Effect of CL316,243, a highly specific beta 3-adrenoceptor agonist, on lipolysis of human and rat adipocytes

Difference in intracellular temperature rise between matured and precursor brown adipocytes in response to uncoupler and β-adrenergic agonist stimuli

Brown adipocytes function to maintain body temperature by heat production. However, direct measurement of heat production at a single cell level remains difficult. Here we developed a method to measure the temperature within primary cultured brown adipocytes using a cationic fluorescent polymeric thermometer. Placement of the thermometer within a matured brown adipocyte and a precursor cell enabled the detection of heat production following uncoupler treatment. The increase in the intracellular temperature due to stimulation with a mitochondrial uncoupler was higher in matured brown adipocytes than in precursor cells. Stimulation with a β-adrenergic receptor (β-AR) agonist, norepinephrine, raised the intracellular temperature of matured brown adipocytes to a level comparable to that observed after stimulation with a β3-AR-specific agonist, CL316.243. In contrast, neither β-AR agonist induced an intracellular temperature increase in precursor cells. Further, pretreatment of brown adipocytes with a β3-AR antagonist inhibited the norepinephrine-stimulated elevation of temperature. These results demonstrate that our novel method successfully determined the difference in intracellular temperature increase between matured brown adipocytes and precursor cells in response to stimulation by an uncoupler and β-AR agonists.

Lack of FFAR1/GPR40 does not protect mice from high-fat diet-induced metabolic disease

OBJECTIVE

FFAR1/GPR40 is a G-protein-coupled receptor expressed predominantly in pancreatic islets mediating free fatty acid-induced insulin secretion. However, the physiological role of FFAR1 remains controversial. It was previously reported that FFAR1 knockout (Ffar1(-/-)) mice were resistant to high-fat diet-induced hyperinuslinemia, hyperglycemia, hypertriglyceridemia, and hepatic steatosis. A more recent report suggested that although FFAR1 was necessary for fatty acid-induced insulin secretion in vivo, deletion of FFAR1 did not protect pancreatic islets against fatty acid-induced islet dysfunction. This study is designed to investigate FFAR1 function in vivo using a third line of independently generated Ffar1(-/-) mice in the C57BL/6 background.

RESEARCH DESIGN AND METHODS

We used CL-316,243, a beta3 adrenergic receptor agonist, to acutely elevate blood free fatty acids and to study its effect on insulin secretion in vivo. Ffar1(+/+) (wild-type) and Ffar1(-/-) (knockout) mice were placed on two distinct high-fat diets to study their response to diet-induced obesity.

RESULTS

Insulin secretion was reduced by approximately 50% in Ffar1(-/-) mice, confirming that FFAR1 contributes significantly to fatty acid stimulation of insulin secretion in vivo. However, Ffar1(+/+) and Ffar1(-/-) mice had similar weight, adiposity, and hyperinsulinemia on high-fat diets, and Ffar1(-/-) mice showed no improvement in glucose or insulin tolerance tests. In addition, high-fat diet induced comparable levels of lipid accumulation in livers of Ffar1(+/+) and Ffar1(-/-) mice.

CONCLUSIONS

FFAR1 is required for normal insulin secretion in response to fatty acids; however, Ffar1(-/-) mice are not protected from high-fat diet-induced insulin resistance or hepatic steatosis.

β3-adrenergic agonists: potential therapeutics for obesity

During the last decade a number of beta3-adrenergic receptor agonists have been advanced to clinical trials. The results of human studies to date have been disappointing with respect to sustained increases in metabolic rate and weight loss. Cloning of beta3-adrenoceptors in a number of species and subsequent pharmacological evaluations revealed these early investigational drugs to be weakly active against the human receptor, suggesting an explanation for the poor performance of these compounds in clinical trials. This information has been integrated into subsequent research efforts resulting in the discovery of agents with activities optimised for the human receptor. This new generation of compounds is in late preclinical/early clinical development and are poised to address the role beta3-adrenoceptor signalling plays in the obese state. Issues related to the potential for beta3-adrenergic agonists to positively impact metabolic parameters in humans are also discussed.

