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

Is the β3-Adrenoceptor a Valid Target for the Treatment of Obesity and/or Type 2 Diabetes?

β3-Adrenoceptors mediate several functions in rodents that could be beneficial for the treatment of obesity and type 2 diabetes. This includes promotion of insulin release from the pancreas, cellular glucose uptake, lipolysis, and thermogenesis in brown adipose tissue. In combination, they lead to a reduction of body weight in several rodent models including ob/ob mice and Zucker diabetic fatty rats. These findings stimulated drug development programs in various pharmaceutical companies, and at least nine β3-adrenoceptor agonists have been tested in clinical trials. However, all of these projects were discontinued due to the lack of clinically relevant changes in body weight. Following a concise historical account of discoveries leading to such drug development programs we discuss species differences that explain why β3-adrenoceptors are not a meaningful drug target for the treatment of obesity and type 2 diabetes in humans.

Antiobesity and Antidiabetic β-agonists: Lessons Learned and Questions to be Answered

In several species of obese animals, a group of phenethanolamine beta-agonists stimulates lipolysis and thermogenesis, resulting in the loss of body fat and weight. Brown adipose tissue is considered to be the major target tissue for the antiobesity activity of these compounds. Independent of this antiobesity activity, some of these compounds are also antidiabetic and increase muscle mass. Based on the pharmacological profile of these compounds, a beta3-receptor was proposed and characterized in mouse, rat, and humans. The beta3-receptor in brown adipose tissue has been suggested to mediate the antiobesity activity of these beta-agonists. Whether this receptor is responsible for the antidiabetic activity and whether there is a linkage between the antiobesity/antidiabetic activity and the nutrient partitioning activity is not clear. Clinical trials with these mixed beta-agonists showed marginal antiobesity effects when caloric intake of subjects was restricted. Insulin sensitivity was also improved in some of the trials designed to test the antidiabetic activity of these compounds. Side effects included tachycardia and tremor. To eliminate these side effects, a second generation of compounds was selected for its agonist activity on rat beta3-receptors. Clinical trials with these compounds have shown little increase of energy expenditure even at high doses. Successful development of an antiobesity and antidiabetic drug from this class of compounds will require the elucidation of the physiological role of the human beta3-receptor and the regulatory mechanism between fuel efficiency and feeding behavior.

UCP1: the only protein able to mediate adaptive non-shivering thermogenesis and metabolic inefficiency

The uniqueness of UCP1 (as compared to UCP2/UCP3) is evident from expression analysis and ablation studies. UCP1 expression is positively correlated with metabolic inefficiency, being increased by cold acclimation (in adults or perinatally) and overfeeding, and reduced in fasting and genetic obesity. Such a simple relationship is not observable for UCP2/UCP3. Studies with UCP1-ablated animals substantiate the unique role of UCP1: the phenomenon of adaptive adrenergic non-shivering thermogenesis in the intact animal is fully dependent on the presence of UCP1, and so is any kind of cold acclimation-recruited non-shivering thermogenesis; thus UCP2/UCP3 (or any other proteins or metabolic processes) cannot substitute for UCP1 physiologically, irrespective of their demonstrated ability to show uncoupling in reconstituted systems or when ectopically expressed. Norepinephrine-induced thermogenesis in brown-fat cells is absolutely dependent on UCP1, as is the uncoupled state and the recoupling by purine nucleotides in isolated brown-fat mitochondria. Although very high UCP2/UCP3 mRNA levels are observed in brown adipose tissue of UCP1-ablated mice, there is no indication that the isolated brown-fat mitochondria are uncoupled; thus, high expression of UCP2/UCP3 does not necessarily confer to the mitochondria of a tissue a propensity for being innately uncoupled. Whereas the thermogenic effect of fatty acids in brown-fat cells is fully UCP1-dependent, this is not the case in brown-fat mitochondria; this adds complexity to the issues concerning the mechanisms of UCP1 function and the pathway from beta(3)-adrenoceptor stimulation to UCP1 activation and thermogenesis. In addition to amino acid sequences conserved in all UCPs as part of the tripartite structure, all UCPs contain certain residues associated with nucleotide binding. However, conserved amongst all UCP1s so far sequenced, and without parallel in all UCP2/UCP3, are two sequences: 144SHLHGIKP and the C-terminal sequence RQTVDC(A/T)T; these sequences may therefore be essential for the unique thermogenic function of UCP1. The level of UCP1 in the organism is basically regulated at the transcriptional level (physiologically probably mainly through the beta(3)-adrenoceptor/CREB pathway), with influences from UCP1 mRNA stability and from the delay caused by translation. It is concluded that UCP1 is unique amongst the uncoupling proteins and is the only protein able to mediate adaptive non-shivering thermogenesis and the ensuing metabolic inefficiency.

