Biochemical characterization of brown adipose tissue beta-adrenergic receptor

Respective degree of expression of beta 1-, beta 2- and beta 3-adrenoceptors in human brown and white adipose tissues

1. The possible existence of a beta 3-adrenoceptor in human brown and white adipose tissues was investigated by mRNA expression and binding studies. 2. The relative amounts of beta 1-, beta 2- and beta 3-adrenoceptor mRNA, as determined by total RNA Northern blot analysis in newborn brown adipose tissue, were 28, 63 and 9% respectively of the total beta-adrenoceptor mRNA. 3. The beta 1/beta 2-adrenoceptors of human brown adipose tissue plasma membranes were characterized using [3H]-CGP 12177 as a ligand. Their Kd and Bmax values were 1.9 nM and 156 fmol mg-1 of membrane proteins, respectively. The beta 3-adrenoceptor was characterized by use of the new specific radioligand [3H]-SB 206606. The binding of this ligand was stereospecifically displaced by the active R,R- or the inactive S,S-enantiomer of BRL 37344 up to a concentration of about 10 microM. The Kd and Bmax values of the brown adipose tissue membrane beta 3-adrenoceptors were 87 nM and 167 fmol mg-1 of proteins, respectively. A low affinity [3H]-CGP 12177 binding site population was also detected in these membranes. 4. In human omental white adipose tissue, no beta 3-adrenoceptor mRNA could be detected in total RNA Northern blots and the beta 1-and beta 2-adrenoceptor mRNAs represented 9 and 91%, respectively of the total beta-adrenoceptor mRNA, and no specific binding of [3H]-SB 206606 could be measured.

Effect of selective beta-adrenoceptor stimulation on UCP synthesis in primary cultures of brown adipocytes

Given the co-existence of the three beta-adrenoceptor (beta AR) subtypes (beta 1AR, beta 2AR and beta 3AR) in brown adipocytes, the present study was undertaken to determine the relative importance of these in the induction of UCP synthesis in mouse BAT precursor cells in primary culture. Cells at different stages of differentiation were exposed to different beta AR agonists: prenalterol (a selective beta 1AR agonist), salbutamol or clenbuterol (selective beta 2AR agonists), or BRL 37344 (a selective beta 3AR agonist). As with the endogenous agonist, noradrenaline, and the non-selective beta AR agonist, isoprenaline, all four beta AR agonists induced UCP in the confluent stage of the cells, but with different potencies, and with the highest induction being seen after clenbuterol or BRL 37344 treatment. Cells in the confluent stage of development were the most sensitive to the effects of the agonists, although clenbuterol and BRL 37344 induced a weak UCP synthesis in pre-confluent cells. None of these beta AR agonists were able to induce UCP synthesis in the post-confluent period. The responses to prenalterol and salbutamol were inhibited by propranolol at relatively low concentrations, suggesting their effects were mediated by beta 1AR and beta 2AR, respectively. However, propranolol was a particularly weak antagonist of BRL 37344 and, unexpectedly, of the clenbuterol UCP responses, which suggests that both induce UCP synthesis via the beta 3AR. In summary, the beta 3AR is the most important adrenoceptor coupled to the induction of UCP synthesis, although both beta 1AR and beta 2AR activation may make a contribution. However, all three beta AR subtypes do not become fully functional until cultured cells become confluent.

Coexisting beta-adrenoceptor subtypes: significance for thermogenic process in brown fat cells

The possible significance of the coexisting beta 1-, beta 2-, and beta 3-adrenoceptors in brown adipose tissue for the thermogenic response was investigated. Oxygen consumption of isolated hamster brown fat cells was analyzed as a measure of thermogenesis. Thermogenesis could be evoked not only by the physiological agent norepinephrine but also by BRL-37344 and CGP-12177. No evidence for biphasic inhibition curves was found with either the selective beta 1-antagonist ICI-89406, the beta 2-antagonist ICI-118551, or the beta 1/beta 2-nonselective beta-antagonist propranolol against 1 microM norepinephrine; pI50 (the negative logarithm of the inhibitory constant for an antagonist, as estimated from the dose-response curve for an antagonist vs. a constant agonist concentration) values for ICI-89406 and ICI-118551 were very low (4-5), implying nonselective inhibition; the pI50 for propranolol was approximately 6 (as expected for the beta 3-receptor). Even with suboptimal norepinephrine, no biphasic inhibition was found. CGP-12177 at concentrations where it is primarily an antagonist to the beta 1-receptor did not influence the dose-response curve for either norepinephrine or BRL-37344. BRL-37344- or CGP-12177-induced thermogenesis was inhibited by the beta-antagonists in a manner similar to norepinephrine-induced thermogenesis. Schild plots for propranolol inhibition of norepinephrine-, isoprenaline-, BRL-37344- and CGP-12177-induced thermogenesis yielded similar pA2 (the negative logarithm of the inhibitory constant for an antagonist, as calculated from a series of agonist dose-response curves at different antagonist concentrations) (approximately 5.5), for interaction with either agonist, implying that the same receptor was stimulated by all agonists. Thus, despite the fact that different beta-receptor subtypes coexist in the tissue, we find no evidence for the participation of beta 1- or beta 2-receptors in the thermogenic response. Within the resolution of the experiments, the results therefore imply that it is predominantly or solely the beta 3-receptor that is coupled to thermogenesis, and it is via this beta-adrenergic receptor that not only norepinephrine but also CGP-12177 and BRL-37344 induce thermogenesis.

