Liver-derived metabolic hormone fibroblast growth factor 21 (FGF21) improves insulin sensitivity and extends lifespan in mice. Aging also compromises the adaptive immune system by reducing T-cell production from the thymus. In this paper, we describe a new immunological function of FGF21 as a regulator of T-cell production from thymus in aging. The overexpression of FGF21 prevents thymic lipoatrophy, which protects the mice from age-induced loss of naïve T cells. FGF21 expression in thymic epithelial cells and signaling in thymic stromal cells support thymic function in aging. Loss of FGF21 in mice increases lethality postirradiation and delays the reconstitution of thymus. Hence, we highlight FGF21 as an immunometabolic regulator that can be harnessed to delay immune senescence.
Irisin and FGF21 Are Cold-Induced Endocrine Activators of Brown Fat Function in Humans
Rediscovery of cold-activated brown adipose tissue (BAT) in humans has boosted research interest in identifying BAT activators for metabolic benefits. Of particular interest are cytokines capable of fat browning. Irisin, derived from FNDC5, is an exercise-induced myokine that drives brown-fat-like thermogenesis in murine white fat. Here we explored whether cold exposure is an afferent signal for irisin secretion in humans and compared it with FGF21, a brown adipokine in rodents. Cold exposure increased circulating irisin and FGF21. We found an induction of irisin secretion proportional to shivering intensity, in magnitude similar to exercise-stimulated secretion. FNDC5 and/or FGF21 treatment upregulated human adipocyte brown fat gene/protein expression and thermogenesis in a depot-specific manner. These results suggest exercise-induced irisin secretion could have evolved from shivering-related muscle contraction, serving to augment brown fat thermogenesis in concert with FGF21. Irisin-mediated muscle-adipose crosstalk may represent a thermogenic, cold-activated endocrine axis that is exploitable in obesity therapeutics development.
The Circulating Metabolic Regulator FGF21 Is Induced by Prolonged Fasting and PPARα Activation in Man
FGF21 is a critical metabolic regulator, pivotal for fasting adaptation and directly regulated by PPARα in rodents. However, the physiological role of FGF21 in man is not yet defined and was investigated in our study. Serum FGF21 varied 250-fold among 76 healthy individuals and did not relate to age, gender, body mass index (BMI), serum lipids, or plasma glucose. FGF21 levels had no diurnal variation and were unrelated to bile acid or cholesterol synthesis. Ketosis induced by a 2 day fast or feeding a ketogenic diet (KD) did not influence FGF21 levels, whereas a 74% increase occurred after 7 days of fasting. Hypertriglyceridemic nondiabetic patients had 2-fold elevated FGF21 levels, which were further increased by 28% during fenofibrate treatment. FGF21 circulates in human plasma and increases by extreme fasting and PPARα activation. The wide interindividual variation and the induction of ketogenesis independent of FGF21 levels indicate that the physiological role of FGF21 in humans may differ from that in mice.