Effects of growth hormone on the function of beta-adrenoceptor subtypes in rat adipocytes

OBJECTIVE

The influence of growth hormone (GH) on the regulation of lipolytic response to specific agonists to beta-adrenoceptors and several post-receptor steps in the lipolytic cascade were investigated.

RESEARCH METHODS AND PROCEDURES

Adipose tissues from rats were incubated with or without GH (1.38 nM). After a 24-hour incubation, isolated adipocytes were prepared for different assays. Rats were hypophysectomized. One week after operation, L-thyroxine and hydrocortisone acetate was given to hypophysectomized rats. One group of rats was treated with GH (1.33 mg/kg, daily). After 1 week of hormonal treatment, adipose tissues were removed for different studies.

RESULTS

GH treatment increased both basal lipolysis and lipolytic sensitivity to dobutamine and CGP 12177 in adipocytes. The lipolytic sensitivity to terbutaline was not influenced by GH treatment. GH treatment increased the maximal lipolytic response to dobutamine and CGP 12177, but not to terbutaline as determined with absolute values of lipolysis. Forskolin-induced lipolysis was increased by addition of GH to tissues. Moreover, GH treatment resulted in enhanced expression of hormone-sensitive lipase. GH treatment in hypophysectomized rats influenced neither the expressions of G alpha s protein and cholera toxin-catalyzed adenosine diphosphate-ribosylation of G alpha s protein, nor cholera toxin-induced 3',5'-cyclic adenosine monophosphate accumulation. However, the expression of G alpha i protein was decreased after GH treatment.

DISCUSSION

These and previous results suggest that GH increases lipolysis in rat adipocytes partly through the beta-adrenergic system, including increases in both beta(1)- and beta(3)-adrenergic receptor function, and partly through enhanced adenylate cyclase function, and expression of hormone-sensitive lipase, perhaps via a decrease in G alpha i protein expression.

Lipolytic effect of BRL 35 135, a beta3 agonist, and its interaction with dietary lipids on the accumulation of fats in rat body

The type of intaked fat and fat uptake mechanisms such as adrenergic-induced lipolysis affect patterns of fat accumulation in animal body. In this study, in vitro lipolytic effect of BRL 35135, a selectivebeta3 agonist, and its interaction with different dietary fats on fat accumulation in animal body (in vivo) were studied. For in vitro study, adipocytes isolated from epididymal fat were incubated with 10(-5) M -10(-9) M of either BRL 35135 or isoproterenol, a non-selectivebeta-agonist. In animal study, two groups of SD-rats, i.e., BRL35135-intaked (dosed at 0.5 mg/kg/day in diet) and control, were divided into 4 sub-groups and fed diets containing 12% of either beef tallow (BT), canola oil (CO), olive oil (OO) or safflower oil(SO) for 6 weeks. In vitro study showed that BRL 35135 was 10 times more potent than isoproterenol in increasing the lipolysis in rat adipocytes. In animal study, inclusion of BRL35135 reduced daily weight gain in CO and SO groups (P < 0.05). Abdominal fat weight in BRL35135-intaked group was significantly lower than control in all dietary sub-groups (CO, OO and SO) except BT (P < 0.05). In BT group, abdominal fat contained significantly higher amount of total saturated fatty acids (SFAs) compared to CO, OO or SO. It was concluded that, although BRL 35135 was very potent in increasing lipolysis in the isolated adipocytes of rat, its preventive effect on lipid accumulation in animal body through the lipolysis could be affected by the type of dietary fat and was lesser when rats fed fats rich in SFAs.

The putative beta4-adrenergic receptor is a novel state of the beta1-adrenergic receptor

The atypical beta3-adrenergic receptor (AR) agonist CGP-12177 has been used to define a novel atypical beta-AR subtype, the putative beta4-AR. Recent evaluation of recombinant beta-AR subtypes and beta-AR-deficient mice, however, has established the identity of the pharmacological beta4-AR as a novel state of the beta1-AR protein. The ability of aryloxypropanolamine ligands like CGP-12177 to independently interact with agonist and antagonist states of the beta1-AR has important implications regarding receptor classification and the potential development of tissue-specific beta-AR agonists.

