Exercise Training in Obese Rats Does Not Induce Browning at Thermoneutrality and Induces a Muscle-Like Signature in Brown Adipose Tissue

Abstract

Aim: Exercise training elicits diverse effects on brown (BAT) and white adipose tissue (WAT) physiology in rodents housed below their thermoneutral zone (i.e., 28-32°C). In these conditions, BAT is chronically hyperactive and, unlike human residence, closer to thermoneutrality. Therefore, we set out to determine the effects of exercise training in obese animals at 28°C (i.e., thermoneutrality) on BAT and WAT in its basal (i.e., inactive) state. Methods: Sprague-Dawley rats (n = 12) were housed at thermoneutrality from 3 weeks of age and fed a high-fat diet. At 12 weeks of age half these animals were randomized to 4-weeks of swim-training (1 h/day, 5 days per week). Following a metabolic assessment interscapular and perivascular BAT and inguinal (I)WAT were taken for analysis of thermogenic genes and the proteome. Results: Exercise attenuated weight gain but did not affect total fat mass or thermogenic gene expression. Proteomics revealed an impact of exercise training on 2-oxoglutarate metabolic process, mitochondrial respiratory chain complex IV, carbon metabolism, and oxidative phosphorylation. This was accompanied by an upregulation of multiple proteins involved in skeletal muscle physiology in BAT and an upregulation of muscle specific markers (i.e., Myod1, CkM, Mb, and MyoG). UCP1 mRNA was undetectable in IWAT with proteomics highlighting changes to DNA binding, the positive regulation of apoptosis, HIF-1 signaling and cytokine-cytokine receptor interaction. Conclusion: Exercise training reduced weight gain in obese animals at thermoneutrality and is accompanied by an oxidative signature in BAT which is accompanied by a muscle-like signature rather than induction of thermogenic genes. This may represent a new, UCP1-independent pathway through which BAT physiology is regulated by exercise training.

Keywords: adipose tissue; browning; exercise training; housing temperature; obesity.

Figure 1

Exercise training (Ex) attenuated weight gain and increased brown adipose tissue (BAT) mass but had no effect on insulin, glucose, triglycerides, or non-esterified fatty acids compared to no training (C). (A) Final body weight, (B) 4 week intervention weight gain, (C) total fat mass, (D) BAT mass, (E) inguinal white adipose tissue (IWAT) mass, (F) respiratory exchange ratio (RER), (G) oxygen consumption (VO₂), (H) ambulatory activity. (I) 24h energy intake (J) serum glucose, (K) insulin, (L) HOMA-IR, (M) triglycerides, (N) NEFA and (O) Corticosterone, (P) liver weight and (Q) hepatic triglycerides. Data (F–I) obtained from CLAMs metabolic cages. All data expressed as mean ± SEM, n = 4–5 per group. For comparison, data was analyzed by either Students t-test (A–E, I–P) or two-way ANOVA (F–H) and Sidak post-hoc tests. Significance denoted as ^*p < 0.05.

Figure 2

Exercise training (Ex) regulates thermogenic genes in a depot-specific manner. (A–C) thermogenic mRNA in BAT, PVAT, and IWAT, (D,E) histological analysis of BAT and lipid droplet size and (F,G) histological analysis of WAT and adipocyte size. Data expressed as mean ± SEM, n = 4–5 per group. Adipocyte/lipid droplet area quantified using Adiposoft (BAT-C, n = 8,949 and Ex, n = 8,234; WAT-C, n = 750 and Ex – n = 555). For comparison, data was analyzed by Students t-test. Significance denoted as ^*p < 0.05 and ^****p < 0.0001.

Figure 3

Overview of enriched metabolic and skeletal muscle related gene ontology (GO) terms in brown adipose tissue (A–G) and expression of myogenic markers (H). Figures created with AdvaitaBio IPathway Guide. N = 4–5/group, for analysis see methods section.

Figure 4

Overview of enriched gene ontology (GO) terms in inguinal white adipose tissue (A–D). Figures created with AdvaitaBio IPathwayGuide. N = 4/group, for analysis see methods section.

Figure 5

Overview of significantly impacted pathways in brown adipose tissue (A–C) and inguinal white adipose tissue (D,E). (A–E) created with AdvaitaBio IPathwayGuide. N = 4/group, for analysis see methods section.

Figure 6

Protein-protein interaction network in BAT created using NetworkAnalyst showing hub proteins (bold) and proteins involved in RNA binding (yellow), steroid dehydrogenase activity (green), neuropeptide hormone activity (orange), and transcription cofactor activity (light blue). The pink/purple dots represent topological representation of other hub/seed proteins. N = 4/group, for analysis see methods section.

Figure 7

Protein-protein interaction network in WAT created using NetworkAnalyst showing hub proteins (bold) and proteins involved in RNA binding (yellow), transcription cofactor activity (light blue), and nucleotide binding (green). The pink/purple dots represent topological representation of other hub/seed proteins. N = 4/group, for analysis see methods section.