FGF 21 Human
Description
Molecular Structure and Classification
FGF21 is a 181-amino acid protein encoded by the FGF21 gene and belongs to the endocrine subfamily of fibroblast growth factors (FGF19, FGF21, FGF23) . Unlike canonical FGFs, endocrine FGFs lack a heparin-binding domain, enabling systemic circulation . Its receptor complex requires β-Klotho (KLB) alongside FGFR1c for signal transduction, restricting activity to tissues expressing KLB (e.g., adipose, liver, pancreas) .
Physiological Roles and Mechanisms
FGF21 regulates energy metabolism through tissue-specific pathways:
Key metabolic effects include:
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Acute action: Rapid glucose lowering via insulin sensitization .
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Chronic action: Weight loss via increased energy expenditure and reduced adiposity .
Obesity and Diabetes
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Obesity: Circulating FGF21 levels are elevated but correlate with resistance due to reduced KLB expression and ERK1/2 phosphorylation .
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Type 2 Diabetes: FGF21 enhances glucose uptake in adipocytes and protects pancreatic β-cells . Paradoxically, elevated levels predict poor outcomes in diabetic cohorts .
Non-Alcoholic Fatty Liver Disease (NAFLD)
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FGF21 reduces hepatic inflammation and fibrosis by suppressing TNF-α and IL-17A . Clinical trials show improved lipid profiles and hepatic steatosis in NAFLD patients .
Preclinical Success
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Animal Models: Recombinant FGF21 reduces body weight (−20%), triglycerides (−50%), and glucose (−30%) in obese rodents .
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Mechanisms: Activates adiponectin secretion and mitochondrial uncoupling .
Clinical Trials
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Limitations: Short half-life, FGF21 resistance in obesity, and proteolytic cleavage by FAP .
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Innovations: Long-acting analogs (e.g., PF-05231023) and FAP inhibitors to stabilize bioactive FGF21 .
Table 1: FGF21 Levels in Metabolic Disorders
| Condition | FGF21 Serum Levels | Clinical Correlation | Source |
|---|---|---|---|
| Obesity | ↑↑↑ | Insulin resistance | |
| NAFLD | ↑↑ | Hepatic fibrosis | |
| Heart Failure | ↑↑↑ | Mortality risk |
Table 2: Genetic and Pharmacological Insights
Future Directions
Product Specs
The FGF family comprises over 20 small (~17–26 kDa) secreted peptides. Initial research on these proteins centered around their capacity to promote fibroblast proliferation, a mitogenic effect mediated by fibroblast growth factor receptors (FGFRs) 1, 2, or 3. A fourth related tyrosine kinase receptor (FGFR4) exhibited FGF binding capabilities but did not trigger a mitogenic response.
FGFs exert their effects on cellular activity through at least five distinct subfamilies of high-affinity FGFRs: FGFR-1, -2, -3, and -4, each possessing intrinsic tyrosine kinase activity and, with the exception of FGFR-4, multiple splice isoforms, and FGFR-5, which lacks an intracellular kinase domain. Evidence suggests that FGFRs may play a significant role in regulating glucose and lipid homeostasis. Mice exhibiting overexpression of a dominant negative form of FGFR-1 develop diabetes, implying that proper FGF signaling is crucial for normal cell function and maintaining glycemic control. FGFR-2 appears to be a key player in pancreatic development. Furthermore, FGFR-4 has been linked to cholesterol metabolism and bile acid synthesis.
FGF-19 has demonstrated the ability to induce resistance to diet-induced obesity and improve glucose and lipid profiles in diabetic rodents, along with desensitization. Given that these effects are at least partially mediated by observed changes in metabolic rates, FGF-19 can be considered a regulator of energy expenditure.
Although FGF-21 is primarily expressed in the liver, a comprehensive understanding of its bioactivity and mechanism of action remains elusive. FGF-21 is a potent stimulator of glucose uptake in adipocytes, offers protection against diet-induced obesity in transgenic mice overexpressing the protein, and reduces blood glucose and triglyceride levels when administered therapeutically to diabetic rodents.
Recombinant Human Fibroblast Growth Factor -21, produced in E. coli, is a single, non-glycosylated polypeptide chain comprising 181 amino acids with a molecular weight of 19.4 kDa.
Purification of FGF-21 is achieved using proprietary chromatographic methods.
For long-term storage, adding a carrier protein (0.1% HSA or BSA) is recommended.
Avoid repeated freeze-thaw cycles.
Purity exceeds 96.0% as determined by:
(a) Reverse-phase high-performance liquid chromatography (RP-HPLC) analysis.
(b) Sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis.
The half-maximal effective concentration (ED50), determined using a thymidine uptake assay with BaF3 cells transfected with FGF receptors, is less than 0.5 µg/ml. This corresponds to a specific activity greater than 2.0 × 103 IU/mg in the presence of 5 µg/ml recombinant MuKlotho-β and 10 µg/ml heparin.
HPIPDS SPLLQFGGQV RQRYLYTDDA QQTEAHLEIR EDGTVGGAAD QSPESLLQLK ALKPGVIQIL GVKTSRFLCQ RPDGALYGSL HFDPEACSFR ELLLEDGYNV YQSEAHGLPL HLPGNKSPHR DPAPRGPARF LPLPGLPPAP PEPPGILAPQ PPDVGSSDPL SMVGPSQGRS PSYAS.
