Western diet (WD) activates pancreatic Toll‐like receptor 4 (TLR4) signaling in female rats: a potential mechanism of WD‐induced insulin resistance
Authors: Mathew Shilpa , Kiernan Risa , Maddie Nicole , Carrillo-Sepulveda Maria Alicia 
Published: April 2020

BackgroundOverconsumption of a WD, a high‐fat and high‐sugar diet, has plagued the United States contributing to the widespread prevalence of type 2 diabetes (T2DM). Insulin resistance (IR) plays a key role in the development of T2DM and is a consistent finding among T2DM patients. The pathogenesis of IR, however, is not well understood. Based on recent evidence that inflammation precedes IR, and previous findings from our lab showing involvement of Toll‐like receptor 4 (TLR4) signaling in diabetic vascular inflammation, we hypothesize that WD leads to pancreatic inflammation through TLR4 signaling activation, contributing to IR.MethodsOur lab has established a model of a WD‐induced metabolic syndrome and IR in female rats. Eight‐week old female Wistar rats were randomized into two experimental groups: Control (n = 9) and WD group (n = 16). The control group received a regular chow diet (5% fat, 48.7% carbohydrates [3.2% sucrose], and 24.1% protein), while the WD group received a WD (40% fat, 43% carbohydrates [34% sucrose], 17% protein), for 28 weeks. Body weight and food consumption were measured weekly. Lipid and glucose metabolism, and intraperitoneal glucose tolerance tests (IPGTT) were measured. Quantitative IR was assessed using Homeostasis Model Assessment of Insulin Resistance (HOMA‐IR), while functional beta cell capacity was assessed via HOMA‐B calculation. At the terminal experiments, pancreas’ were isolated for molecular (TLR4 signaling) and histological (hematoxylin and eosin, H&E) analysis.ResultsWD group exhibited greater body weight (415.1 ± 30.9 vs. 323.0 ± 24.0 g controls, p<0.01), and increased daily caloric consumption (130.5 ± 7.36 vs.110.7 ± 7.0 kcals controls, p<0.05) in spite of a significant reduction in daily food intake (23.7 ± 1.5 vs. 31.5 ± 1.8 g controls, p<0.01). WD negatively interfered in the glucose metabolism as evidenced by increased blood glucose area under the curve during IPGTT (17531.8 ± 574.3 vs. 12381.25 ± 1480.6 a.u. controls, p<0.001). IR in the WD group was confirmed by increased HOMA‐IR (8.09 ± 1.90 vs. 2.05 ± .042 a.u. controls, p<0.001), which was accompanied by hyperinsulinemia (0.93 ± 0.1 vs. 0.30 ± 0.09 mg/dL controls, p<0.01). No changes were observed in the HOMA‐B, suggesting that β cells remain functional. Moreover, no evidence of vacuolation, irregular outlining and size of the Langerhans islets, which are all markers of β cell dysfunction, were observed in the H&E stained pancreatic section from the WD group. Pancreatic inflammation in the WD group was confirmed by a 42% increase in TNF‐α expression (p<0.05), an inflammatory marker. Strikingly, pancreatic TLR4 (1.8‐fold increased, p<0.05) and its downstream molecule TNF‐receptor associated factor 6 (TRAF6) (3.1‐fold increased, p<0.001) were markedly upregulated in the WD group in comparison to controls, indicating TLR4/TRAF6 signaling activation in the pancreas.ConclusionOur findings suggest that a WD can induce IR along with activation of pancreatic TLR4 signaling‐related inflammation. These results can provide new insights into how high fat and fat sugar diet choices can causes inflammation in the pancreas.Support or Funding InformationNIH