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

Excess nutrients are typically stored in adipose tissue; when this storage capacity is exceeded or maladaptive, lipids accumulate ectopically in non-adipose tissues, contributing to insulin resistance and metabolic disease. Subcutaneous adipose tissue expansion (often via hyperplasia) has been associated with metabolic protection, whereas increased visceral adiposity correlates with metabolic syndrome and insulin resistance. Mechanisms underlying depot-specific risk include differences in inflammation, lipolysis, fatty acid storage, adipokine secretion, and gene expression. Peripancreatic adipose tissue (PAT) is a visceral fat depot distinct from intrapancreatic adipocytes. Intrapancreatic adipocytes typically arise under adipogenic conditions (e.g., HFD) and may influence pancreatic lipid content and islet function. In contrast, PAT contains adipocytes and diverse stromal-vascular cells and has been implicated in crosstalk with islets and in inflammatory processes. However, its intrinsic characteristics and role in diabetes pathogenesis are largely unknown. The aim of this study was to define the morphology, cellular composition, and metabolic function of PAT in lean and HFD-induced obese mice, and to determine whether PAT contributes to systemic metabolic outcomes, including hepatic steatosis and insulin resistance.

Prior research indicates that visceral adiposity is a stronger predictor of metabolic syndrome and insulin resistance than subcutaneous fat, with interventions such as partial visceral lipectomy improving metabolic profiles and visceral fat transplantation promoting atherogenesis in mice. Depot-specific differences in inflammation, lipolysis, fatty acid storage, adipokine secretion, and gene expression have been documented. Intrapancreatic adipocytes increase under HFD and may modulate pancreatic fat content and islet function via adipokines and fatty acids. Emerging data suggest PAT engages in crosstalk with pancreatic islets: PAT and islet omics in rats implicate coordinated changes during obesity, and PAT (but not GWAT/IWAT) secretome can enhance beta-cell proliferation in vitro. PAT in mice and humans contains inflammatory foci (lymphoid-like structures) and may contribute to HFD/obesity-driven acceleration of pancreatic neoplasia in a cancer model. Despite these observations, fundamental characterization of PAT and its role in metabolic disease remained limited prior to this study.

Animal model and diets: Adult male C57BL/6J mice were maintained with ad libitum access to water and standard chow. For diet-induced obesity, mice were randomly assigned to chow or high-fat diet (HFD; 60% fat, 20% protein, 20% carbohydrate; D12492) for the last 1, 4, 8, or 16 weeks before tissue collection (age 16 weeks, except 16-week chow and HFD groups harvested at 24 weeks). Animals were fasted 4 h before collection. Tissues collected included unilateral GWAT, unilateral IWAT, MWAT, PAT, pancreas, and liver; weights recorded; samples snap-frozen for molecular analyses or processed for histology. Measurements and assays: - Histology: H&E and perilipin-1 immunostaining to assess adipocyte morphology and to distinguish PAT from intrapancreatic adipocytes. - Adipocyte size quantification in PAT, MWAT, IWAT, GWAT (n=7–10 chow; n=4–5 after 8-week HFD). - Stromal vascular fraction (SVF) cellular composition by flow cytometry (FACS) including CD45+ leukocytes and subsets (fibroblasts, macrophages, dendritic cells, lymphocytes) (n=5–6/group). - Ex vivo adipocyte function: basal and insulin-stimulated de novo lipogenesis; basal and CL-316,243 (β3-agonist)-stimulated lipolysis (n=3/group). - Gene expression: quantitative real-time PCR across depots for adipokines, lipogenesis, insulin sensitivity, lipolysis, inflammation, macrophage polarization markers, and mitochondrial function markers (n=6–10/group); normalization to Actb (and Tbp for HFD time-course) and expression reported relative to PAT or fold-change vs chow. - In vivo metabolism: radioactive tracers to compare lipid and glucose uptake/metabolism across depots (n=5–11/group). PAT removal experiment: - Surgical PAT-ectomy: Littermate male C57BL/6J mice (8–10 weeks) underwent left-side abdominal incision under isoflurane anesthesia; PAT identified using spleen as landmark and resected (5±0.4 mg) in PAT-ectomy group; sham controls had PAT cut but not removed. Peritoneum sutured, skin stapled; postoperative analgesia with buprenorphine (Temgesic, 0.05 mg/kg i.p.) and thermal support provided. - Post-surgery, mice were challenged with HFD for 16 weeks. Outcomes assessed: oral glucose tolerance; basal and glucose-stimulated insulin levels; hepatic and pancreatic triglyceride/steatosis; pancreatic islet morphology; gene expression (n=8–10/group). Statistics: Data presented as mean±SEM; comparisons by one- or two-way ANOVA or two-tailed Student’s t-test with variance similarity confirmed; Pearson correlation for depot vs liver weights; log-transformation as needed; significance at p<0.05.

