Fibroblast Growth Factor 21 (FGF21) Protects against High Fat Diet Induced Inflammation and Islet Hyperplasia in Pancreas

Abstract

Fibroblast growth factor 21 (FGF21) is an important endocrine metabolic regulator expressed in multiple tissues including liver and adipose tissue. Although highest levels of expression are in pancreas, little is known about the function of FGF21 in this tissue. In order to understand the physiology of FGF21 in the pancreas, we analyzed its expression and regulation in both acinar and islet tissues. We found that acinar tissue express 20-fold higher levels than that observed in islets. We also observed that pancreatic FGF21 is nutritionally regulated; a marked reduction in FGF21 expression was noted with fasting while obesity is associated with 3-4 fold higher expression. Acinar and islet cells are targets of FGF21, which when systemically administered, leads to phosphorylation of the downstream target ERK 1/2 in about half of acinar cells and a small subset of islet cells. Chronic, systemic FGF21 infusion down-regulates its own expression in the pancreas. Mice lacking FGF21 develop significant islet hyperplasia and periductal lymphocytic inflammation when fed with a high fat obesogenic diet. Inflammatory infiltrates consist of TCRb+ Thy1+ T lymphocytes with increased levels of Foxp3+ regulatory T cells. Increased levels of inflammatory cells were coupled with elevated expression of cytokines such as TNFα, IFNγ and IL1β. We conclude that FGF21 acts to limit islet hyperplasia and may also prevent pancreatic inflammation.

Fig 1. Expression and nutritional regulation of FGF21 in pancreas.

FGF21 is regulated by the nutritional status in fed, fasted and diet-induced obesity (DIO). Fasting reduced FGF21 mRNA in whole pancreas (A). β-klotho, the co-receptor of FGF21, is expressed in the pancreas and is inversely regulated by fasting (B). FGF21 protein was only detected in pancreas compared to liver and inguinal white adipose tissue (IWAT) (C). Pancreas FGF21 protein level was reduced by fasting in both chow fed and high fat diet fed conditions (D, E). Separation of pancreatic fractions to evaluate the individual contribution of islets and acinar tissue on FGF21 expression show that acinar pancreas contributes to the majority of FGF21 levels while islet cells have very low FGF21 levels and are not nutritionally regulated (F). Acinar expression of FGF21 is regulated with fasting (F, G). n = 6 per group. Each western blot lane represents an individual animal.

Fig 2. Induction of ERK1/2 phosphorylation in pancreas with acute FGF21 exposure and Co-immunostaining with endocrine hormones.

Acute FGF21 exposure induces ERK1/2 signaling events therefore phosphorylation of ERK1/2 (pERK1/2) has been chosen as a marker of FGF21 signaling. Up to 50% of acinar cells show strong nuclear pERK1/2 staining (brown) after FGF21 administration (A i, iii). Islets represent a very unique pattern with pERK1/2 only on the peripheral cells (A ii, iv). Percentage of pERK1/2-labeled acinar cells with saline and FGF21 administration (B). Western blot of pERK1/2 and total ERK1/2 from the same pancreas samples (C). Epididymal white adipose tissue (EWAT) is a positive control for FGF21 signaling (C). S = saline and F = FGF21. Confocal photomicrographs show FGF21-induced pERK1/2 immunoreactivity in islets (D). Absence of pERK1/2 staining (green) after saline treatment (Di; Inset, islet outline is indicated by insulin-immunoreactivity shown in red). FGF21 did not elicit pERK1/2 in insulin (red)-producing β-cells (D ii). Merged images show pERK1/2 co-localization in glucagon-positive α cells (D iii) and somatostatin-positive δ cell (D iv). Split panels show glucagon (iiia) and pERK (iiib) labeling or somatostatin (iva) and pERK (ivb) labeling in single cells. A robust and specific pERK1/2 staining was observed in almost 10% of total islet nuclei mostly at the periphery. Outlined region (dashed line) in the inset indicates the region represented in each panel. Representative pERK1/2 labelled cells are indicated by white arrows. n = 4 per group. Scale: 200 μm, A; 20 μm, D.

Fig 3. Effect of exogenous treatment of FGF21 in pancreas.

Chronic FGF21 treatment for 3 days by mini osmotic pumps led to the reduction in FGF21 expression in pancreas (A). FGF21 infusion down-regulated insulin but glucagon, somatostatin or amylase expression. While FGF21 infusion did not alter serum glucose levels (B), it reduced the serum levels of pancreatic hormones, insulin (C) and glucagon (D) without affecting the amylase activity (E). n = 10 per group.

Fig 4. Long term high fat diet feeding caused severe islet hyperplasia in FGF21 KO animals.

WT and FGF21 KO mice on chow diet have similar body weight (A), glucose tolerance (B) and insulin secretion in response to glucose (C), despite notable differences in their pancreatic morphology (D i, iii). After consuming high fat diet for 16 weeks, FGF21 KO mice (D iv) had increased islet hyperplasia compared with WT mice (D ii). Islet cell surface area was measured and represented (E). Islet cell proliferation markers were substantially over expressed in obese FGF21 KO animals (F). Islet hyperplasia leads to increased serum insulin (G) in FGF21 KO animals with a slight reduction in blood glucose (H) (n = 7 per group). Scale: 1 mm.

Fig 5. FGF21 KO animals demonstrate large perivascular inflammation on obesogenic diet.

HFD consumption caused severe pancreatic inflammation in FGF21 KO mice (A). H&E staining showing, that compared to WT animals (i), FGF21 KO mice developed severe pancreatic periductal inflammation (outlined area in ii-iii) when consuming a HFD diet for 16 weeks. Lymphocytic nature of inflammatory cells is shown in a higher magnification image (iii). (B) Immunohistochemical analysis for the lymphocytic marker CD3 on the pancreas of WT (i) and FGF21 KO animals (ii-iii) is represented. FGF21 KO showed higher number of CD3, T cell receptor antigen. n = 7 per group. Scale: 1 mm, Ai-ii; 2 mm, Bi-ii; 200 μm, iii.

Fig 6. Predominant T lymphocytic inflammation in FGF21 KO obese mice.

(A) Representative cytofluorometric dot plots of isolated lymphocytes from WT and FGF21 KO animals consuming high fat diet for 16 weeks. FGF21 KO mice show elevated CD45+ (i), TCRb+ and Thy1+ T lymphocytes (ii) and Foxp3+ Treg cells. CD19+ B lymphocytes were not significantly altered (iii). Corresponding summary data is shown in the right panel. The experiment was repeated twice (n = 4 per group). (B) Gene expression analysis of cytokines is presented. n = 7 per group.