Growth hormone induces hepatic production of fibroblast growth factor 21 through a mechanism dependent on lipolysis in adipocytes

Abstract

Fibroblast growth factor (FGF) 21 and growth hormone (GH) are metabolic hormones that play important roles in regulating glucose and lipid metabolism. Both hormones are induced in response to fasting and exert their actions on adipocytes to regulate lipolysis. However, the molecular interaction between these two hormones remains unclear. Here we demonstrate the existence of a feedback loop between GH and FGF21 on the regulation of lipolysis in adipocytes. A single bolus injection of GH into C57 mice acutely increases both mRNA and protein expression of FGF21 in the liver, thereby leading to a marked elevation of serum FGF21 concentrations. Such a stimulatory effect of GH on hepatic FGF21 production is abrogated by pretreatment of mice with the lipolysis inhibitor niacin. Direct incubation of either liver explants or human HepG2 hepatocytes with GH has no effect on FGF21 expression. On the other hand, FGF21 production in HepG2 cells is significantly induced by incubation with the conditioned medium harvested from GH-treated adipose tissue explants, which contains high concentrations of free fatty acids (FFA). Further analysis shows that FFA released by GH-induced lipolysis stimulates hepatic FGF21 expression by activation of the transcription factor PPARα. In FGF21-null mice, both the magnitude and duration of GH-induced lipolysis are significantly higher than those in their wild type littermates. Taken together, these findings suggest that GH-induced hepatic FGF21 production is mediated by FFA released from adipose tissues, and elevated FGF21 in turn acts as a negative feedback signal to terminate GH-stimulated lipolysis in adipocytes.

FIGURE 1.

Effect of GH on the circulating concentrations of FGF21 in mouse. Sera were harvested at different time points after mice were treated with GH (1.5 mg/kg body weight) or PBS, and then subjected to FGF21 measurement using ELISA. **, p < 0.01 versus PBS group (n = 6).

FIGURE 2.

Effect of GH on gene expression and protein levels of FGF21 in mouse liver. Livers were harvested 6 h after mice were treated with GH (1.5 mg/kg body weight) or PBS (as a vehicle control), followed by determination of FGF21 mRNA expression and protein levels using real-time PCR and ELISA. (A) relative FGF21 mRNA abundance in liver. (B) FGF21 protein concentrations in liver. Relative FGF21 mRNA abundance in (C) parenchymal cells (PC) and (D) non-parenchymal cells (NPC). *, p < 0.05 versus PBS group (n = 6).

FIGURE 3.

Effect of GH on FGF21 gene expression and protein secretion in liver explants and human HepG2 hepatocytes. Liver explants from C57 male mice or HepG2 cells were treated with GH (500 ng/ml), fenofibrate (Feno, 500 μm) or PBS for 6 h and subjected to (A and C) real-time PCR analysis for FGF21 gene expression. The concentrations of FGF21 protein released in the conditioned medium were analyzed by ELISA (B and D). *, p < 0.05 versus PBS group (n = 5).

FIGURE 4.

Effect of the lipolysis inhibitor niacin on GH-induced elevation of serum FGF21 in mice. Sera were collected at different time points after GH treatment (1.5 mg/kg body weight) in the presence or absence of niacin (100 mg/kg body weight) as shown in (A) and then subjected to the analysis for FFA (B) and FGF21 levels (C) by using FFA Half Micro Test and ELISA, respectively. **, p < 0.01 versus niacin+GH group at the same time point, and #, p < 0.05; ##, p < 0.01 versus PBS+GH group at 0 h, respectively (n = 6).

FIGURE 5.

The conditioned medium from GH-treated adipose tissue explants induces hepatic FGF21 expression in human HepG2 hepatocytes. Conditioned medium from white adipose tissue (WAT) explants culture were collected at different time points after treatment with GH (500 ng/ml), GH + niacin (2 mm), isopeterenol (10 μm, as a positive control) or PBS, and then subjected to analysis for glycerol (A) and FFA (B) (24-h treatment levels). The conditioned medium collected at 24 h after various treatments were then used to treat human HepG2 hepatocytes for another 24 h. The FGF21 mRNA expression (C) and protein secretion (D) were measured by real-time PCR and ELISA, respectively. *, p < 0.05 versus PBS group or GH + niacin group, respectively (n = 6).

FIGURE 6.

Effect of the PPARα antagonist GW-6471 on induction of hepatic FGF21 production by the conditioned medium from GH-treated WAT explants. HepG2 cells were treated with conditioned medium from WAT explants treated with GH or PBS as in Fig. 5, in the presence or absence of the PPARα antagonist (GW6471, 10 μm) for 24 h. FGF21 mRNA expression (A) and its protein release in the conditioned medium (B) were measured as in Fig. 5. *, p < 0.05; **, p < 0.01 versus GH group (n = 5).

FIGURE 7.

The impact of FGF21 deficiency on GH-induced lipolysis in mice. Sera were collected at different time points after injection of GH (1.5 mg/kg body weight) or PBS in FGF21 knock-out (KO) mice or wild type (WT) littermates, and then subjected to analysis for glycerol (A) and FFA levels (B). *, p < 0.05; **, p < 0.01 versus WT-PBS group or WT-GH group, respectively, and #, p < 0.05; ##, p < 0.01 versus WT-PBS group (n = 5).

FIGURE 8.

A proposed feedback model whereby GH and FGF21 fine-tune lipolysis in adipocytes. During fasting, the secretion of growth hormone from the somatotropes in anterior pituitary is increased, leading to the induction of lipolysis in white adipose tissue. The lipolysis products including FFA induce FGF21 gene expression and its protein secretion in the liver, which in turn serves as a negative feedback signal to terminate lipolysis initiated by GH in adipocytes.