Central Fibroblast Growth Factor 21 Browns White Fat via Sympathetic Action in Male Mice

Abstract

Fibroblast growth factor 21 (FGF21) has multiple metabolic actions, including the induction of browning in white adipose tissue. Although FGF21 stimulated browning results from a direct interaction between FGF21 and the adipocyte, browning is typically associated with activation of the sympathetic nervous system through cold exposure. We tested the hypothesis that FGF21 can act via the brain, to increase sympathetic activity and induce browning, independent of cell-autonomous actions. We administered FGF21 into the central nervous system via lateral ventricle infusion into male mice and found that the central treatment increased norepinephrine turnover in target tissues that include the inguinal white adipose tissue and brown adipose tissue. Central FGF21 stimulated browning as assessed by histology, expression of uncoupling protein 1, and the induction of gene expression associated with browning. These effects were markedly attenuated when mice were treated with a β-blocker. Additionally, neither centrally nor peripherally administered FGF21 initiated browning in mice lacking β-adrenoceptors, demonstrating that an intact adrenergic system is necessary for FGF21 action. These data indicate that FGF21 can signal in the brain to activate the sympathetic nervous system and induce adipose tissue thermogenesis.

Figure 1.

FGF21 induces Erk phosphorylation and increases sympathetic activity. Mice were injected ip with vehicle (n = 6) or FGF21 (n = 6). Representative images of PVH-containing coronal brain sections immunohistochemically labeled for p-Erk are shown (A) vehicle and (B) FGF21. Boxed regions are enlarged below each respective image (C and D). Mean number of p-Erk-positive cell counts taken over 2 consecutive central PVH sections for each experimental animal (E). Hypothalamic slice culture showed p-Erk in the PVH of FGF21-treated (right) but not vehicle-treated (left) (F). *, P < .05 by Student's t test.

Figure 2.

A low central FGF21 dose of 0.4 μg/d into WT mice causes an increase in O₂ consumption. Mice (n = 8 per group) administered icv-FGF21 have a higher metabolic rate of VO₂ (A), yet do not consume more food (vehicle, n = 8, FGF21, n = 7) (B) or have increased activity ambulatory activity (n = 14 per group) (C). Mice given the circulation-matching, 1.6-μg/d sc-FGF21 dose (vehicle, n = 6, FGF21 n = 8) do not show an increase in VO₂ consumption (D). Graphs are shown ± SEM. *, P < .05; **, P < .01; by Student's t test, food consumption not significant by repeat measures ANOVA, P = .3193.

Figure 3.

Browning profile of inguinal adipose tissue, Ucp1 protein expression in BAT, and NETO in adipose depots: icv-FGF21 (0.4 μg/d) 5-day infusion into mice (n = 15 per group) causes increases in expression of BAT-specific markers and markers of thermogenesis and fatty acid oxidation in IWAT (A), whereas a circulation-matching sc-FGF21 dose (1.6 μg/d) (n = 7 per group) fails to induce expression of Ucp1 or Dio2 after 5 days of treatment (B). Histologic examinations (vehicle, n = 8, FGF21, n = 7) of IWAT revealed that 5-day icv-FGF21 administration (0.4 μg/d) caused increased protein expression of Ucp1 as assessed immunohistochemistry (C–F). IBAT total Ucp1 protein is increased after 5-day icv-FGF21 (0.4 μg/d)-treated mice (vehicle, n = 7, FGF21, n = 6) (G). NETO increased after a 5-day icv-FGF21 administration in IWAT (vehicle, n = 16, FGF21 n = 19), EWAT (vehicle n = 16, FGF21, n = 20), and IBAT (vehicle, n = 17, FGF21, n = 20) (H). Carnitine palmitoyltransferase 1b (Cpt1β). Graphs are shown as mean ± SEM. *, P < .05; **, P < .01; ***, P > .001 by Student's t test.

Figure 4.

Central FGF21 is dependent on the SNS. The β-blocker propranolol counters the influence of FGF21 on IWAT and β-adrenoceptors are required for FGF21-mediated induction of BAT-specific markers, thermogenesis, and fatty acid oxidation in IWAT. IWAT of 5-day icv administration (0.4 μg/d) of FGF21 fails to induce gene expression of a panel of markers of browning in IWAT if the mice (n = 7 per group) are treated with the general β-blocker propranolol (A), but this is largely overcome if administered a large sc-FGF21 dose (24 μg/d) (vehicle, n = 6, FGF21 n = 8) plus propranolol (B). Mice lacking the β1, β2, and β3 adrenoceptors (β-less mice) fail to induce a browning profile in IWAT if given FGF21 (0.4 μg/d) icv (vehicle, n = 9, FGF21, n = 8) (C) or if administered a large sc-FGF21 dose (24 μg/d) to β-less mice (vehicle, n = 7, FGF21, n = 6) (D). Primary IWAT culture derived from β-less mice was unresponsive to vehicle or FGF21 treatment but did induce Ucp1 when treated with forskolin (E). Graphs are shown as mean ± SEM. *, P < .05; **, P < .01 by Student's t test.

Figure 5.

Physiologic model of FGF21 action. Fgf21 is increased systemically via increased production in the liver, such as through consumption of a KD. This Fgf21 can act directly on adipose tissue, including WAT where it can induce browning. Fgf21 can also cross the blood-brain barrier and act on the PVH, which activates a cholinergic signal from the IML to increase β-adrenergic signaling in sympathetic ganglia to increase adrenergic activity in target thermogenic tissues such as BAT and WAT. When animals are exposed to cold, SNS activation occurs via pathways independent of central or peripheral FGF21. Increased β-adrenergic signaling in target tissue induces Fgf21 expression in WAT and BAT. In this later case, Fgf21 acts as a local autocrine/paracrine factor to drive thermogenesis.