Chronic glucocorticoid exposure causes brown adipose tissue whitening, alters whole-body glucose metabolism and increases tissue uncoupling protein-1

Abstract

Adipose tissue (AT) has been found to exist in two predominant forms, white and brown. White adipose tissue (WAT) is the body's conventional storage organ, and brown adipose tissue (BAT) is responsible for non-shivering thermogenesis which allows mammals to produce heat and regulate body temperature. Studies examining BAT and its role in whole-body metabolism have found that active BAT utilizes glucose and circulating fatty acids and is associated with improved metabolic outcomes. While the beiging of WAT is a growing area of interest, the possibility of the BAT depot to "whiten" and store more triglycerides also has metabolic and health implications. Currently, there are limited studies that examine the effects of chronic stress and its ability to induce a white-like phenotype in the BAT depot. This research examined how chronic exposure to the murine stress hormone, corticosterone, for 4 weeks can affect the whitening process of BAT in C57BL/6 male mice. Separate treatments with mirabegron, a known β3-adrenergic receptor agonist, were used to directly compare the effects of corticosterone with a beiging phenotype. Corticosterone-treated mice had significantly higher body weight (p ≤ 0.05) and BAT mass (p ≤ 0.05), increased adipocyte area (p ≤ 0.05), were insulin resistant (p ≤ 0.05), and significantly elevated expressions of uncoupling protein 1 (UCP-1) in BAT (p ≤ 0.05) while mitochondrial content remained unchanged. This whitened phenotype has not been previously associated with increased uncoupling proteins under chronic stress and may represent a compensatory mechanism being initiated under these conditions. These findings have implications for the study of BAT in response to chronic glucocorticoid exposure potentially leading to BAT dysfunction and negative impacts on whole-body glucose metabolism.

Keywords: UCP-1; corticosterone; insulin resistance; metabolic syndrome; mirabegron; obesity.

FIGURE 1

Representative image of the corticosterone‐treated mice. Corticosterone mice appeared to have oily coats

FIGURE 2

Chronic corticosterone treatment resulted in mice drinking significantly (p ≤ 0.05) more of the treatment water during the course of the experiment. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (d) represents significantly (p ≤ 0.05) different from the mirabegron high treatment

FIGURE 3

Summary of changes in body weight during four weeks of treatment. The corticosterone‐treated mice gained significantly (p ≤ 0.05) more weight than both the naïve and vehicle controls, and both mirabegron treatments. This illustrates the change in body weight from the beginning of the experiment during the treatment. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, (d) represents significantly (p ≤ 0.05) different from the mirabegron high treatment, (e) represents the vehicle group being significantly (p ≤ 0.05) different than the naïve control group and (f) represents the mirabegron high group being significantly (p ≤ 0.05) different than the naïve control group. Error bars represent SEM

FIGURE 4

Fasting body weights at the end of treatment. Corticosterone‐treated mice were significantly (p ≤ 0.05) heavier than all other treatment groups. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (d) represents significantly (p ≤ 0.05) different from the mirabegron high treatment

FIGURE 5

Comparison of the BAT from each treatment. The corticosterone treatment increased BAT weights to be significantly (p ≤ 0.05) heavier than all other treatment and control groups in this study. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (d) represents significantly (p ≤ 0.05) different from the mirabegron high treatment

FIGURE 6

Representative images of BAT at 10× magnification from each treatment where the scalebar represents 200 μm in the large field of view (left) and the magnified image scalebar represents 100 μm (right)

FIGURE 7

Comparison of mean adipocyte area from each treatment. The corticosterone group had significantly (p ≤ 0.05) larger adipocyte areas than all other treatments. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the corticosterone treatment, (d) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (e) represents significantly (p ≤ 0.05) different from the mirabegron high treatment

FIGURE 8

Plasma glucose (a) and insulin (b) measurements, and (c) HOMA‐IR for each treatment. The corticosterone group illustrated significantly (p ≤ 0.05) elevated insulin concentrations and insulin resistance (HOMA‐IR) compared to all other treatment groups. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (d) represents significantly (p ≤ 0.05) different from the mirabegron high treatment

FIGURE 9

BAT UCP‐1 (a and c) and Citrate Synthase (b and d) protein expression under each treatment and relative ratio (e). The corticosterone group illustrated a significant (p ≤ 0.05) increase in the amount of UCP‐1 (a and c) expressed in each BAT when compared to the naïve and vehicle control groups (corresponding ponceau S stain is below the blot). The citrate synthase protein expressed (b and d) in each BAT remained the same amongst all treatments (corresponding ponceau S stain is below the blot). The observed size of citrate synthase is 45 kDa while the protein size is 51 kDa according to the manufacturer. The ratio of UCP‐1 to citrate synthase (e) is significantly (p ≤ 0.05) greater in both the corticosterone and mirabegron high treatments compared to both the naïve and vehicle controls. (a) represents significantly (p ≤ 0.05) different than the naïve control group, (b) represents significantly (p ≤ 0.05) different from the vehicle control group, (c) represents significantly (p ≤ 0.05) different from the corticosterone treatment, (d) represents significantly (p ≤ 0.05) different from the mirabegron low treatment, and (e) represents significantly (p ≤ 0.05) different from the mirabegron high treatment