FGF21 induces PGC-1alpha and regulates carbohydrate and fatty acid metabolism during the adaptive starvation response

Abstract

The liver plays a crucial role in mobilizing energy during nutritional deprivation. During the early stages of fasting, hepatic glycogenolysis is a primary energy source. As fasting progresses and glycogen stores are depleted, hepatic gluconeogenesis and ketogenesis become major energy sources. Here, we show that fibroblast growth factor 21 (FGF21), a hormone that is induced in liver by fasting, induces hepatic expression of peroxisome proliferator-activated receptor gamma coactivator protein-1alpha (PGC-1alpha), a key transcriptional regulator of energy homeostasis, and causes corresponding increases in fatty acid oxidation, tricarboxylic acid cycle flux, and gluconeogenesis without increasing glycogenolysis. Mice lacking FGF21 fail to fully induce PGC-1alpha expression in response to a prolonged fast and have impaired gluconeogenesis and ketogenesis. These results reveal an unexpected relationship between FGF21 and PGC-1alpha and demonstrate an important role for FGF21 in coordinately regulating carbohydrate and fatty acid metabolism during the progression from fasting to starvation.

Fig. 1.

FGF21 regulates hepatic lipid and glucose metabolism. (A and B) Metabolic parameters of energy expenditure and lipid and glucose metabolism were determined by NMR in livers from fed and fasted 16- to 20-week-old WT and FGF21 transgenic (FGF21-TG) male mice (n = 5 per group). Livers were perfused with nonrecirculating media, and absolute fluxes were determined from the NMR data and rate of glucose production. Liver glycogen content (B) was analyzed in fed and fasted WT and FGF21-TG liver samples (n = 6 per group) independent of those analyzed by NMR. Data are presented as mean ± SEM (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).

Fig. 2.

FGF21 induces gluconeogenic gene expression. (A) Hepatic metabolic gene expression in fed WT and FGF21-TG male mice (n = 5 per group). (B) Western blot analysis of PGC-1α protein in fed WT and FGF21-TG mouse liver. (C) Hepatic metabolic gene expression after injection of vehicle or FGF21 into fed WT mice for the indicated times (n = 4 per group). (D) Groups of WT and PGC-1α-KO mice (n = 4 per group except for WT/fasted vehicle group, where n = 3) were injected with FGF21 in the fed or fasted states as indicated. Hepatic gene expression was analyzed by real-time QPCR. Data are presented as mean ± SEM (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).

Fig. 3.

Generation and characterization of FGF21-KO mice. (A) Genotyping of Fgf21^−/− mice by genomic PCR. Primer triplex includes one set flanking the 5′ loxP site and a third primer downstream of the 3′ loxP site. (B) Global deletion in the germ line by Meox-cre removes all three Fgf21 exons as demonstrated by semiquantitative RT-PCR using RNA prepared from liver of fed or 24-h-fasted WT or FGF21-KO mice (n = 5 per group). Primers are as indicated in Fig. S2. Cyclophilin served as a loading control. (C) FGF21 mRNA levels were measured by real-time QPCR using RNA prepared from liver of fed or 24-h-fasted WT or FGF21-KO mice (n = 5 per group). (D) FGF21 protein levels were measured by RIA in the plasma of fed and fasted WT and FGF21-KO mice (n = 5 per group). (E) Rate of ketone body production was measured in 24-h-fasted, 18- to 20-week-old WT and FGF21-KO male mice challenged with sodium octanoate injection. Tail blood was collected at the indicated times for analysis of plasma β-hydroxybutyrate concentrations. (C–E) Data are presented as mean ± SEM (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).

Fig. 4.

FGF21 regulates genes involved in hepatic carbohydrate and lipid metabolism during fasting. Shown is real-time QPCR analysis of gene expression in WT or FGF21-KO liver under fed or fasted conditions (n = 5 per group). Data are presented as mean ± SEM. (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).

Fig. 5.

FGF21 is required for inducing hepatic lipid and glucose metabolism during fasting. (A and B) Metabolic parameters of energy expenditure and lipid and glucose metabolism were determined by NMR in livers from fed and fasted 16- to 20-week-old WT and FGF21-KO male mice (n = 6 per group). Livers were perfused with nonrecirculating media, and absolute fluxes were determined from the NMR data and rate of glucose production. Liver glycogen content (B) was analyzed in fed and fasted WT and FGF21-KO liver samples (n = 6 per group) independent of those analyzed by NMR. Data are presented as mean ± SEM (a, P < 0.05; b, P < 0.01; c, P < 0.005; d, P < 0.001).