Whole Body and Regional Quantification of Active Human Brown Adipose Tissue Using <sup>18</sup>F-FDG PET/CT

In endothermic animals, brown adipose tissue (BAT) is activated to produce heat for defending body temperature in response to cold. BAT&#39;s ability to ...

National Institutes of Health

Human brown adipose tissue (BAT) presence, metabolic activity, and estimated mass are typically measured by imaging [18F]fluorodeoxyglucose (FDG) uptake in response to cold exposure in regions of the body expected to contain BAT, using positron emission tomography combined with X-ray computed tomography (FDG-PET/CT). Efforts to describe the epidemiology and biology of human BAT are hampered by diverse experimental practices, making it difficult to directly compare results among laboratories. An expert panel was assembled by the National Institute of Diabetes and Digestive and Kidney Diseases on November 4, 2014 to discuss minimal requirements for conducting FDG-PET/CT experiments of human BAT, data analysis, and publication of results. This resulted in Brown Adipose Reporting Criteria in Imaging STudies (BARCIST 1.0). Since there are no fully validated best practices at this time, panel recommendations are meant to enhance comparability across experiments, but not to constrain experimental design or the questions that can be asked.

Brown Adipose Reporting Criteria in Imaging STudies (BARCIST 1.0): Recommendations for Standardized FDG-PET/CT Experiments in Humans.

Human brown adipose tissue (BAT) can be activated to increase glucose uptake and energy expenditure, making it a potential target for treating obesity and metabolic disease. Data on the functional and anatomic characteristics of BAT are limited, however. In 20 healthy young men [12 lean, mean body mass index (BMI) 23.2 ± 1.9 kg/m; 8 obese, BMI 34.8 ± 3.3 kg/m] after 5 h of tolerable cold exposure, we measured BAT volume and activity by F-labeled fluorodeoxyglucose positron emission tomography/computerized tomography (PET/CT). Obese men had less activated BAT than lean men (mean, 130 vs. 334 mL) but more fat in BAT-containing depots (mean, 1,646 vs. 855 mL) with a wide range (0.1-71%) in the ratio of activated BAT to inactive fat between individuals. Six anatomic regions had activated BAT-cervical, supraclavicular, axillary, mediastinal, paraspinal, and abdominal-with 67 ± 20% of all activated BAT concentrated in a continuous fascial layer comprising the first three depots in the upper torso. These nonsubcutaneous fat depots amounted to 1.5% of total body mass (4.3% of total fat mass), and up to 90% of each depot could be activated BAT. The amount and activity of BAT was significantly influenced by region of interest selection methods, PET threshold criteria, and PET resolutions. The present study suggests that active BAT can be found in specific adipose depots in adult humans, but less than one-half of the fat in these depots is stimulated by acute cold exposure, demonstrating a previously underappreciated thermogenic potential.

Mapping of human brown adipose tissue in lean and obese young men.

Brown adipose tissue (BAT) in adult humans has been recently rediscovered and intensively investigated as a new potential therapeutic target for obesity and type 2 diabetes (T2D). However, reliable assessment of BAT mass in vivo represents a considerable challenge. The purpose of this investigation is to demonstrate for the first time that human BAT depots can be imaged with a translocator protein (TSPO)-specific positron emission tomography (PET) tracer [C]PBR28 under thermoneutral conditions.

PET Imaging of Human Brown Adipose Tissue with the TSPO Tracer [C]PBR28.

Brown adipose tissue (BAT) has been identified as a potential target in the treatment and prevention of obesity and metabolic disease. The precise kinetics of BAT activation and the duration of stimulus required to recruit metabolically active BAT, and its subsequent deactivation, are not well-understood. In this clinical trial, 19 healthy adults (BMI: 23.7 ± 0.7 kg/m, Age: 31.2 ± 2.8 year, 12 female) underwent three different cooling procedures to stimulate BAT glucose uptake, and active BAT volume was determined using F-Fluorodeoxyglucose (FDG) PET/CT imaging. We found that 20 min of pre-injection cooling produces activation similar to the standard 60 min (39.9 mL vs. 44.2 mL, p = 0.52), indicating that BAT activity approaches its peak function soon after the initiation of cooling. Furthermore, upon removal of cold exposure, active BAT volume declines (13.6 mL vs. 44.2 mL, p = 0.002), but the deactivation process persists even hours following cessation of cooling. Thus, the kinetics of human BAT thermogenesis are characterized by a rapid increase soon after cold stimulation but a more gradual decline after rewarming. These characteristics reinforce the feasibility of developing mild, short-duration cold exposure to activate BAT and treat obesity and metabolic disease.

Kinetics of human brown adipose tissue activation and deactivation.

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 [F]-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 [SUV] ⋅ 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.

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

Beige/brite adipose tissue displays morphological characteristics and beneficial metabolic traits of brown adipose tissue. Previously, we showed that TGF-β signaling regulates the browning of white adipose tissue. Here, we inquired whether TGF-β signals regulated presumptive beige progenitors in white fat and investigated the TGF-β regulated mechanisms involved in beige adipogenesis.

Aaron M. Cypess

Diabetes,
Endocrinology,
and Obesity Branch,
National Institute of Diabetes and Digestive and Kidney Diseases,
Diabetes, Endocrinology, and Obesity Branch, National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health

Whole Body and Regional Quantification of Active Human Brown Adipose Tissue Using ¹⁸F-FDG PET/CT

Aaron M. Cypess

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Diabetes,Endocrinology,and Obesity Branch,National Institute of Diabetes and Digestive and Kidney Diseases,Diabetes, Endocrinology, and Obesity Branch, National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health

Whole Body and Regional Quantification of Active Human Brown Adipose Tissue Using 18F-FDG PET/CT

Diabetes,
Endocrinology,
and Obesity Branch,
National Institute of Diabetes and Digestive and Kidney Diseases,
Diabetes, Endocrinology, and Obesity Branch, National Institute of Diabetes and Digestive and Kidney Diseases

Whole Body and Regional Quantification of Active Human Brown Adipose Tissue Using ¹⁸F-FDG PET/CT