- PAT characteristics: PAT is a very small depot in chow-fed mice (5–10 mg; ~0.2% of total fat mass). PAT adipocytes are smaller than those in MWAT, IWAT, and GWAT under chow conditions; maximal adipocyte size is also smaller. After 8 weeks of HFD, adipocyte size differences across depots diminish. - Cellular composition and gene expression: PAT shows lower mRNA levels for leptin, adiponectin, lipogenesis, insulin sensitivity, and lipolysis genes compared with other depots, and higher expression of several inflammatory markers. Despite this, PAT adipocytes exhibit comparable basal/insulin-stimulated de novo lipogenesis and greater CL-316,243–stimulated lipolysis vs MWAT. PAT contains approximately 10-fold more stromal vascular cells per gram than IWAT or GWAT, including more fibroblasts, macrophages, dendritic cells, and lymphocytes. - Metabolic activity: In vivo, PAT exhibits increased glucose uptake and fatty acid oxidation compared with other depots in both lean and obese mice. - HFD time course and transcriptional responses: Fat pad weight gain over 1–8 weeks on HFD is similar across depots; by 16 weeks, IWAT shows the greatest gain, while PAT increases more than MWAT and GWAT. PAT’s adiponectin mRNA declines only after 16 weeks on HFD, whereas other depots show early (1-week) reductions. Pro-inflammatory Tnfa shows modest early increases in PAT and IWAT; GWAT shows marked increases at 16 weeks. Pan-macrophage marker F4/80 rises in PAT and MWAT with HFD duration. - Liver association: Among depots, only PAT weight correlates positively with liver weight in obese mice (R=0.65; p=0.009). - PAT removal outcomes: Surgical PAT-ectomy followed by 16-week HFD aggravates hepatic steatosis (p=0.008) and increases both basal (p<0.05) and glucose-stimulated (p<0.01) insulin levels, indicating worsened insulin resistance/hyperinsulinemia. PAT removal enlarges pancreatic islets and increases pancreatic expression of markers related to glucose-stimulated insulin secretion and islet development (p<0.05).

This study addresses whether the peripancreatic adipose tissue (PAT) depot has distinct metabolic and immunologic features and whether it contributes to systemic metabolic homeostasis during diet-induced obesity. The findings demonstrate that PAT, despite its small size, has a unique composition with many stromal-vascular and immune cells and adipocytes that are highly responsive to lipolytic stimulation. PAT’s elevated glucose uptake and fatty acid oxidation suggest it serves as a metabolically active sink for substrates in proximity to the pancreas. The observation that PAT weight correlates with liver weight in obese mice, coupled with the exacerbation of hepatic steatosis and hyperinsulinemia after PAT removal, indicates that PAT exerts a protective role against HFD-induced hepatic lipid accumulation and insulin resistance. The delayed suppression of adiponectin and the nuanced inflammatory gene responses in PAT relative to other depots suggest depot-specific adaptation to HFD rather than overt dysfunction. Enlarged islets and increased expression of genes related to insulin secretion and islet development after PAT-ectomy imply that PAT modulates pancreatic endocrine function, potentially through local paracrine or substrate-mediated mechanisms. Overall, the data reveal PAT as a metabolically active, protective visceral depot whose absence worsens key features of metabolic disease in obesity.

PAT is a distinct, small visceral fat depot characterized by high metabolic activity, smaller adipocytes, and a rich stromal-vascular/immune cell milieu. In vivo it shows elevated glucose uptake and fatty acid oxidation compared with other depots. Importantly, removal of PAT prior to HFD feeding exacerbates hepatic steatosis and hyperinsulinemia and alters pancreatic islet morphology and gene expression, supporting a protective role for PAT against obesity-associated hepatic lipid accumulation and insulin resistance. Future studies should identify the specific PAT-derived mediators and cell types responsible for these effects, delineate mechanisms of PAT–pancreas–liver crosstalk, and assess whether augmenting PAT function can mitigate metabolic disease progression.