OBJECTIVE: Reports of increased circulating fibroblast growth factor 21 (FGF21) levels in obesity indicate that FGF21 may be implicated in body weight homeostasis. We sought to investigate the existence of FGF21 in human cerebrospinal fluid (CSF) and, if present, the relationship between CSF FGF21 with body adiposity and metabolic parameters. RESEARCH DESIGN AND METHODS: CSF and corresponding plasma FGF21 were measured by an enzyme-linked immunosorbent assay (18 men and 20 women, aged 19-80 years, and BMI 16.2-38.1 kg/m(2)) and correlated to body adiposity and metabolic parameters.
RESULTS: CSF and plasma FGF21 increased in particular with rising BMI and fat mass. In CSF, FGF21 was detectable at concentrations ~40% that of plasma levels. CSF and plasma FGF21 levels were significantly positively correlated with BMI and fat mass, body weight, plasma insulin, and homeostasis model assessment of insulin resistance. Plasma FGF21 levels were significantly negatively correlated with plasma adiponectin. When subjected to multiple regression analysis, only fat mass was predictive of plasma FGF21 (ß = 0.758; P = 0.004) and CSF FGF21 (ß = 0.767; P = 0.007). The CSF-to-plasma FGF21 ratio was significantly negatively correlated with BMI, fat mass, and plasma FGF21. Subjects in the highest plasma FGF21 quintile had a lower CSF-to-plasma FGF21 ratio (12.7% [9.7-14.9]) compared with those in the lowest plasma FGF21 quintile (94.7% [37.3-99.8]) (P < 0.01).
CONCLUSIONS: Our observations have important implications with respect to the potential central actions of FGF21. Future research should seek to clarify whether FGF21 would be beneficial in the management of obesity and its metabolic complications.

Tan BK, Hallschmid M, Adya R, Kern W, Lehnert H, Randeva HS. Fibroblast growth factor 21 (FGF21) in human cerebrospinal fluid: relationship with plasma FGF21 and body adiposity. Diabetes. 2011;60(11):2758-62.

BACKGROUND: Muscle biopsy is the gold standard for diagnosis of mitochondrial disorders because of the lack of sensitive biomarkers in serum. Fibroblast growth factor 21 (FGF-21) is a growth factor with regulatory roles in lipid metabolism and the starvation response, and concentrations are raised in skeletal muscle and serum in mice with mitochondrial respiratory chain deficiencies. We investigated in a retrospective diagnostic study whether FGF-21 could be a biomarker for human mitochondrial disorders.
METHODS: We assessed samples from adults and children with mitochondrial disorders or non-mitochondrial neurological disorders (disease controls) from seven study centres in Europe and the USA, and recruited healthy volunteers (healthy controls), matched for age where possible, from the same centres. We used ELISA to measure FGF-21 concentrations in serum or plasma samples (abnormal values were defined as >200 pg/mL). We compared these concentrations with values for lactate, pyruvate, lactate-to-pyruvate ratio, and creatine kinase in serum or plasma and calculated sensitivity, specificity, and positive and negative predictive values for all biomarkers.
FINDINGS: We analysed serum or plasma from 67 patients (41 adults and 26 children) with mitochondrial disorders, 34 disease controls (22 adults and 12 children), and 74 healthy controls. Mean FGF-21 concentrations in serum were 820 (SD 1151) pg/mL in adult and 1983 (1550) pg/mL in child patients with respiratory chain deficiencies and 76 (58) pg/mL in healthy controls. FGF-21 concentrations were high in patients with mitochondrial disorders affecting skeletal muscle but not in disease controls, including those with dystrophies. In patients with abnormal FGF-21 concentrations in serum, the odds ratio of having a muscle-manifesting mitochondrial disease was 132.0 (95% CI 38.7-450.3). For the identification of muscle-manifesting mitochondrial disease, the sensitivity was 92.3% (95% CI 81.5-97.9%) and specificity was 91.7% (84.8-96.1%). The positive and negative predictive values for FGF-21 were 84.2% (95% CI 72.1-92.5%) and 96.1 (90.4-98.9%). The accuracy of FGF-21 to correctly identify muscle-manifesting respiratory chain disorders was better than that for all conventional biomarkers. The area under the receiver-operating-characteristic curve for FGF-21 was 0.95; by comparison, the values for other biomarkers were 0.83 lactate (p=0.037, 0.83 for pyruvate (p=0.015), 0.72 for the lactate-to-pyruvate ratio (p=0?002), and 0.77 for creatine kinase (p=0.013).
INTERPRETATION: Measurement of FGF-21 concentrations in serum identified primary muscle-manifesting respiratory chain deficiencies in adults and children and might be feasible as a first-line diagnostic test for these disorders to reduce the need for muscle biopsy.

