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

Classical brown adipocytes such as those found in interscapular brown adipose tissue (iBAT) represent energy-burning cells, which have been postulated to play a pivotal role in energy metabolism. Brown adipocytes can also be found in white adipose tissue (WAT) depots (e.g. inguinal WAT - iWAT) following adrenergic stimulation and they have been referred to as "beige" adipocytes. Whether the presence of these adipocytes, which gives iWAT a "beige" appearance, can confer a white depot with some thermogenic activity remains to be seen. In consequence, we designed the present study to investigate the metabolic activity of iBAT, iWAT and epididymal white depots in mice. Mice were either (i) kept at thermoneutrality (30oC), (ii) kept at 30oC and treated daily for 14 days with an adrenergic agonist (CL) or (iii) housed at 10oC for 14 days. Metabolic activity was assessed using PET imaging with 18F-fluorodeoxyglucose (glucose uptake), 18F-fluoro-thia-heptadecanoic acid (fatty acid uptake) and 11C-acetate (oxidative activity). In each group, substrate uptakes and oxidative activity were measured in anaesthetized mice in response to acute CL. Our results revealed iBAT as a major site of metabolic activity, which exhibited enhanced glucose and NEFA uptakes and oxidative activity in response to chronic cold and CL. On the other hand, "beige" adipose tissue failed to exhibit appreciable increased in oxidative activity in response to chronic cold and CL. Altogether, our results suggest that the contribution of "beige" fat to acute-CL-induced metabolic activity is low compared to that of iBAT, even after sustained adrenergic stimulation.