

High expression of monoamine oxidases in human white adipose tissue: evidence for their involvement in noradrenaline clearance

The clearance of plasma adrenaline and noradrenaline by human adipose tissue suggests the expression of the catecholamine-degrading enzyme monoamine oxidases and of catecholamine transport systems in adipocytes. In the present study, we identified and characterized the monoamine oxidases and an extraneuronal noradrenaline transporter expressed in human adipocytes. Enzyme assays using the monoamine oxidase A/B substrate [14C]tyramine showed that abdominal and mammary human adipocytes contain one of the highest monoamine oxidase activities in the body. Characterization of the enzyme isoforms by inhibition profiles of [14C]tyramine oxidation and Western and Northern blot analyses showed that mRNAs and proteins related to both monoamine oxidases A and B were expressed in adipocytes. Quantification of each enzyme isoform performed by enzyme assay and Western blot showed that monoamine oxidase A was predominant, representing 70-80% of the total enzyme activity. In uptake experiments, the monoamine oxidase substrate [3H]noradrenaline was transported into white adipocytes (Vmax 0.81+/-0.3 nmol/30 min/100 mg of lipid, Km 235+/-104 microM). The inhibition of [3H]noradrenaline uptake by specific inhibitors indicated that white human adipocytes contain an extraneuronal-type noradrenaline transporter. Competition studies of [14C]tyramine oxidation showed that noradrenaline is metabolized by monoamine oxidases in intact cells. In conclusion, the concomitant expression of monoamine oxidases and of a noradrenaline transporter in human white adipocytes supports the role of the adipose tissue in the clearance of peripheral catecholamines. These results suggest that adipocytes should be considered as a previously unknown potential target of drugs acting on monoamine oxidases and noradrenaline transporters.