Molecular cloning and functional characterization of the promoter region of the human uncoupling protein-2 gene

As a member of the uncoupling protein family, UCP2 is ubiquitously expressed in rodents and humans, implicating a major role in thermogenesis. To analyze promoter function and regulatory motifs involved in the transcriptional regulation of UCP2 gene expression, 3.3 kb of 5'-flanking region of the human UCP2 (hUCP2) gene have been cloned. Sequence analysis showed that the promoter region of hUCP2 lacks a classical TATA or CAAT box, however, appeared GC-rich resulting in the presence of several Sp-1 motifs and Ap-1/-2 binding sites near the transcription initiation site. Functional characterization of human UCP2 promoter-CAT fusion constructs in transient expression assays showed that minimal promoter activity was observed within 65 bp upstream of the transcriptional start site (+1). 75 bp further upstream (from nt -141 to -66) a strong cis-acting regulatory element (or enhancer) was identified, which significantly enhanced basal promoter activity. The regulation of human UCP2 gene expression involves complex interactions among positive and negative regulatory elements distributed over a minimum of 3.3 kb of the promoter region.

Tissue-dependent upregulation of rat uncoupling protein-2 expression in response to fasting or cold

The control of uncoupling protein-2 (UCP2) mRNA expression in rat brown adipose tissue (BAT), heart and skeletal muscles was examined. Cold exposure (48 h) increased UCP2 mRNA in BAT, heart and soleus muscle by 2.4-, 4.3- and 2.6-fold, respectively. Fasting (48 h) had no effect on UCP2 mRNA expression neither in BAT nor in heart, but markedly increased it in skeletal muscles. While the upregulation of UCP2 mRNA in response to cold exposure is in line with a putative uncoupling role for this protein in thermoregulatory thermogenesis, the unexpected upregulation of UCP2 in skeletal muscles in response to fasting seems inconsistent with its role as an uncoupling protein involved in dietary regulation of thermogenesis.

Adrenergic receptors and fat cells: differential recruitment by physiological amines and homologous regulation

The control of fat cell lipolysis by the catecholamines involves at least four different adrenoceptor subtypes; three beta (beta 1-, beta 2-, and beta 3-ARs) and one alpha 2-adrenoceptor (alpha 2-AR). The physiological importance of the beta- and alpha 2A-ARs varies according to the species, the sex, the age, the anatomical location of fat deposits and the degree of obesity in humans and animals. The physiological amines operate through differential recruitment of these sites on the basis of their relative affinities. This point has been assessed by in vitro studies and has partly been confirmed in in vivo experiments using selected alpha/beta-AR antagonists and in situ microdialysis. The affinity of the beta 3-AR for catecholamines is less than that of the classical beta 1- and beta 2-ARs in the various species investigated. Conversely, it is the alpha 2-AR which exhibit the highest affinity for the physiological amines in all fat cells. The relative order of affinity of the various fat cell ARs for the physiological amines defined in binding studies and in vitro assays is alpha 2 > beta 1 > or = beta 2 > beta 3 for norepinephrine and alpha 2 > beta 2 > beta 1 > beta 3 for epinephrine. When considering differential beta-AR recruitment by catecholamines, it is the beta 1-AR which is always activated at the lowest norepinephrine levels, whatever the species, while the activation of the beta 3-AR requires higher norepinephrine levels. In addition to the differential recruitment, differential regulation by hormones could also occur for each fat cell AR subtype. The alpha 2-and beta 3-ARs are less prone to desensitization and down-regulation by comparison with the beta 1- and beta 2-AR.

