Fibroblast growth factor 21 (FGF-21) is a member of the FGF gene family, which contains
22 mammalian members. Based on its structure, it is further classified as a member of the
FGF-19 subfamily, which also includes FGF-19 and FGF-23 (1-4). FGF family members contain a
120 amino acid (aa) core FGF domain that exhibits a beta -trefoil structure. FGF-19 subfamily
members, unlike other FGFs, lack one strand of the beta -trefoil and bind poorly to extracellular
matrix molecules such as heparin (3). They are consequently more diffusible than other FGFs
and are considered endocrine rather than paracrine (1-4). All three subfamily members impact
some aspect of metabolism; all three are induced by a nuclear receptor heterodimer that
includes RXR (retinoid X receptor), and all three bind FGF receptors (FGF R) indirectly through
co-receptors of the klotho family (5-9). FGF-21 binds to beta -Klotho via its C-terminal sequence.
This binding, along with amino acids at the N-terminus, is required for signaling through FGF R
(7, 8). FGF-21 is selective for FGF R1 isoform 1c, with varying reports of using isoforms 2c or 3c
(10-12). Presence of the required klotho and FGF R family members determines tissue
specificity of FGF-19 subfamily members, and thus concentrates FGF-21 activity within adipose
tissue (3, 9-11). Mature human FGF-21 shares 81% aa sequence identity with mouse and rat
FGF-21.
FGF-21 is produced by hepatocytes in response to free fatty acid (FFA) stimulation of a
PPAR alpha /RXR dimeric complex (4, 13-15). This situation occurs during starvation, diabetic ketosis,
or following the ingestion of a high-fat/low-carbohydrate or ketogenic diet (5, 14-16). Upon
FGF-21 secretion, white adipose tissue is induced to release FFAs from triglyceride stores. Once
FFAs reach the hepatocytes, they are oxidized and reduced to acetyl-CoA (16). The acetyl-CoA is
recombined into 4-carbon ketone bodies (acetoacetate and beta -hydroxybutyrate), released, and
transported to peripheral tissues for energy generation (5, 15, 16). FGF-21 production is also induced upon differentiation of human or mouse fibroblasts to
adipocytes (17, 18). In adipose tissue, FGF-21 induces glucose uptake by signaling in synergy
with PPAR gamma to increase production of the glucose transporter, GLUT1 (10, 12, 19).
FGF-21
production follows a circadian pattern in mice (20). It diffuses across the blood-brain barrier
and this may facilitate induction of a state of torpor, or decreased activity, in response to
increased FGF-21 (16, 21). These characteristics appear to induce a hibernation-like state during
fasting and short days in winter (22). In diet-induced obese mice and mouse models of
diabetes such as db/db and ob/ob, administration or transgenic overexpression of FGF-21
restores circulating glucose and triglyceride values to near normal and increases insulin
sensitivity (5, 6, 14, 23, 24). In some of these states and in human obesity and type II diabetes,
FGF-21 is already elevated prior to treatment, suggesting that resistance to FGF-21 is possible
(17, 25, 26). Although FGF-21 administration corrects obesity in mice, it is unclear whether the
same benefit would be seen in humans (2, 3, 17, 26-28).