Why the study?
Conflicting results exist regarding brown adipose tissue 18F-FDG uptake in mice with and without UCP1, potentially due to variations in animal handling techniques.
18F-FDG uptake in BAT during adrenergic stimulation increases consistently in wildtype mice but is independent of β3 adrenergic stimulation in UCP1-knockout mice, indicating high variability that precludes using a simple SUV threshold to differentiate the two.
Animal BAT PET/CT findings remain hypothesis-generating; leaves open translation to human thermogenic assessment.
Background Brown adipose tissue (BAT) is a fat tissue found in most mammals that helps regulate energy balance and core body temperature through a sympathetic process known as non-shivering thermogenesis. BAT activity is commonly detected and quantified in 18 F-fluorodeoxyglucose ( 18 F-FDG) positron emission tomography/computed tomography (PET/CT) scans, and radiotracer uptake in BAT during adrenergic stimulation is often used as a surrogate measure for identifying thermogenic activity in the tissue. BAT thermogenesis is believed to be contingent upon the expression of the protein UCP1, but conflicting results have been reported in the literature concerning 18 F-FDG uptake within BAT of mice with and without UCP1. Differences in animal handling techniques such as feeding status, type of anesthetic, type of BAT stimulation, and estrogen levels were identified as possible confounding variables for 18 F-FDG uptake. In this study, we aimed to assess differences in BAT 18 F-FDG uptake between wildtype and UCP1-knockout mice using a protocol that minimizes possible variations in BAT stimulation caused by different stress responses to mouse handling. Results 18 F-FDG PET/CT scans were ran on mice that were anesthetized with pentobarbital after stimulation of non-shivering thermogenesis by norepinephrine. While in wild type mice 18 F-FDG uptake in BAT increased significantly with norepinephrine stimulation of BAT, there was no consistent change in 18 F-FDG uptake in BAT of mice lacking UCP1. Conclusions 18 F-FDG uptake within adrenergically stimulated BAT of wildtype and UCP1-knockout mice can significantly vary such that an 18 F-FDG uptake threshold cannot be used to differentiate wildtype from UCP1-knockout mice. However, while an increase in BAT 18 F-FDG uptake during adrenergic stimulation is consistently observed in wild type mice, in UCP1-knockout mice 18 F-FDG uptake in BAT seems to be is independent of β 3 adrenergic stimulation of non-shivering thermogenesis.
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McHugh et al. (2020) studied this question.
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