Thermogenic capacity is antagonistically regulated in classical brown and white subcutaneous fat depots by high fat diet and endurance training in rats: impact on whole-body energy expenditure

Abstract

This study investigated the regulation of thermogenic capacity in classical brown adipose tissue (BAT) and subcutaneous inguinal (SC Ing) white adipose tissue (WAT) and how it affects whole-body energy expenditure in sedentary and endurance-trained rats fed ad libitum either low fat or high fat (HF) diets. Analysis of tissue mass, PGC-1α and UCP-1 content, the presence of multilocular adipocytes, and palmitate oxidation revealed that a HF diet increased the thermogenic capacity of the interscapular and aortic brown adipose tissues, whereas exercise markedly suppressed it. Conversely, exercise induced browning of the SC Ing WAT. This effect was attenuated by a HF diet. Endurance training neither affected skeletal muscle FNDC5 content nor circulating irisin, but it increased FNDC5 content in SC Ing WAT. This suggests that locally produced FNDC5 rather than circulating irisin mediated the exercise-induced browning effect on this fat tissue. Importantly, despite reducing the thermogenic capacity of classical BAT, exercise increased whole-body energy expenditure during the dark cycle. Therefore, browning of subcutaneous WAT likely exerted a compensatory effect and raised whole-body energy expenditure in endurance-trained rats. Based on these novel findings, we propose that exercise-induced browning of the subcutaneous WAT provides an alternative mechanism that reduces thermogenic capacity in core areas and increases it in peripheral body regions. This could allow the organism to adjust its metabolic rate to accommodate diet-induced thermogenesis while simultaneously coping with the stress of chronically increased heat production through exercise.

Keywords: AMP-activated Kinase (AMPK); Adipose Tissue Metabolism; Exercise; Obesity; Peroxisome Proliferator-activated Receptor γ Coactivator 1-α (PGC-1a) (PPARGC1A); UCP-1, FNDC5, Irisin, Brown Adipocytes, ATGL.

FIGURE 1.

Chronic endurance training and HF diet exert antagonistic effects on mass and unilocular lipid content in the iBAT of rats. A, picture of iBATs dissected from Sed and Ex rats fed either LF or HF diets at week 8. B, respective microscopic images (×20 magnification) of H&E staining of iBAT samples from all groups of animals. Average iBAT mass (C) and unilocular adipocyte area (D) are present within the iBAT of all four groups of animals. *, p < 0.05 versus Sed LF; #, p < 0.05 versus Sed LF, Ex LF, and Ex HF; †, p < 0.05 versus Ex LF and Sed HF; ‡, p < 0.05 versus all other conditions. Two-way ANOVA (n = 8).

FIGURE 2.

Chronic endurance training reduces and HF diet increases PGC-1α and UCP-1 content, as well as palmitate oxidation in the iBAT and aBAT, respectively, brown adipose tissues. iBAT and aBAT were extracted from Sed or 8-week Ex rats fed either a LF or a HF diet. Representative blots (A and B) and densitometric analysis of PGC-1α (C and D) and UCP-1 (E and F) contents and the assessment of palmitate oxidation (G and H) in iBAT and aBAT, respectively, are shown. β-Actin was used as loading control. *, p < 0.05 versus Sed LF; #, p < 0.05 versus all other conditions. One-way ANOVA (n = 8).

FIGURE 3.

Chronic endurance training induces browning and increases the number of multilocular adipocytes in the SC Ing fat depot of LF- and HF-fed rats. Pictures of left and right SC Ing fat depots from Sed and Ex rats fed either a LF (A and B) or a HF (G and H) diet for 8 weeks. Samples of the upper extremities and middle regions of the SC Ing fat depots, as indicated by the black circles, were used for H&E staining and microscopy analyses. Representative images (×20 magnification) of adipocytes from Sed (C and D) and Ex (E and F) rats fed a LF diet or from Sed (I and J) and Ex (K and L) rats fed a HF diet are shown.

FIGURE 4.

Chronic endurance training reduces unilocular adipocyte area in the upper (A) and middle (B) regions of the SC Ing fat depots, whereas a HF diet increases unilocular adipocyte area only in the upper region. Samples of the upper and middle regions of the SC Ing fat depot (as indicated in Fig. 3) from Sed and Ex rats fed either a LF or a HF diet for 8 weeks were extracted for microscopic analysis and determination of mean unilocular adipocyte area. *, p < 0.05 versus Sed LF; #, p < 0.05 versus all other conditions, One-way ANOVA (n = 8).

FIGURE 5.

Chronic endurance training increases and HF diet reduces PGC-1α (A and B), UCP-1 (A and C), and ATGL (D and E) contents, as well as AMPK phosphorylation (F and G) and palmitate oxidation (H) in the SC Ing fat depot. The middle region of the SC Ing fat was extracted from Sed or 8-week Ex rats fed either an LF or a HF diet. β-Actin was used as loading control. *, p < 0.05 versus Sed LF; #, p < 0.05 versus all other conditions. One-way ANOVA (n = 8).

FIGURE 6.

Neither Ex nor HF diet affect UCP1 content in the Epid and Retro fat pads. The Epid and Retro fat pads were excised from Sed and 8-week endurance-trained rats fed either a HF or a LF diet. The whole tissues were used for the determination by Western blot of UCP-1 content. β-Actin was used as loading control. A sample containing 10 μg of protein extracted from the iBAT was used as a positive control for UCP-1. All samples from Epid and Retro fat pads contained 50 μg of protein.

FIGURE 7.

Chronic endurance training increases PGC-1α and FNDC5 content in SC Ing fat, but neither affects soleus muscle FNDC5 content nor circulating irisin levels. Soleus muscles were extracted from Sed and 8-week Ex rats fed either a LF or a HF diet and assayed for PGC-1α (A and B) and FNDC5 content (A and C), as well as palmitate oxidation (D). Irisin was measured in the plasma under resting conditions 24 h after the last bout of exercise (E) and immediately after exercise at week 6 (F). The middle region of the SC Ing fat depot was used for the determination of FNDC5 content (G and H). β-Actin and GAPDH were used as loading control. *, p < 0.05 versus Sed LF; #, p < 0.05 versus all other conditions. One-way ANOVA (n = 8).

FIGURE 8.

Effects of chronic Ex and HF diet on spontaneous physical activity and whole-body energy expenditure. Ambulatory activity (A and B) and energy expenditure (C and D) were measured after the last bout of exercise at the end of the 8-week endurance training period. The rats were allowed to rest for 24 h prior to being placed in the CLAMS. *, p < 0.05 versus Sed LF; #, p < 0.05 versus all other conditions. Two-way ANOVA (n = 18).