Stereochemical aspects of beta-adrenoceptor antagonist-receptor interaction in adipocytes. Differentiation of beta-adrenoceptors in human and rat adipocytes

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.

Longitudinal trends and subgroup analysis in publication patterns for preclinical data of newly approved drugs

Having observed a large variation in the number and type of original preclinical publications for newly registered drugs, we have explored whether longitudinal trends and/or factors specific for certain drugs or their manufacturers may explain such variation. Our analysis is based on 1954 articles related to 170 newly approved drugs. The number of preclinical publications per compound declined from a median of 10.5 in 1991 to 3 in 2011. A similar trend was observed for the number of in vivo studies in general, but not in the subset of in vivo studies in animal models of disease. The percentage of compounds with studies using isolated human cells or cell lines almost doubled over time from 37 to 72%. Number of publications did not exhibit major differences between compounds intended for human versus veterinary use, therapeutic areas, small molecules versus biologicals, or innovator versus follow-up compounds; however, some companies may publish fewer studies per compound than others. However, there were qualitative differences in the types of models being used depending on the therapeutic area; specifically, compounds for use in oncology very often used isolated cells and cell lines, often from human origin. We conclude that the large variation in number and type of reported preclinical data is not easily explained. We propose that pharmaceutical companies should consistently provide a comprehensive documentation of the preclinical data they generate as part of their development programs in the public domain to enable a better understanding of the drugs they intend to market.

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

The discovery of drugs for obesity, the metabolic effects of leptin and variable receptor pharmacology: perspectives from beta3-adrenoceptor agonists

Although beta3-adrenoceptor (beta3AR) agonists have not become drugs for the treatment of obesity or diabetes, they offer perspectives on obesity drug discovery, the physiology of energy expenditure and receptor pharmacology. beta3AR agonists, some of which also stimulate other betaARs in humans, selectively stimulate fat oxidation in rodents and humans. This appears to be why they improve insulin sensitivity and reduce body fat whilst preserving lean body mass. Regulatory authorities ask that novel anti-obesity drugs improve insulin sensitivity and reduce mainly body fat. Drugs that act on different targets to stimulate fat oxidation may also offer these benefits. Stimulation of energy expenditure may be easy to detect only when the sympathetic nervous system is activated. Leptin resembles beta3AR agonists in that it increases fat oxidation, energy expenditure and insulin sensitivity. This is partly because it raises sympathetic activity, but it may also promote fat oxidation by directly stimulating muscle leptin receptors. The beta1AR and beta2AR can, like the beta3AR, display atypical pharmacologies. Moreover, the beta3AR can display variable pharmacologies of its own, depending on the radioligand used in binding studies or the functional response measured. Studies on the beta3AR demonstrate both the difficulties of predicting the in vivo effects of agonist drugs from in vitro data and that there may be opportunities for identifying drugs that act at a single receptor but have different profiles in vivo.

Stereoselectivity for interactions of agonists and antagonists at mouse, rat and human β3-adrenoceptors

This study examines the stereoselectivity profile of recombinant mouse, rat and human beta(3)-adrenoceptors expressed in Chinese Hamster Ovary (CHO-K1) cells using radioligand binding, in comparison with endogenously expressed beta(3)-adrenoceptors mediating relaxation responses in mouse ileum. The enantiomeric ratios for several beta-adrenoceptor agonists and antagonists at the cloned mouse, rat and human beta(3)-adrenoceptor were less than those reported at the cloned beta(1)-/beta(2)-adrenoceptor but higher than those reported in previous studies. The degree of stereoselectivity was relatively low for the agonists isoprenaline and noradrenaline but higher for antagonists and, in particular, tertatolol and propranolol. In mouse ileum, stereoselectivity of propranolol and tertatolol was observed under beta(1)-/beta(2)-adrenoceptor blockade. The (-)-enantiomers of propranolol and tertatolol were more effective at antagonism of (-)-isoprenaline-mediated relaxation of mouse ileum than their (+)-enantiomers. The recombinant mouse, rat and human beta(3)-adrenoceptors display stereoselective interactions for agonists and antagonists similar to the stereoselective profile of beta(3)-adrenoceptors in mouse ileum. The degree of stereoselectivity varied between species and the human beta(3)-adrenoceptor displayed higher affinities and enantiomeric ratios than the mouse or rat beta(3)-adrenoceptors.

