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

Abstract

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 1) kept at thermoneutrality (30°C), 2) kept at 30°C and treated daily for 14 days with an adrenergic agonist [CL-316,243 (CL)], or 3) housed at 10°C for 14 days. Metabolic activity was assessed using positron emission tomography imaging with fluoro-[(18)F]deoxyglucose (glucose uptake), fluoro-[(18)F]thiaheptadecanoic acid (fatty acid uptake), and [(11)C]acetate (oxidative activity). In each group, substrate uptakes and oxidative activity were measured in anesthetized mice in response to acute CL. Our results revealed iBAT as a major site of metabolic activity, which exhibited enhanced glucose and nonesterified fatty acid uptakes and oxidative activity in response to chronic cold and CL. On the other hand, beige adipose tissue failed to exhibit appreciable increase 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 with that of iBAT, even after sustained adrenergic stimulation.

Keywords: beige adipose tissue; brown adipose tissue; energy metabolism; oxidative metabolism; positron emission tomography.

Fig. 1.

Chronic adrenergic stimulation promotes the formation of multilocular and uncoupling protein 1 (UCP1)-expressing cells. Black bars represent interscapular brown adipose tissue (iBAT), gray bars represent interscapular white adipose tissue (iWAT), and open bars represent epididymal white adipose tissue (eWAT) (n = 6 mice for each experimental condition). A: Ucp1 gene expression. B: ratio of mitochondrial DNA to nuclear DNA was determined by quantitative PCR of nuclear (Actin) and mitochondrial (Nd1) genes. C: UCP1-immunostained brown adipose tissue (BAT), iWAT, and eWAT. Representative sections from indicated tissues and conditions are shown. Data are expressed as means ± SE. ***P < 0.001 vs. 30°C and ##P < 0.01 and ###P < 0.001 vs. iBAT, assessed by post hoc Bonferroni test following 2-way ANOVA.

Fig. 2.

Glucose metabolism in various adipose depots. Black bars represent iBAT, orange bars represent iWAT, and open bars represent eWAT [n = 5 mice for 30°C at basal state, n = 4 mice for 30°C and 30°C-chronic CL-316,243 (CL), and n = 5 mice for 10°C following acute CL]. A: fractional glucose uptake determined using the Patlak graphic approach following a 30-min dynamic scan. B: total dynamic glucose uptake corrected for tissue weight. C: representative coronal computed tomography (CT) images. Brown arrows show iBAT region. D: representative positron emission tomography (PET)-CT coregistration following chronic cold exposure. Brown arrows show positive fluoro-[¹⁸F]deoxyglucose (¹⁸FDG) uptake in iBAT. E: representative coronal CT images. Orange arrows show iWAT region. F: representative PET-CT coregistration following chronic cold exposure. Orange arrows show positive ¹⁸FDG uptake in iWAT. Data are expressed as means ± SE. ***P < 0.001 vs. 30°C + acute CL and ###P < 0.001 vs. iBAT, assessed by post hoc Bonferroni test following 2-way ANOVA.

Fig. 3.

Nonesterified fatty acid (NEFA) metabolism in various adipose depots. Black bars represent iBAT, orange bars represent iWAT, and open bars represent eWAT (n = 5 mice for 30°C at basal state, n = 4 mice for 30°C and 30°C-chronic CL, and n = 6 mice for 10°C following acute CL). A: fractional NEFA uptake determined using the Patlak graphic approach following a 30-min dynamic scan. B: total dynamic NEFA uptake corrected for tissue weight. C: representative coronal CT images. Brown arrows show iBAT region. D: representative PET-CT coregistration following chronic cold exposure. Brown arrows show positive 14(R,S)-fluoro-[¹⁸F]-6-thiaheptadecanoic acid (¹⁸FTHA) uptake in iBAT. E: representative coronal CT images. Orange arrows show iWAT region, and pink arrows show eWAT region. F: representative PET-CT coregistration following chronic cold exposure. Orange arrows show positive ¹⁸FTHA uptake in iWAT, and pink arrows show positive ¹⁸FTHA uptake in eWAT. Data are expressed as means ± SE. ***P < 0.001 vs. 30°C + acute CL, ##P < 0.01 and ### P < 0.001 vs. iBAT, and ‡P < 0.001 vs. iWAT assessed by post hoc Bonferroni test following 2-way ANOVA.

Fig. 4.

Effects of chronic adrenergic stimulation on blood flow and total oxidative activity index of various adipose depots. Mean standard uptake value (SUV) time-activity curves for 30°C mice (A), 30°C-chronic CL mice (B), and 10°C mice (C). Red dots show iBAT, orange squares show iWAT, and pink triangles show eWAT. Black bars represent iBAT, gray bars represent iWAT, and open bars represent eWAT (n = 4 mice for 30°C, n = 5 mice for 30°C-chronic CL, and n = 6 mice for 10°C). Total tissue blood flow (D) and total oxidative activity index (E) were calculated based on a 3-compartment kinetic model following a 20-min dynamic scan and corrected for respective tissue weight. Data are expressed as means ± SE. ***P < 0.001 vs. 30°C and ###P < 0.001 vs. iBAT, assessed by post hoc Bonferroni test following 2-way ANOVA.