Regulation of Human Adipose Tissue Activation, Gallbladder Size, and Bile Acid Metabolism by a β3-Adrenergic Receptor Agonist

Abstract

β3-adrenergic receptor (AR) agonists are approved to treat only overactive bladder. However, rodent studies suggest that these drugs could have other beneficial effects on human metabolism. We performed tissue receptor profiling and showed that the human β3-AR mRNA is also highly expressed in gallbladder and brown adipose tissue (BAT). We next studied the clinical implications of this distribution in 12 healthy men given one-time randomized doses of placebo, the approved dose of 50 mg, and 200 mg of the β3-AR agonist mirabegron. There was a more-than-dose-proportional increase in BAT metabolic activity as measured by [18F]-2-fluoro-D-2-deoxy-d-glucose positron emission tomography/computed tomography (medians 0.0 vs. 18.2 vs. 305.6 mL ⋅ mean standardized uptake value [SUVmean] ⋅ g/mL). Only the 200-mg dose elevated both nonesterified fatty acids (68%) and resting energy expenditure (5.8%). Previously undescribed increases in gallbladder size (35%) and reductions in conjugated bile acids were also discovered. Therefore, besides urinary bladder relaxation, the human β3-AR contributes to white adipose tissue lipolysis, BAT thermogenesis, gallbladder relaxation, and bile acid metabolism. This physiology should be considered in the development of more selective β3-AR agonists to treat obesity-related complications.

Trial registration: ClinicalTrials.gov NCT01950520.

© 2018 by the American Diabetes Association.

Figures

Figure 1

Study design. Subjects entered the metabolic chamber at 0800 h and remained there until 1400 h, after which they were transported to the PET/CT suite. Blood was drawn before treatment (tx) at 0800 h and then just prior to 18F-FDG administration at 1300 h. The black bars above the diagrams refer to the 30-min still periods during which REE was measured. PK, pharmacokinetic.

Figure 2

Mirabegron activation of BAT and tissue β3-AR expression. A: PET images demonstrating the dose-dependent pattern of BAT activation in a representative subject. The supraclavicular BAT depot is identified by the white arrowhead. B: Mirabegron-induced BAT metabolic activity in the 12 subjects who had detectable cold-activated BAT after a single dose of placebo, 50 mg mirabegron, and 200 mg mirabegron. For the 50-mg and 200-mg doses, the wider lines are the group medians and the narrower upper and lower lines show the interquartile ranges. The P values for the nonparametric Wilcoxon signed rank tests are shown. mRNA expression of the β3-AR (Adrb3) (C) and Ucp1 (D) in mouse iWAT, epididymal WAT (eWAT), interscapular BAT (iBAT), skeletal muscle (SkMs), heart myocardium (Heart), gallbladder (GBdr), and urinary bladder (UBdr). Expression was normalized to the iWAT depot geometric mean; n = 4 for each of the sites. mRNA expression of the β3-AR (ADRB3) (E) and UCP1 (F) in human subcutaneous WAT (Scu), visceral WAT (Vis), supraclavicular BAT (Scl), skeletal muscle, heart myocardium, gallbladder, and urinary bladder. Expression was normalized to the human subcutaneous WAT depot geometric mean. n = 3–12 postmortem subjects. Black bars are the geometric means. #P values <0.05 compared with other depots for all paired comparisons using Tukey-Kramer honest significant difference tests.

Figure 3

Mirabegron pharmacokinetic and pharmacodynamic profiles. A: Mean plasma concentrations of mirabegron in the 12 subjects who had detectable cold-activated BAT after taking a single dose of placebo, 50 mg mirabegron, and 200 mg mirabegron. Changes in plasma glucose (n = 11) (B) and insulin (n = 6) (C) after oral administration of placebo and 50 and 200 mg of the β3-AR agonist mirabegron. The baseline (BSL) levels are shown in the inset table on the left and the changes in area under the curve (AUC) in the inset table on the right. Values are mean ± SEM. For individual time points, * indicates paired t tests with P < 0.05.

Figure 4

Effects of mirabegron and BAT activity on REE. Change in REE as measured in a metabolic chamber during 20-min still periods as described in research design and methods after dosing with placebo (triangles) (A), 50 mg mirabegron (squares) (B), and 200 mg mirabegron (circles) (C). P values shown are for the paired Student t tests comparing pretreatment (Pre) (0800 h) with posttreatment (Post) (1300 h, the time of 18F-FDG injection). D: Relationship between the change in REE and BAT metabolic activity. P values were determined using a linear mixed-effects model to account for each subject taking three different doses of medication. n = 34.

Figure 5

Metabolomic analysis after exposure to the β3-AR agonist mirabegron. A: Volcano plot of 443 metabolites comparing the fold induction before and then after oral administration of 200 mg mirabegron. The vertical lines indicate changes log2-fold >1.0 or <1.0. The horizontal line indicates –log10P values >1.3 (P < 0.05) based on paired Student t tests. Metabolites meeting those criteria are shown by magenta circles, with bile acids (decreased, blue circles) and long-chain fatty acids (increased, red circles) named. n = 13. B: Quantitative enrichment analysis of the metabolic pathways most affected by treatment with 200 mg mirabegron. n = 13. C: Relationship between mirabegron dose and gallbladder size. Red bars are sample means. D–G: Dose-response effects of mirabegron on plasma bile acid levels. For glycochenodeoxycholate (D), glycocholic acid (E), glycodeoxycholate (F), and taurodeoxycholic acid (G), the effects of fasting are shown through unpaired Student t tests comparing baseline (n = 36) with placebo (n = 11). The effects of mirabegron dose are shown through one-way means comparisons between the average baseline levels of the bile acids combining the placebo, 50-mg, and 200-mg days (n = 36) and the posttreatment levels of the three different treatment days, shown individually. n = 11–13. In the box plots, the middle lines are the group medians and the upper and lower lines show the interquartile ranges. a.u., arbitrary units; FSH/LH, follicle-stimulating hormone/leutinizing hormone; PGD2, prostaglandin D2.

Figure 6

Bile acid processing in humans and the potential role of β3-AR agonists. For bile acid synthesis and enterohepatic cycling in humans, the hepatic enzyme that is the rate-determining step for production from cholesterol is CYP7A1. Its cellular activity is reflected by plasma C4. Primary unconjugated (black ovals) cholic acid (CA) and chenodeoxycholic acid (CDCA); primary conjugated (black rectangles) glycochenodeoxycholate (G-CDCA) and glycocholate (G-CA); secondary unconjugated (gray oval) deoxycholic acid (DCA); and secondary conjugated (gray rectangles) glycodeoxycholate (G-DCA) and taurodeoxycholate (T-DCA). The rate of ileal transepithelial bile salt flux is reflected by plasma levels of FGF19. Bile acids return to the liver; those that are not taken up are detected in the peripheral circulation, which the bile acids enter via the inferior vena cava (IVC). Closed arrowheads indicate the movement of bile acids, and open arrowheads indicate flow of blood. The * indicates a conjugation step, the addition of either choline or taurine by liver hepatocytes.