Peripancreatic adipose tissue protects against high-fat-diet-induced hepatic steatosis and insulin resistance in mice

Abstract

Background/objectives: Visceral adiposity is associated with increased diabetes risk, while expansion of subcutaneous adipose tissue may be protective. However, the visceral compartment contains different fat depots. Peripancreatic adipose tissue (PAT) is an understudied visceral fat depot. Here, we aimed to define PAT functionality in lean and high-fat-diet (HFD)-induced obese mice.

Subjects/methods: Four adipose tissue depots (inguinal, mesenteric, gonadal, and peripancreatic adipose tissue) from chow- and HFD-fed male mice were compared with respect to adipocyte size (n = 4-5/group), cellular composition (FACS analysis, n = 5-6/group), lipogenesis and lipolysis (n = 3/group), and gene expression (n = 6-10/group). Radioactive tracers were used to compare lipid and glucose metabolism between these four fat depots in vivo (n = 5-11/group). To determine the role of PAT in obesity-associated metabolic disturbances, PAT was surgically removed prior to challenging the mice with HFD. PAT-ectomized mice were compared to sham controls with respect to glucose tolerance, basal and glucose-stimulated insulin levels, hepatic and pancreatic steatosis, and gene expression (n = 8-10/group).

Results: We found that PAT is a tiny fat depot (~0.2% of the total fat mass) containing relatively small adipocytes and many "non-adipocytes" such as leukocytes and fibroblasts. PAT was distinguished from the other fat depots by increased glucose uptake and increased fatty acid oxidation in both lean and obese mice. Moreover, PAT was the only fat depot where the tissue weight correlated positively with liver weight in obese mice (R = 0.65; p = 0.009). Surgical removal of PAT followed by 16-week HFD feeding was associated with aggravated hepatic steatosis (p = 0.008) and higher basal (p < 0.05) and glucose-stimulated insulin levels (p < 0.01). PAT removal also led to enlarged pancreatic islets and increased pancreatic expression of markers of glucose-stimulated insulin secretion and islet development (p < 0.05).

Conclusions: PAT is a small metabolically highly active fat depot that plays a previously unrecognized role in the pathogenesis of hepatic steatosis and insulin resistance in advanced obesity.

Fig. 1. Anatomical and morphological characterization of PAT in chow-diet-fed male mice.

a Anatomical location of mouse PAT. Representative b H&E and c perilipin-1 (red) stained sections of mouse pancreas including PAT versus intrapancreatic adipocytes (IA). Adipocyte size of PAT, MWAT, IWAT and GWAT in d chow-diet-fed mice (n = 7–10 mice) and e 8-week HFD-fed mice (n = 4–5 mice). All values are expressed as mean ± SEM. *p < 0.05 and **p < 0.01 for the indicated comparisons. PAT peripancreatic adipose tissue, MWAT mesenteric adipose tissue, IWAT inguinal adipose tissue, and GWAT gonadal adipose tissue.

Fig. 2. Gene expression profile assessed by quantitative real-time PCR in PAT, MWAT, IWAT, and GWAT.