Effects of the enantiomers of R,S-salbutamol on incompletely fused tetanic contractions of slow- and fast-twitch skeletal muscles of the guinea-pig

1. The effects of racemic R,S-salbutamol, and its individual enantiomers have been studied on incompletely fused (sub-tetanic) contractile responses of fast- and slow-contracting isolated skeletal muscles of the guinea-pig. 2. R,S-salbutamol (2-4 microM) decreased the peak force of sub-tetani in the slow-contracting soleus muscle and increased the peak force of sub-tetani in the fast-contracting peroneus longus muscle. It also increased the force of the first twitch of sub-tetani in both muscles. The decrease in the peak force of sub-tetani in the soleus muscle was due to defusion of the individual twitches caused by a shortening of their time course. 3. The effects of 4 microM of the racemate on both fast- and slow-contracting muscles were mimicked by 2 microM R-salbutamol (levalbuterol). However, 2 microM S-salbutamol was devoid of activity in both muscles. 4. We concluded that all the effects of R,S-salbutamol on guinea-pig skeletal muscles are due to the activity of the R-enantiomer. Thus there is a common enantiomeric profile for the skeletal muscle and bronchorelaxant activity of the compound.

Adrenergic control of lipolysis in swine adipose tissue

Most potential adrenergic compounds did not stimulate lipolysis in swine adipose tissue slices. Most of the sympatholytic agents antagonized lipolysis. Most beta 1- and beta 2-adrenergic agonists were not active but many were active with rat adipose tissue. Catecholamines (epinephrine, norepinephrine and isoproterenol), the resorcinol containing beta 2-agonists (terbutaline, metaproterenol and fenoterol) and the beta 1-agonist, dobutamine were active. The beta 1-antagonists were generally more potent and efficacious than the beta 2-antagonists. The swine adipose tissue adrenoceptor was not readily classified as either beta 1- or beta 2-specific.

Identification and subclassification of rat adipocyte beta-adrenoceptors using (+/-)-[125I]cyanopindolol

The ligand binding characteristics of beta-adrenoceptors on membranes derived from rat whole fat pads and isolated adipocytes have been compared to those present on lung membranes using the specific ligand (+/-)-[125I]cyanopindolol [(125I]CYP). The equilibrium dissociation constant (KD) of [125I]CYP was similar in all three preparations, whilst the degree of stereospecificity displayed by the isomers of propranolol varied between lung and isolated adipocyte membranes. The rank order of potency in displacing [125I]CYP binding was isoprenaline greater than adrenaline greater than noradrenaline in lung and whole fat pads, suggesting an overall beta 2-receptor subtype, and isoprenaline greater than noradrenaline greater than adrenaline in isolated adipocytes, suggesting a beta 1-receptor. Pharmacological characterisation of receptor subtype with selective beta-adrenoceptor agents indicated that all three preparations contained a heterogeneous receptor population (lung 80% beta 2, 20% beta 1; whole fat pad 62% beta 2, 38% beta 1; isolated adipocyte 15% beta 2, 85% beta 1). In view of the high proportion of beta 2-receptors on whole fat pad membranes, the overall amount of beta 2-receptors on isolated adipocytes may reflect contamination from other cell types. The nature of the beta 1-receptors present on fat cells is discussed in relation to the apparently atypical beta-receptor involved in mediating the functional lipolytic response.

Blockade of isoprenaline-induced changes in plasma free fatty acids, immunoreactive insulin levels and plasma renin activity in healthy human subjects, by propranolol, pindolol, practolol, atenolol, metoprolol and acebutolol

1 The effects of intravenously administered propranolol 0.01 and 0.03, pindolol 0.001 and 0.003, practolol 0.12 and 0.36, atenolol 0.03 and 0.09, metoprolol 0.045 and 0.135 and acebutolol 0.12 and 0.36 mg/kg, on isoprenaline-induced changes in heart rate, blood pressure, plasma free fatty acids, immunoreactive insulin plasma levels and plasma renin activity were determined in six healthy human subjects.

2 Propranolol, atenolol and metoprolol had a stronger effect on resting heart rate than practolol, acebutolol and pindolol, probably reflecting differences in intrinsic β-sympathomimetic activity. Antagonist potencies against isoprenaline-induced changes in heart rate and blood pressure suggested cardioselectivity for practolol, atenolol, metoprolol and the lower dose of acebutolol and non-cardioselectivity for propranolol, pindolol and the higher dose of acebutolol.

3 All six β-adrenoceptor blocking agents were able, to a varying extent, to antagonize the isoprenaline-induced increases in plasma free fatty acids and plasma immunoreactive insulin levels. In general, the cardioselective agents were relatively less effective antagonists than the non-cardioselective agents.

4 Resting plasma renin activity was reduced by all six β-adrenoceptor blocking agents, suggestive of the presence of β₁-adrenoceptors mediating renin release, but the non-cardioselective agents propranolol and pindolol seemed relatively more effective in antagonizing isoprenaline-induced increases in plasma renin activity than the cardioselective agents, which indicates that β₂-adrenoceptors might also be involved.

5 The results are compatible with the hypothesis that both β₁- and β₂-adrenoceptors are involved in the regulation of lipolysis, insulin release and renin release.