Fibroblast growth factor 21 as a novel stress-responsive hormone during starvation and physical exercise

Abstract

FGF21 is a stress-inducible hormone predominantly secreted from the liver. FGF21 acts as a downstream target gene of hepatic transcription factor PPARα that plays an obligatory role in mediating metabolic adaptation responses to prolonged fasting. However, the physiological roles of FGF21 in regulating glucose homeostasis during adaptive starvation responses and its underlying mechanisms remain unknown. Furthermore, FGF21 is induced by both acute and chronic exercise training in both rodents and human. The physiological significances and the underlying mechanisms for exercise-induced FGF21 production have not been explored so far.

Therefore, this study aims to investigate: 1) the mechanisms whereby FGF21 mediates the role of PPARα activation in modulating hepatic gluconeogenesis in response to prolonged fasting; 2) the physiological roles and mechanisms whereby FGF21 regulates exercise capacity and insulin sensitivity via its actions in the muscle during exercise; 3) the roles of FGF receptor-1 (FGFR1) and co-receptor βKlotho in mediating the metabolic effects of FGF21 during fasting and exercise.

Our results show thatFGF21 is induced by fasting in the liver and enters into the brain. Both FGF21knockout (KO) mice and PPARα KO mice exhibit severe hypoglycemia and defective hepatic gluconeogenesis during prolonged fasting, and these changes are accompanied by impaired activation of the hypothalamic-pituitary-adrenal (HPA)axis and blunted release of corticosterone from adrenalgland. Moreover, intracerebroventricular injection of recombinant FGF21 reverses fasting hypoglycemia and impairment in hepatic gluconeogenesis by restoring corticosterone production in both FGF21 KO and PPARα KO mice. These effects are abrogated by blockage of hypothalamic FGFR1or by pharmacological inhibition of ERK1/2in the hypothalamus. In addition,FGF21 acts directly on the hypothalamic neurons to activate FGFR1/βKlotho-ERK1/2-CREBsignaling pathway, thereby leading to the transcriptional activation of corticotropin-releasing hormone (CRH) and subsequent activation of the HPA axis.

FGF21 is also induced in the liver by exercise and FGF21 KO mice have decreased exercise capacity associated with lower mitochondrial content and thus lower insulin sensitivity compared to wildtype (WT)mice after exercise training.AMPK-PGC-1α signaling pathway is impaired in the muscle of FGF21 KO mice during exercise.FGF21treatmentincreasesmitochondrial content in myotubes, while inhibition of AMPK and knockdown of FGFR1, βKlotho and PGC-1αblocksthis effect. Moreover, FGF21 regulates genes and proteins involved in different steps of the autophagy process and these alterations are mediated by FGFR1/βKlotho-PGC-1α signaling cascade. Furthermore, replenishment of FGF21 reverses the reduced mitochondrial content and insulin sensitivity by restoring PGC-1α expression and subsequent increase in mitochondrial biogenesis and suppression of autophagy in the muscle of FGF21 KO mice.

In conclusion, our results identify FGF21 as a critical hormonal regulator of glucose homeostasis during prolonged fasting, by coupling hepatic PPARα activation to corticosterone release via stimulation of the HPA axis in the brain. Moreover, FGF21 acts in the muscle through two distinctive mechanism to maintain mitochondrial homeostasis during exercise training, involving both biogenesis of new mitochondria and decreased clearance of stressed mitochondria and thus to mediate the beneficial effects of exercise on lipid and glucose metabolism.