¹⁸F-FDG PET/CT Monitoring of β3 Agonist–Stimulated Brown Adipocyte Recruitment in White Adipose Tissue

Abstract

There is rising interest in recruitment of brown adipocytes into white adipose tissue (WAT) as a means to augment energy expenditure for weight reduction. We thus investigated the potential of 18 F-FDG uptake as an imaging biomarker that can monitor the process of WAT browning. Methods: C57BL/6 mice were treated daily with the β3 agonist CL316,243 (5-[(2R)-2-[[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl] amino]propyl]-1,3-benzodioxole-2,2-dicarboxylic acid disodium salt), whereas controls received saline. 18 F-FDG small-animal PET/CT was serially performed at 1 h after CL316,243 injection. After sacrifice, interscapular brown adipose tissue (BAT) and WAT depots were extracted, weighed, and measured for 18 F-FDG uptake. Tissues underwent immunostaining, and UCP1 content was quantified by Western blotting. Results: PET/CT showed low 18 F-FDG uptake in both BAT and inguinal WAT at baseline. BAT uptake was substantially increased by a single stimulation with CL316,243. Uptake in inguinal WAT was only modestly elevated by the first stimulation uptake but gradually increased to BAT level by prolonged stimulation. Ex vivo measurements recapitulated the PET findings, and measured 18 F-FDG uptake in other WAT depots was similar to inguinal WAT. WAT browning by prolonged stimulation was confirmed by a substantial increase in uncoupling protein 1 (UCP1), cytochrome-c oxidase 4 (COX4), and PR domain containing 16 (PRDM16) staining as markers of brown adipocytes. UCP1 content, which served as a measure for extent of browning, was low in baseline inguinal WAT but linearly increased over 10 d of CL316,243 injection. Finally, image-based and ex vivo-measured 18 F-FDG uptake in inguinal WAT correlated well with UCP1 content. Conclusion: 18 F-FDG PET/CT has the capacity to monitor brown adipocyte recruitment into WAT depots in vivo and may thus be useful for screening the efficacy of strategies to promote WAT browning. Subcut aneous and visceral white adipose tissue (WAT) contributes to obesity by storing excess energy as intracellular lipid (1,2). By contrast, brown adipose tissue (BAT) can have a negative influence on weight gain by dissipating energy as heat (3,4). This thermoregulatory response is mediated through norepinephrineactivated b3 adrenergic receptor (b3AR) signaling. The exciting recent revelation that functional BAT is present not only in animals and young children but also in human adults (5,6) has fueled attempts to exploit its energy-consuming property to counter obesity (7,8). Unfortunately, however, obese patients have insufficient amounts of BAT, limiting the value of constitutive BAT for weight reduction (9).On the other hand, obese subjects have abundant amounts WAT. It has recently become known that WAT can recruit clusters of adipocytes with a brownlike phenotype. This process, called WAT browning, can be seen in mice stimulated by prolonged cold exposure or by b3AR agonists as a mimetic of cold stress (10,11). Brownlike cells recruited by WAT browning are referred to as beige (pale brown) adipocytes (12)(13)...

Metabolic activity of brown, “beige,” and white adipose tissues in response to chronic adrenergic stimulation in male mice

“…There is also evidence that its thermogenic potential is much less than that of iBAT (16,30). Recently, Park et al (35) reported that browning of iWAT following chronic CL treatment (10 days) was associated with an increase in 18 FDG fractional uptake in response to acute CL, a finding that tends to be corroborated by the 18 FDG uptake results of the present study. However, one has to consider that 18 FDG uptake, in contrast to [ 11 C]acetate, does not represent a direct marker of tissue oxidative/thermogenic activity.…”

“…It has been known for years that cold exposure (24 -26, 54) and ␤ 3 -adrenergic agonism (9,17,35,53) both lead to the browning of iWAT. Our results confirm the ability to iWAT to undergo browning after chronic CL and cold and to exhibit increased mtDNA content.…”

“…Brown adipocytes can also develop in white adipose tissue (WAT) depots [e.g., inguinal WAT (iWAT)] under sustained adrenergic stimulation led to, for instance, by cold exposure (24 -26, 54) or ␤ 3 -adrenergic agonism (9,17,35,53). Development of brown adipocytes in WAT (13,47) led to the browning of white fat, giving it a "beige" appearance.…”

Preliminary evaluation of β3-adrenoceptor agonist-induced 18F-FDG metabolic activity of brown adipose tissue in obese Zucker rat

“…Activated BAT may thus have therapeutic potential to combat both diabetes and obesity with its ability to reduce plasma triglyceride levels [2]. CL316,243, a β3-adrenoceptor selective agonist, has been shown to enhance the uptake of 18 F-FDG in BAT of normal rats [3] and mice [4,5]. More recently another β3-adrenoceptor agonist, mirabegron, has been shown to activate BAT in rats [6] and humans [7].…”

Role of Brown Fat in Lipoprotein Metabolism and Atherosclerosis

“…33,34 After its uptake by the glucose transporter-1 and glucose transporter-4, 29,30 glucose is used for both de novo lipogenesis and ATP generation that supports general adipocyte function. 35 Indeed, BAT activation increases the expression of genes involved in glycolysis and the pentose phosphate pathway; pathways that provide ATP and reducing equivalents for de novo lipogenesis, respectively.…”