Modulation of obese gene expression in rat brown and white adipose tissues

The ob gene mRNA expression in rat brown adipose tissue (BAT) and epididymal white adipose tissue (WAT) was measured on Northern blots hybridized with a rat ob gene probe. The level of ob gene mRNA in BAT was about 40% of that in WAT. Fasting (36 h) or semi-starvation (10 days) decreased the ob gene mRNA level in both tissues by 62-68%, and cold exposure at 6 degrees C (24 h) decreased it in BAT (-84%) but not in WAT. Acute administration of the beta 3-adrenergic agonist Ro 16-8714 decreased the ob gene mRNA level in BAT (-51%) and WAT (-28%) of lean Zucker rats and only in BAT (-74%) of obese falfa rats. This study demonstrates that, in the rat, the ob gene is not only expressed in WAT but also in BAT, and suggests that in these two tissues, the modulation of the ob gene expression might be more closely associated with known alterations in cell lipid content than with changes in sympathetic activity.

Modulation in vivo of beta-adrenergic-receptor subtypes in rat brown adipose tissue by the thermogenic agonist Ro 16-8714

The number of beta 3-adrenergic receptors (AR) in plasma membranes from interscapular brown adipose tissue (IBAT) was decreased by 62% in lean Zucker rats treated with the thermogenic beta-adrenergic agonist Ro 16-8714 as compared with controls after 72 h of treatment. The loss of beta 3-AR number was preceded by a 93% decrease in the steady-state level of beta 3-AR mRNA at 30 h. Similar results were obtained in obese (fa/fa) Zucker rats. Ro 16-8714 had no effect on the number of beta 1- and beta 2-ARs in IBAT. This is the first report to demonstrate that the beta 3-AR in IBAT can be specifically down-regulated in vivo by exposure to a thermogenic agonist.

Hibernoma formation in transgenic mice and isolation of a brown adipocyte cell line expressing the uncoupling protein gene

Transgenic mice were produced containing the adipocyte-specific regulatory region from the adipocyte P2 (aP2) gene linked to the simian virus 40 transforming genes. Most of the transgenic mice developed brown fat tumors (hibernomas) in their interscapular brown adipose tissue. Hibernoma formation was noticeable in some of the mice as early as 1 day after birth and most of the mice developed very large tumors by 1 month of age. All of the tumor tissue expressed the brown fat-specific uncoupling protein (UCP) gene as well as the aP2 gene. Several of the tumors have been used to establish cultured cell lines and at least one of these lines can be induced to differentiate into brown adipocytes. The cultured adipocytes express mRNA for UCP upon stimulation with N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate, norepinephrine, isoproterenol or D7114, a beta 3 adrenergic agonist. Thus, regulation of the key thermogenic gene UCP can now be studied in an established cell line.

Hypoglycemic effects of a beta-agonist, Ro 16-8714, in streptozotocin-diabetic rats: decreased hepatic glucose production and increased glucose utilization in oxidative muscles

Streptozotocin (STZ)-induced diabetic rats are glycosuric, hyperglycemic, hyperketonemic, overproduce glucose, and have a decreased glucose utilization in oxidative muscles. Treatment with a beta-agonist, Ro 16-8714, decreases the glycosuria, hyperglycemia, hyperketonemia, and hepatic glucose production. Tissue glucose utilization was unchanged, except in oxidative muscles, where it was increased.