Numerous synthetic agonists selectively stimulate beta3-adrenoceptors (ARs). The endogenous catecholamines, noradrenaline and adrenaline, however, stimulate all the beta-AR subtypes, and no selective physiological agonist for beta3-ARs has been described so far. The aim of this study was to investigate whether any naturally occurring amine can stimulate selectively beta3-ARs. Since activation of lipolysis is a well-known beta-adrenergic function, the efficacy and potency of various biogenic amines were compared with those of noradrenaline, isoprenaline, and beta3-AR agonists 4-(-{[2-hydroxy-(3-chlorophenyl)ethyl]-amino} propyl)phenoxyacetate (BRL 37,344) and (R,R)-5-(2-{[2-(3-chlorophenyl )-2-hydroxyethyl]-amino} propyl)-1,3-benzo-dioxole-2,2-dicarboxylate (CL 316,243) by testing their lipolytic action in white fat cells. Five mammalian species were studied: rat, hamster and dog, in which selective beta-AR agonists act as full lipolytic agents, and guinea-pigs and humans, in which beta3-AR agonists are less potent activators of lipolysis. Several biogenic amines were inefficient (e.g. dopamine, tyramine and beta-phenylethylamine) while others (synephrine, phenylethanolamine, epinine) were partially active in stimulating lipolysis in all species studied. Their actions were inhibited by all the beta-AR antagonists tested, including those selective for beta1- or beta2-ARs. Octopamine was the only amine fully stimulating lipolysis in rat, hamster and dog fat cells, while inefficient in guinea-pig or human fat cells, like the beta3-AR agonists. In rat white fat cells, beta-AR antagonists inhibited the lipolytic effect of octopamine with a relative order of potency very similar to that observed against CL 316,243. Competitive antagonism of octopamine effect resulted in the following apparent pA2 [-log(IC50), where IC50 is the antagonist concentration eliciting half-maximal inhibition] values: 7.77 (bupranolol), 6.48 [3-(2-ethyl-phenoxy)-1[(1 S)-1,2,3,4-tetrahydronaphth-1-ylaminol]-(2S)2-propanol oxalate, SR 59230A, a beta3-selective antagonist], 6.30[erythro-D,L-1(7-lethylindan-4-yloxy)-3-isopropylamino-+ ++butan-2-ol, ICI 118,551, a beta2-selective antagonist] and 4.71 [(+/-)-[2-(3-carbomyl-4-hydroxyphenoxy)-ethylamino]-3-[4-(1- methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]2-propanolmethane sulphonate, CGP 20712A, a beta1-selective antagonist]. Octopamine had other properties in common with beta3-AR agonists: stimulation of oxygen consumption in rat brown fat cells and very low affinity in displacing [3H]CGP 12,177 binding to [beta1- or beta2-ARs in dog and rat adipocyte membranes. In Chinese hamster ovary (CHO) cells expressing human beta3-ARs, octopamine inhibited [125I]ICYP binding with only twofold less affinity than noradrenaline while it exhibited an affinity around 200-fold lower than noradrenaline in CHO cells expressing human beta1- or beta2-ARs. These data suggest that, among the biogenic amines metabolically related to catecholamines, octopamine can be considered as the most selective for beta3-ARs.

We have previously reported that substrates of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) exert short-term insulin-like effects in rat adipocytes, such as stimulation of glucose transport. In the present work, we studied whether these substrates could also mimic long-term actions of insulin. Adipose differentiation of 3T3 F442A cells, which is highly insulin-dependent, served as a model to test the effects of sustained administration of amine oxidase substrates. Daily treatment of confluent cells with 0.75 mM tyramine (a substrate of MAO and SSAO) or benzylamine (a substrate of SSAO) over 1 week caused the acquisition of typical adipocyte morphology. The stimulation of protein synthesis and triacylglycerol accumulation caused by tyramine or benzylamine reached one half of that promoted by insulin. This effect was insensitive to pargyline (an MAO inhibitor), but was inhibited by semicarbazide (an SSAO inhibitor) and by N-acetylcysteine (an antioxidant agent), suggesting the involvement of the H # O #

We have previously reported that substrates of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) exert short-term insulin-like effects in rat adipocytes, such as stimulation of glucose transport. In the present work, we studied whether these substrates could also mimic long-term actions of insulin. Adipose differentiation of 3T3 F442A cells, which is highly insulin-dependent, served as a model to test the effects of sustained administration of amine oxidase substrates. Daily treatment of confluent cells with 0.75 mM tyramine (a substrate of MAO and SSAO) or benzylamine (a substrate of SSAO) over 1 week caused the acquisition of typical adipocyte morphology. The stimulation of protein synthesis and triacylglycerol accumulation caused by tyramine or benzylamine reached one half of that promoted by insulin. This effect was insensitive to pargyline (an MAO inhibitor), but was inhibited by semicarbazide (an SSAO inhibitor) and by N-acetylcysteine (an antioxidant agent), suggesting the involvement of the H(2)O(2) generated during SSAO-dependent amine oxidation. Chronic administration of amine oxidase substrates also induced the emergence of adipose conversion markers, such as aP2, glycerol-3-phosphate dehydrogenase, the glucose transporter GLUT4, and SSAO itself. Moreover, cells treated with amines acquired the same insulin sensitivity regarding glucose transport as adipocytes classically differentiated with insulin. In all, most of the adipogenic effects of amines were additive to insulin. Our data reveal that amine oxidase substrates partially mimic the adipogenic effect of insulin in cultured preadipocytes. Furthermore, they suggest that SSAO not only represents a novel late marker of adipogenesis, but could also be directly involved in the triggering of terminal adipocyte differentiation.

No abstract

Biogenic amines like tyramine, methylamine and the non-naturally occuring amine, benzylamine, have been described to promote adipose conversion of murine 3T3 preadipocytes. To further investigate these novel effects of amines, we studied whether they selectively mimic the long-term adipogenic action of insulin. To this aim, we decided to use the 3T3-L1 cell line since this model needs a complex combination of inducers to trigger the differentiation programme: insulin, isobutylmethylxanthine (IBMX, an activator of cAMP-signal transduction pathway) and the synthetic glucocorticoid, dexamethasone. A cell culture protocol was designed, by which each component of the differentiation cocktail was replaced with either benzylamine or tyramine, in order to determine whether these amine oxidase substrates could substitute any of the differentiation inducers in 3T3-L1 cells. The incomplete lipid accumulation found in cells grown under IBMX- or dexamethasone-free conditions was not improved by the daily addition of amines to the culture medium. Insulin was the only component of adipose differentiation cocktail of 3T3-L1 that could be replaced, although partially, by tyramine or benzylamine. When used at 0.5 mM, these amines resulted in a significant increase of triacylglycerol accumulated eight days after confluence, when compared to cells kept without insulin. This partial insulin replacement was totally abolished by SSAO-inhibitors, while MAO-blockade did not reduce lipid accumulation. As previously reported for other insulin-sensitive processes, such as stimulation of glucose transport or lipolysis inhibition in mature adipocytes, the stimulation of adipogenesis by tyramine and benzylamine was an SSAO-dependent mechanism that apparently shared common signaling pathways with insulin.