Subcutaneous adipose tissue accumulation protects systemic glucose tolerance and muscle metabolism

Abstract

The protective effects of lower body subcutaneous adiposity are linked to the depot functioning as a "metabolic sink" receiving and sequestering excess lipid. This postulate, however, is based on indirect evidence. Mechanisms that mediate this protection are unknown. Here we directly examined this with progressive subcutaneous adipose tissue removal. Ad libitum chow fed mice underwent sham surgery, unilateral or bilateral removal of inguinal adipose tissue or bilateral removal of both inguinal and dorsal adipose tissue. Subsequently mice were separated into 5 week chow or 5 or 13 week HFD groups (N = 10 per group). Primary outcome measures included adipocyte distribution, muscle and liver triglycerides, glucose tolerance, circulating adipocytokines and muscle insulin sensitivity. Subcutaneous adipose tissue removal caused lipid accumulation in femoral muscle proximal to excision, however, lipid accumulation was not proportionally inverse to adipose tissue quantity excised. Accumulative adipose removal was associated with an incremental reduction in systemic glucose tolerance in 13 week HFD mice. Although insulin-stimulated pAkt/Akt did not progressively decrease among surgery groups following 13 weeks of HFD, there was a suppressed pAkt/Akt response in the non-insulin stimulated (saline-injected) 13 week HFD mice. Hence, increases in lower body subcutaneous adipose removal resulted in incremental decreases in the effectiveness of basal insulin sensitivity of femoral muscle. The current data supports that the subcutaneous depot protects systemic glucose homeostasis while also protecting proximal muscle from metabolic dysregulation and lipid accumulation. Removal of the "metabolic sink" likely leads to glucose intolerance because of decreased storage space for glucose and/or lipids.

Keywords: adipose tissue distribution; lipectomy; metabolic sink; muscle; subcutaneous adipose tissue; triglyceride; visceral adipose tissue.

Figure 1.

Adipocyte Cell Size and Abundance, 13 week HFD only: Intra-abdominal adipocyte size distribution. Specifically, adipocyte cell size as measured by adipocytes in specific cell size bin, total adipocyte number and mean adipocyte size. A-C) Visceral adipocyte distribution and mean were not altered by Unilateral, Bilateral or All adipose tissue removal. D-F) Unilateral and All removal did not alter distribution of Epididymal adipocytes, however Bilateral IngX did. Specifically, Bilateral IngX caused significant decreases in E) total adipocyte number (* p = 0.02) that resulted from lower adipocyte numbers in all size bins, with significant decreases in the 45-54 um and 55-64 um bins (* p ≤ 0.04).

Figure 2.

Glucose and Insulin Response: One week prior to termination. A 1.5 g/kg bolus of dextrose was injected (ip) and glucose measurements were made at 0, 15, 30, 45, 60, and 120 minutes. Insulin measurements were made at 0, 15, and 120 minutes. Area under the curve (AUC) represents the total glucose/insulin excursion from 0–120 minutes. A-B) Adipose tissue removal in 5 week chow and HFD mice did not alter glucose tolerance. C) Glucose intolerance was exacerbated in 13 week HFD mice with subcutaneous adipose tissue removed. Specifically, Bilateral removal of IWAT significantly increased glucose AUC compared with Sham Control (p = 0.04). This difference was further exaggerated by the addition of dorsal adipose tissue removal causing the All removal group to have the largest significant difference in glucose AUC compared with Sham Control (p = 0.0005). In general, Insulin AUC was not altered by subcutaneous adipose tissue removal.

Figure 3.

Femoral Muscle Triglyceride Concentration. Intramuscular triglycerides were measured in mg/g. For unilateral inguinal removal, muscles were measured separately for removed and non-removed side. Subcutaneous adipose tissue removal was associated with a significant increase in femoral muscle triglyceride concentration. B) This occurred in the femoral muscle of 5 week HFD mice (One-way ANOVA surgery effect; P = 0.036). Specifically, Unilateral (both removed and non-removed), Bilateral INGx and All removal caused a ~ 2 fold increase in femoral muscle triglycerides, however only the femoral muscle of the removed side in the Unilateral inguinal surgery was significantly greater (~ 3 fold) than the respective control (p = 0.01). C) Femoral muscle triglyceride concentration from mice fed HFD for 13 weeks was not different among groups, with the exception of the muscle from the non-removed IWAT side from the Unilateral IngX. The muscle next to the non-excised adipose depot had significantly lower triglyceride concentration than its removed counterpart (p = 0.005). A) 5 Week chow mice did not differ from controls.

Figure 4.

Femoral Muscle Insulin Sensitivity. The ratio of phosphorylated Akt to total Akt was used to measure muscle insulin sensitivity. For unilateral inguinal removal, muscles were measured separately for removed and non-removed side. Corresponding western blots are shown next to each graph. A-C) Subcutaneous adipose tissue removal did decrease insulin sensitivity in some groups, but a progressive reduction in insulin-stimulated pAkt did not occur at 5 or 13 weeks. C) The 13 week HFD All group had a significant increase in insulin stimulated pAkt/Akt compared with respective controls (p = 0.0001) making it the only surgical group to have any pAkt/Akt response to the insulin injection. Instead, effects of adipose tissue removal on pAkt/Akt measures predominately occurred in saline-injected mice. C) Adipose tissue removal caused a significant decrease in pAkt/Akt of the femoral muscle of saline-injected (basal) 13 week HFD mice (One-way ANOVA surgery effect; p = 0.005). Specifically, the Unilateral (non-fat removed side) and Bilateral IWAT removal as well as the All group had significantly lower pAkt/Akt compared with control (p = 0.004, p = 0.002, p = 0.003).