Circulating FGF21 is liver derived and enhances glucose uptake during refeeding and overfeeding

Abstract

Fibroblast growth factor (FGF)21 is an endocrine hormone that is expressed in multiple tissues and functions physiologically to maintain energy homeostasis. FGF21 is being pursued as a therapeutic target for diabetes and obesity because of its rapid and potent effects on improving insulin sensitivity. However, whether FGF21 enhances insulin sensitivity under physiologic conditions remains unclear. Here, we show that liver-derived FGF21 enters the circulation during fasting but also remains present and functional during the early stage of refeeding. After a prolonged fast, FGF21 acts as an insulin sensitizer to overcome the peripheral insulin resistance induced by fasting, thereby maximizing glucose uptake. Likewise, FGF21 is produced from the liver during overfeeding and mitigates peripheral insulin resistance. DIO FGF21 liver-specific knockout, but not FGF21 adipose-specific knockout, mice have increased insulin resistance and decreased brown adipose tissue-mediated glucose disposal. These data are compatible with the concept that FGF21 functions physiologically as an insulin sensitizer under conditions of acute refeeding and overfeeding.

Figure 1

Generation and characterization of FGF21 LivKO mice. A–F: 12- to 14-week-old male WT (FGF21^fl/fl) and FGF21 LivKO mice were either ad libitum–fed chow, fasted 24 h, or fasted 24 h and then refed for 15 min via oral gavage (n = 5–8/group). Levels of hepatic Fgf21 mRNA (A), eWAT Fgf21 mRNA (B), plasma FGF21 protein (C), plasma glucose (D), plasma insulin (E), and plasma adiponectin (F) in the indicated mice are shown. G: Levels of plasma glucose and area under the curve (AUC) from WT and FGF21 LivKO mice during a GTT (n = 5–6/group). Data are presented as mean ± SEM. Different lowercase letters represent statistical significance between genotypes (a, P < 0.05; b, P < 0.01; d, P < 0.001). *Significance between control mice in different conditions (P < 0.05).

Figure 2

Loss of liver-derived FGF21, but not adipose-derived FGF21, impairs glucose homeostasis and insulin sensitivity in DIO mice. A–D: DIO WT, FGF21 total knockout (KO), FGF21^fl/fl, FGF21 LivKO, and FGF21 AdipoKO mice were fed an HFD for 16 weeks. Levels of hepatic Fgf21 mRNA (A), eWAT Fgf21 mRNA (B), BAT Fgf21 mRNA (C), and plasma FGF21 protein (D) in the indicated mice are shown. E–K: Age-matched male FGF21^fl/fl, FGF21 LivKO, and FGF21 AdipoKO mice were individually caged and maintained on chow diet until 10 weeks of age. At 10 weeks of age, all mice were placed on HFD for 10 weeks (n = 8–9/group). FGF21^fl/fl control mice were compared only with their knockout littermates. Levels of plasma glucose (E and F) and plasma insulin (G and H) during a GTT (n = 6/group). GTTs were performed by administering 2 g glucose/kg lean body wt i.p. to mice that had been fasted for 16 h. I and J: Plasma glucose levels during an ITT (n = 5–6/group). ITTs were performed by administering 0.75 units insulin/kg body wt i.p. to mice fasted for 4–6 h. K: Oil Red O staining of livers and hematoxylin-eosin staining of eWAT. Data are presented as mean ± SEM. Different lowercase letters represent statistical significance (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).

Figure 3

Loss of hepatic FGF21 impairs glucose uptake into BAT in vivo. Plasma glucose area under the curve (AUC) (A), plasma insulin levels (B), and rates of glucose uptake (C) in unanesthetized male WT and FGF21 LivKO fed HFD for 6 weeks. All mice were administered [³H]2-deoxyglucose after an overnight fast; glucose uptake was measured in BAT, eWAT, subcutaneous WAT, heart, and skeletal muscle (tibialis anterior) (n = 5–7/group). Glucose uptake assays in primary brown adipocytes (D) and primary white adipocytes (E) treated with vehicle (Veh), insulin (100 nmol/L), FGF21 (1 µg/mL), or FGF21 plus insulin. Experiments were performed in triplicate twice. Data are presented as mean ± SEM. Different lowercase letters represent statistical significance (a, P < 0.05; b, P < 0.01; d, P < 0.001).