Isomeric-activity ratios of trimetoquinol enantiomers on beta-adrenergic receptor subtypes: functional and biochemical studies

Trimetoquinol [1-(3',4',5'-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4- tetrahydroisoquinoline , TMQ] exists as two enantiomers, and the (-)-(S)-isomer is a potent beta-adrenergic receptor (beta-AR) agonist. Experiments were conducted to examine the functional and biochemical potencies of the (S)-and (R)-enantiomers of TMQ for interaction with beta-AR subtypes in tissues, membrane fractions, and cell systems. The isomeric-activity ratios (IARs) of the TMQ isomers [(S)-isomer > > (R)-isomer] for stimulation of beta 1- and beta 2-AR of guinea pig right atria and trachea were 224 and 1585, respectively; these IARs were similar to those observed on atypical beta-AR systems of rat distal colon (575), rat brown adipocytes (398), but differed from that of rat esophageal smooth muscle (2884) in the presence of pindolol. In the absence of pindolol, the potencies for the TMQ enantiomers were slightly increased; however, the IARs remained unchanged in rat distal colon, rat brown adipocytes, and rat esophageal smooth muscle. Similarly, radioligand binding studies demonstrated that the TMQ isomer beta-AR affinities were stereoselective for the (-)-(S)-isomer in membranes of guinea pig left ventricle (beta 1) and lung (beta 2) giving IARs of 115 and 389, respectively; and in E. coli expressing human beta 1- and beta 2-AR giving IARs of 661 and 724, respectively. Corresponding IARs of receptor affinities and stimulation of cAMP accumulation in Chinese hamster ovary cells expressing human beta 2-AR and rat beta 3-AR were 331 and 282, and 118 and 4678, respectively. These results indicate that the (-)-(S)-isomer of TMQ exhibits high affinity, and is a potent and highly stereoselective agonist for each beta-AR subtype, that the isomers generally fail to differentiate between the beta-AR subtypes, and that, based upon differences in IAR within beta 3-AR containing systems, subtypes of atypical beta (or beta 3)-AR may exist in adipose tissue and smooth muscle.

Radioligand binding studies of the atypical beta 3-adrenergic receptor in rat brown adipose tissue using [3H]CGP 12177

Two populations of [3H]CGP 12177 binding sites exist in rat interscapular brown adipose tissue (IBAT) plasma membranes. The majority of binding sites are of low affinity with a Kd of 31 nM, a value in close agreement with that for the Kd of [3H]CGP 12177 binding to a cloned rat beta 3-adrenergic receptor (AR) expressed in CHO cells (44 nM). Competition binding studies demonstrate that the Ki values of the cloned rat beta 3-AR and of the low affinity sites in IBAT are 45 and 29 nM, respectively, for BRL 37344 and 1.4 and 1.0 microM, for (-)-propranolol. These findings strongly suggest that the low affinity [3H]CGP 12177 binding site measured in IBAT plasma membranes represents the atypical beta 3-AR in this tissue.

Coexistence of three beta-adrenoceptor subtypes in white fat cells of various mammalian species

The nature of the beta-adrenoceptors (beta-ARs) of the white fat cells of five mammalian species (rat, hamster, rabbit, dog and humans) was reassessed. The coexistence of at least three beta-ARs on the fat cell (except human) was demonstrated. Comparative binding and lipolysis studies were performed, using recently synthesized compounds selective for the atypical beta-AR of the rat brown fat cell and of the rat colon. beta 1- and beta 2-ARs have previously been identified in all the mammalian white fat cells using [125I]cyanopindolol ([125]CYP) or [3H]dihydroalprenolol. In addition to these receptors, we now demonstrated the existence of a third beta-AR directly involved in adrenergic-mediated lipolysis, and identified it in the white fat cells of the most commonly studied animal species, except humans. This receptor is not detected by the classically used beta-antagonist radioligands, explaining the discrepancies in reports on the nature of the beta-ARs of the adipose tissue. Pharmacological delineation of the third type of beta-AR-induced lipolysis showed this receptor to be rather similar to the previously proposed atypical beta-AR of brown and white rat fat cells. Its pharmacological properties were clarified, using new selective full agonists and partial agonists also acting as non-selective beta 1/beta 2-antagonists. The limits of [125]CYP as a radioligand were reported and the usefulness of BRL 37344, (+/-)-CGP 12177 and phenylethanolaminotralines derivatives (having an atypical beta-activity on intestinal motility) as major tools usable for atypical beta-AR activation was demonstrated. Moreover, confirming our previous results about the nature of the beta-ARs (beta 1- and beta 2-ARs) located in the fat cells of women (Mauriège et al., J. Lipid Res., 1987, 17, 156), no atypical beta-AR-mediated lipolysis was identified in abdominal adipose tissue from healthy women. The possible differences and similarities between this receptor and the recently cloned beta 3-AR are discussed.

The novel thermogenic beta-adrenergic agonist Ro 16-8714 increases the interscapular brown-fat beta-receptor-adenylate cyclase and the uncoupling-protein mRNA level in obese (fa/fa) Zucker rats

In obese (fa/fa) rats both the total (-)-isoprenaline- and NaF-stimulated adenylate cyclase activities of interscapular brown-adipose-tissue (IBAT) plasma membranes were decreased as compared with lean rats by 40 and 38% respectively. An acute treatment with Ro 16-8714 increased (-)-isoprenaline- and NaF-stimulated adenylate cyclase activities by 2.5- and 2.0-fold respectively, and beta-adrenoceptor number 2.8-fold in obese rat IBAT, but it had no effect on these parameters in lean rats. It increased mRNA for mitochondrial uncoupling protein in both lean and obese rats to the same extent.