Activation of natriuretic peptides and the sympathetic nervous system following Roux-en-Y gastric bypass is associated with gonadal adipose tissues browning

Abstract

Objective: Roux-en-Y gastric bypass (RYGB) is an effective method of weight loss and remediation of type-2 diabetes; however, the mechanisms leading to these improvements are unclear. Additionally, adipocytes within white adipose tissue (WAT) depots can manifest characteristics of brown adipocytes. These 'BRITE/beige' adipocytes express uncoupling protein 1 (UCP1) and are associated with improvements in glucose homeostasis and protection from obesity. Interestingly, atrial and B-type natriuretic peptides (NPs) promote BRITE/beige adipocyte enrichment of WAT depots, an effect known as "browning." Here, we investigate the effect of RYGB surgery on NP, NP receptors, and browning in the gonadal adipose tissues of female mice. We propose that such changes may lead to improvements in metabolic homeostasis commonly observed following RYGB.

Methods: Wild type, female, C57/Bl6 mice were fed a 60% fat diet ad libitum for six months. Mice were divided into three groups: Sham operated (SO), Roux-en-Y gastric bypass (RYGB), and Weight matched, sham operated (WM-SO). Mice were sacrificed six weeks following surgery and evaluated for differences in body weight, glucose homeostasis, adipocyte morphology, and adipose tissue gene expression.

Results: RYGB and calorie restriction induced similar weight loss and improved glucose metabolism without decreasing food intake. β3-adrenergic receptor expression increased in gonadal adipose tissue, in addition to Nppb (BNP), and NP receptors, Npr1, and Npr2. The ratio of Npr1:Npr3 and Npr2:Npr3 increased in RYGB, but not WM-SO groups. Ucp1 protein and mRNA, as well as additional markers of BRITE/beige adipose tissue and lipolytic genes increased in RYGB mice to a greater extent than calorie-restricted mice.

Conclusions: Upregulation of Nppb, Npr1, Npr2, and β3-adrenergic receptors in gonadal adipose tissue following RYGB was associated with increased markers of browning. This browning of gonadal adipose tissue may underpin the positive effect of RYGB on metabolic parameters and may in part be mediated through upregulation of natriuretic peptides.

Keywords: Atrial natriuretic peptide (ANP); B-type natriuretic peptide (BNP); Browning; High fat diet (HFD); Natriuretic peptide receptor; Roux-en-Y gastric bypass (RYGB).

Figure 1

Effects of Roux-en-Y gastric bypass surgery on body weight and feeding efficiency. Three experimental groups received either sham operation (SO), Roux-en-Y gastric bypass (RYGB), or sham operation with a calorie-restricted diet (WM-SO). (A) Body weight changes over six weeks post-surgery as percentage of pre-operative body weight (SO n = 11, RYGB n = 5, WM-SO n = 6). Data presented as mean ± s.d. (B) Body weight as percentage of pre-operative body weight at post-operative week six. Each data point represents a single subject, horizontal lines denote mean and s.d. of the group (C) Fat mass as determined by nuclear magnetic resonance imaging at time of sacrifice (SO n = 6, RYGB n = 5, WM-SO n = 6). (D) Food intake during post-operative week four (SO n = 7, RYGB n = 5). (E) Feeding efficiency (calculated as average daily change in body weight divided by average daily food intake) during post-operative week four (SO n = 7, RYGB n = 5). ^∗P < 0.05 RYGB or WM-SO versus SO. ^ˆ P < 0.05 WM-SO only versus SO. ^#P < 0.05 RYGB versus WM-SO.

Figure 2

Effects of Roux-en-Y gastric bypass surgery on glucose homeostasis and adipocyte size. At post-operative week six, SO (n = 10), RYGB (n = 4), and WM-SO (n = 6) groups underwent tests of glucose homeostasis. All data presented as mean ± s.e.m. (A) Oral glucose tolerance test, plasma glucose concentrations. (B) Normalized area under curve graph for Oral glucose tolerance test. (C) Insulin tolerance test, percentage of baseline plasma glucose. (D) Normalized area under curve graph for Insulin tolerance test. (E) Representative immunohistochemical images and (F) Quantification of average adipocyte size in gonadal adipose tissues from SO, RYGB, and WM-SO groups. ^∗P < 0.05 RYGB or WM-SO versus SO. ^ˆ P < 0.05 WM-SO only versus SO. ^#P < 0.05 RYGB versus WM-SO.

Figure 3

Roux-en-Y gastric bypass surgery increases markers of beige adipocytes, natriuretic peptides and natriuretic peptide receptors in gonadal adipose tissue. (A) Representative immunohistochemical image of UCP1 protein in gonadal adipose tissue taken from SO, RYGB, and WM-SO mice. Original magnification, 20x (B) Relative expression determined by RT-qPCR of Ucp1 mRNA in gonadal, inguinal, and brown adipose tissues taken from SO (n = 6), RYGB (n = 4) and WM-SO (n = 6) mice. (B) Relative expression determined by RT-qPCR of (C) Adrb3 mRNA (D) Atgl and Hsl mRNA, and (E) Nppa (ANP), Nppb (BNP), Npr1, Npr2, Npr3 mRNA in gonadal adipose tissue taken from SO (n = 6), RYGB (n = 4) and WM-SO (n = 6) mice. (F) Ratio of Npr1:Npr3 mRNA expression and Npr2:Npr3 mRNA expression in gonadal adipose tissue taken from SO (n = 6), RYGB (n = 4) and WM-SO (n = 6) mice. All data presented as mean ± SEM. ^∗P < 0.05 RYGB or WM-SO versus SO. ^#P < 0.05 RYGB versus WM-SO.