a Relative mRNA levels in PAT, MWAT, IWAT, and GWAT on chow-diet conditions (normalized to Actb). Data are expressed relative to the level in PAT. b FACS analysis of CD45⁺ (leukocytes) and CD45⁻ cells in SVF isolated from PAT, IWAT, and GWAT. c Tissue weights of PAT, MWAT, IWAT, and GWAT in mice after 1-, 4-, 8- and 16-week HFD feeding, represented as fold-change from chow-diet-fed aged matched controls. d Relative mRNA levels in PAT, MWAT, IWAT, and GWAT in mice after 1-, 4-, 8- and 16-week HFD feeding (normalized to Actb and Tbp), represented as fold-change from chow diet. All values are expressed as mean ± SEM (n = 6 mice/group in 1, 4, and 8 weeks on HFD vs. CD; n = 10 mice/group in 16 weeks on HFD vs. CD). *p < 0.05, **p < 0.01, and ***p < 0.001 for PAT vs. other depots; †p < 0.05, ††p < 0.01, and †††p < 0.001 for mice-fed high-fat diet vs. chow diet. Adipoq adiponectin, Lep leptin, Scd1 stearoyl-coenzyme A desaturase 1, Fasn fatty acid synthase, Srebp1c sterol regulatory element-binding transcription factor-1c, Chrebp carbohydrate-responsive element-binding protein, Insr insulin receptor, Pparg peroxisome proliferator-activated receptor gamma, Glut4 glucose transporter type 4, B3ar adrenoceptor beta 3, Tnfa tumor necrosis factor alpha, Mcp1 monocyte chemoattractant protein-1, Il1b interleukin-1 beta, Arg1 arginase 1, Cd206 macrophage mannose receptor C-type 1, Ucp1 uncoupling protein-1, Cpt1b carnitine palmitoyltransferase 1b, Atp6 ATP synthase 6, PAT peripancreatic adipose tissue, MWAT mesenteric adipose tissue, IWAT inguinal adipose tissue, and GWAT gonadal adipose tissue.

Fig. 3. Lipid and glucose metabolism in PAT, MWAT, IWAT, and GWAT in chow- and 16-week HFD-fed male mice.

a, d Total lipid uptake, b, e triglyceride accumulation (organic phase), and c, f lipid oxidation (aqueous phase) in different fat depots in respectively, chow (n = 8–11 mice/group) and 16-week HFD-fed mice (n = 5–6 mice/group). g, j Total glucose uptake, h, k de novo lipogenesis from glucose (organic phase), and i, l glucose catabolism (aqueous phase) in respectively, chow (n = 8 mice/group) and 16-week HFD-fed mice (n = 5–6 mice/group). All values are expressed as mean ± SEM. Values that do not share a common letter (a, b, and c) are statistically different. PAT peripancreatic adipose tissue, MWAT mesenteric adipose tissue, IWAT inguinal adipose tissue, and GWAT gonadal adipose tissue.

Fig. 4. Pearson correlations between different fat depots and liver weight in chow and 16-week HFD-fed male mice.

Pearson correlation was used to measure the degree of correlation between liver weight and, respectively, a PAT weight, b IWAT weight, c MWAT weight, and d GWAT weight. Black squares indicate mice fed with chow diet and white circles indicate mice fed with HFD. A p value < 0.05 indicates that the Pearson’s correlations coefficient (R) is significantly different from zero. PAT peripancreatic adipose tissue, MWAT mesenteric adipose tissue, IWAT inguinal adipose tissue, and GWAT gonadal adipose tissue.

Fig. 5. Oral glucose tolerance tests in male PAT-ectomized and sham control mice on chow diet and after 8- and 16-week HFD-feeding.

Circulating glucose levels at indicated time points and AUC in response to oral glucose load in PAT-ectomized and sham controls mice on a chow diet (CD; n = 7–9 mice/group), after b 8 weeks on HFD (n = 8–10 mice/group), and c 16 weeks on HFD (n = 8–10 mice/group). Circulating insulin levels at indicated time points and AUC in response to oral glucose load in PAT-ectomized and sham controls mice on d chow diet, after e 8 weeks on HFD, and f 16 weeks on HFD. All values are expressed as mean ± SEM. *p < 0.05.

Fig. 6. Effect of PAT-ectomy on body weight and, hepatic and pancreatic ectopic lipid deposition and gene expression after 16-week HFD feeding in male mice.

a Body weight, b pancreas weight, c pancreatic triglyceride content, and d pancreatic gene expression in PAT-ectomized and sham control mice (n = 8–10 mice/group). e Average pancreatic islet area in PAT-ectomized and sham control mice and representative H&E sections (n = 3–4 mice/group). f Liver weight, g hepatic triglyceride content, h hepatic lipid droplet distribution and representative H&E section (n = 4–6 mice/group), and i hepatic gene expression in PAT-ectomized and sham control mice (n = 8–10 mice/group). All values are expressed as mean ± SEM. *p < 0.05 and **p < 0.01.