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.

beta 3-Adrenergic agonist induces a functionally active uncoupling protein in fat and slow-twitch muscle fibers

The mitochondrial uncoupling protein (UCP) has usually been found only in brown adipose tissue. We recently observed that a chronic administration of the beta 3-adrenergic agonist CL-316,243 (CL) induced the ectopic expression of UCP in white fat and skeletal muscle in genetic obese yellow KK mice. The aim of the present study was to examine whether UCP could be induced in nongenetic obese animals produced by neonatal injections of monosodium L-glutamate (MSG). The daily subcutaneous injection of CL (0.1 mg/kg) to MSG-induced obese mice for 2 wk caused significant reductions of body weight (15%) and white fat pad weight (58%). Northern and Western blot analyses showed that CL induced significant expressions of UCP in the white fat and muscle, as well as in brown fat. Immunohistochemical observations revealed that the UCP stains in white fat were localized on multilocular cells and that those in muscle were localized on slow-twitch fibers rich in mitochondria. Immunoelectron microscopy confirmed the mitochondrial localization of UCP in the myocytes. The guanosine 5'-diphosphate (GDP) binding to mitochondria in brown fat doubled after the CL treatment. Moreover, significant GDP binding was detected in the white fat and muscle of the CL-treated mice, at about one-fourth and one-thirteenth the activity of brown fat, respectively, suggesting that ectopically expressed UCP is functionally active. We concluded that the beta 3-adrenergic agonist CL can induce functionally active UCP in white fat and slow-twitch muscle fibers of obese mice.

Lipolytic effects of conventional beta 3-adrenoceptor agonists and of CGP 12,177 in rat and human fat cells: preliminary pharmacological evidence for a putative beta 4-adrenoceptor

1. The nature of rat and human fat cell beta 3-adrenoceptors was investigated by studying the effects of the new beta 3-adrenoceptor selective antagonist, SR 59,230A, on lipolysis induced by the conventional beta 3-adrenoceptor agonists, CL 316,243 and SR 58,611A, and by the non-conventional partial beta 3-adrenoceptor agonist CGP 12,177 (a potent beta 1- and beta 2-adrenoceptor antagonist with partial beta 3-adrenoceptor agonist property). 2. In rat fat cells, the rank order of potency of agonists was: CL 316,243 > isoprenaline > SR 58,611A > CGP 12,177. The three former agents were full agonists whereas CGP 12,177 was a partial agonist (intrinsic activity of 0.70). In human fat cells, the lipolytic effect of CGP 12,177 reached 25% of isoprenaline effect. CL 316,243 was a poor inducer of lipolysis and SR 58,611A was ineffective. 3. In rat fat cells, lipolysis induced by CL 316,243 and SR 58,611A was competitively antagonized by SR 59,230A. Schild plots were linear with pA2 value of 6.89 and 6.37, respectively. Conversely, 0.1, 0.5 and 1 microM SR 59,230A did not modify the concentration-response curve of CGP 12,177. A rightward shift of the curve was however observed with 10 and 100 microM of SR 59,230A. The apparent pA2 value was 5.65. The non-selective beta-adrenergic antagonist, bupranolol, competitively displaced the concentration-response curve of CGP 12,177 and CL 316,243. Schild plots were linear with pA2 values of 6.70 and 7.59, respectively. CL316,243-mediated lipolytic effect was not antagonized by CGP 20,712A. In human fat cells, CGP 12,177-mediated lipolytic effect was antagonized by bupranolol and CGP 20,712A. SR 59,230A (0.1, 1 and 10 microM) did not modify the concentration-response curve of CGP 12,177. A rightward shift was however observed at 100 microM leading to an apparent pA2 value of 4.32. 4. The results suggest that the non-conventional partial agonist CGP 12,177 can activate lipolysis in fat cells through the interaction with a beta-adrenoceptor pharmacologically distinct from the beta 3-adrenoceptor, i.e. through a putative beta 4-adrenoceptor. They suggest that the two subtypes coexist in rat fat cells whereas only the putative beta 4-adrenoceptor mediates lipolytic effect of CGP12,177 in human fat cells.