Suomalainen A, Elo JM, Pietiläinen KH, et al. FGF-21 as a biomarker for muscle-manifesting mitochondrial respiratory chain deficiencies: a diagnostic study. Lancet Neurol. 2011;10(9):806-18.

OBJECTIVE: Fibroblast growth factor (FGF)-21 improves insulin sensitivity and lipid metabolism in obese or diabetic animal models, while human studies revealed increased FGF-21 levels in obesity and type 2 diabetes. Given that FGF-21 has been suggested to be a peroxisome proliferator-activator receptor (PPAR) alpha-dependent regulator of fasting metabolism, we hypothesized that free fatty acids (FFAs), natural agonists of PPARalpha, might modify FGF-21 levels. RESEARCH DESIGN AND METHODS: The effect of fatty acids on FGF-21 was investigated in vitro in HepG2 cells. Within a randomized controlled trial, the effects of elevated FFAs were studied in 21 healthy subjects (13 women and 8 men). Within a clinical trial including 17 individuals, the effect of insulin was analyzed using an hyperinsulinemic-euglycemic clamp and the effect of PPARgamma activation was studied subsequently in a rosiglitazone treatment trial over 8 weeks. RESULTS: Oleate and linoleate increased FGF-21 expression and secretion in a PPARalpha-dependent fashion, as demonstrated by small-interfering RNA-induced PPARalpha knockdown, while palmitate had no effect. In vivo, lipid infusion induced an increase of circulating FGF-21 in humans, and a strong correlation between the change in FGF-21 levels and the change in FFAs was observed. An artificial hyperinsulinemia, which was induced to delineate the potential interaction between elevated FFAs and hyperinsulinemia, revealed that hyperinsulinemia also increased FGF-21 levels in vivo, while rosiglitazone treatment had no effect.
CONCLUSIONS: The results presented here offer a mechanism explaining the induction of the metabolic regulator FGF-21 in the fasting situation but also in type 2 diabetes and obesity.

Mai K, Andres J, Biedasek K, et al. Free fatty acids link metabolism and regulation of the insulin-sensitizing fibroblast growth factor-21. Diabetes. 2009;58(7):1532-8.

Diabetes mellitus is a major health concern, affecting more than 5% of the population. Here we describe a potential novel therapeutic agent for this disease, FGF-21, which was discovered to be a potent regulator of glucose uptake in mouse 3T3-L1 and primary human adipocytes. FGF-21–transgenic mice were viable and resistant to diet-induced obesity. Therapeutic administration of FGF-21 reduced plasma glucose and triglycerides to near normal levels in both ob/ob and db/db mice. These effects persisted for at least 24 hours following the cessation of FGF-21 administration. Importantly, FGF-21 did not induce mitogenicity, hypoglycemia, or weight gain at any dose tested in diabetic or healthy animals or when overexpressed in transgenic mice. Thus, we conclude that FGF-21, which we have identified as a novel metabolic factor, exhibits the therapeutic characteristics necessary for an effective treatment of diabetes.

Kharitonenkov A, Shiyanova TL, Koester A, et al. FGF-21 as a novel metabolic regulator. J Clin Invest. 2005;115(6):1627-35.