Impact of aging and caloric restriction on fibroblast growth factor 21 signaling in rat white adipose tissue

Caloric restriction (CR) suppresses age-related pathophysiology and extends lifespan. We recently reported that metabolic remodeling of white adipose tissue (WAT) plays an important role in the beneficial actions of CR; however, the detailed molecular mechanisms of this remodeling remain to be established. In the present study, we aimed to identify CR-induced alterations in the expression of fibroblast growth factor 21 (FGF21), a regulator of lipid and glucose metabolism, and of its downstream signaling mediators in liver and WAT, across the lifespan of rats. We evaluated groups of rats that had been either fed ad libitum or calorie restricted from 3 months of age and were euthanized at 3.5, 9, or 24 months of age, under fed and fasted conditions. The expression of FGF21 mRNA and/or protein increased with age in liver and WAT. Interestingly, in the WAT of 9-month-old fed rats, CR further upregulated FGF21 expression and eliminated the aging-associated reductions in the expression of FGFR1 and beta-klotho (KLB; FGF21 receptor complex). It also enhanced the expression of FGF21 targets, including glucose transporter 1 and peroxisome proliferator-activated receptor (PPAR)gamma coactivator-1 alpha. The analysis of transcriptional regulators of Fgf21 suggested that aging and CR might upregulate Fgf21 expression via different mechanisms. In adipocytes in vitro, constitutive FGF21 overexpression upregulated the FGF21 receptor complex and FGF21 targets at the mRNA or protein level. Thus, both aging and CR induced FGF21 expression in rat WAT; however, only CR activated FGF21 signaling. Our results suggest that FGF21 signaling contributes to the CR-induced metabolic remodeling of WAT, likely activating glucose uptake and mitochondrial biogenesis.