Q&A
What is the primary source of circulating FGF21 in humans?
Unlike in rodent models where FGF21 expression is more widespread, the FGF21 gene in humans is nearly exclusively expressed in the liver under basal conditions. While weak expression signals have been detected in the pancreas, there is minimal expression in muscle tissue . During cold exposure, brown adipose tissue (BAT) may contribute to circulating FGF21 levels, though the magnitude of this contribution in humans requires further investigation . When designing tissue-specific studies, researchers should prioritize liver samples for baseline FGF21 expression analysis while considering adipose tissue for condition-specific investigations.
How does the FGF21 signaling pathway function in humans?
FGF21 primarily signals through a complex of fibroblast growth factor receptor isoforms (specifically FGFR1c and FGFR3c) and the essential co-factor beta-Klotho (KLB) . While the receptors are ubiquitously expressed, KLB expression is restricted primarily to liver, adipose tissue, breast, bone marrow, and brain, with weak expression in human pancreas . This tissue-specific co-factor expression pattern explains the selective metabolic actions of FGF21. Experimental protocols examining FGF21 function should therefore assess both receptor and co-factor expression in target tissues to accurately interpret signaling competence.
What are the critical factors to consider when measuring circulating FGF21 levels?
When designing protocols for FGF21 measurement, researchers should address several methodological considerations:
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Timing of collection: FGF21 demonstrates circadian oscillation patterns, necessitating standardized collection times
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Fasting state: Unlike in mice, short-term fasting/feeding status minimally affects FGF21 levels in humans
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Recent dietary composition: Particularly fructose intake, which can rapidly and significantly elevate FGF21 levels
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Physical activity: Higher levels of physical activity correlate with lower FGF21 concentrations
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Sleep patterns: Total sleep duration positively correlates with FGF21 levels (r = 0.34)
Researchers should standardize these variables when possible or record them as potential confounders in analysis.
Why do human FGF21 levels show such high variability between individuals?
The extraordinary inter-individual variability in FGF21 levels (250-fold range in some studies) represents a significant challenge for researchers . Several factors contribute to this variability:
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Genetic factors: Account for approximately 40% of the variation
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Diet composition: Responsible for up to 57% of variance in some studies, particularly protein intake
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Metabolic health status: Obesity and metabolic dysfunction alter baseline levels
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Physical activity levels: More active individuals typically show lower circulating FGF21
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Sex differences: Some studies suggest differential regulation based on biological sex
To address this variability, researchers should consider study designs with repeated measurements, larger sample sizes, and statistical approaches that account for these known confounding variables.
How do nutritional factors modulate circulating FGF21 levels in humans?
Unlike rodent models, human FGF21 regulation shows distinct nutritional response patterns:
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Fasting: Short-term fasting (24-48 hours) does not significantly increase FGF21 in humans, unlike the robust response seen in mice. Prolonged fasting (7+ days) is required to elevate FGF21 by 75-400%
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Protein restriction: Low protein diets increase FGF21 levels by 1.7-fold after 4 weeks and 2-fold after 6 weeks
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Carbohydrates: Fructose ingestion rapidly (within 2 hours) increases FGF21 by 3-4 fold in a dose-dependent manner
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Fat intake: Saturated fat intake shows negative correlation (r = -0.37) with FGF21 levels
These differential responses highlight the importance of precisely controlling and documenting dietary variables in human FGF21 studies.
What is the relationship between physical activity and FGF21 levels?
Research indicates a consistent inverse relationship between physical activity and circulating FGF21 levels. While acute exercise transiently increases FGF21, individuals with higher daily physical activity and better cardiorespiratory fitness demonstrate lower baseline FGF21 concentrations . The magnitude of this relationship increases with activity intensity:
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Light activity: Weak negative correlation
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Moderate activity: Moderate negative correlation
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Vigorous activity: Stronger negative correlation
Researchers studying FGF21 should record physical activity levels and potentially stratify subjects based on fitness levels to control for this variable.
What explains the paradoxical elevation of FGF21 in metabolically unhealthy states?
One of the most intriguing aspects of FGF21 biology is the observation that obese individuals generally exhibit higher circulating FGF21 levels despite FGF21's beneficial metabolic effects when administered pharmacologically . This apparent contradiction has led to the "FGF21 resistance" hypothesis, whereby elevated levels represent a compensatory mechanism in response to impaired FGF21 signaling .
When investigating this phenomenon, researchers should consider:
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Receptor and co-receptor expression: Assess potential downregulation of FGFR1c/3c or beta-Klotho in target tissues
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Post-receptor signaling: Examine potential defects in downstream signaling cascades
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FGF21 bioactivity: Measure the ratio of active to inactive FGF21 forms
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FGF21 clearance: Evaluate potential alterations in FGF21 degradation or excretion
Understanding these mechanisms requires integrated approaches combining circulating level measurements with tissue-specific signaling assessments.
How does FGF21 inactivation by fibroblast activation protein (FAP) impact research findings?
Recent research has identified fibroblast activation protein (FAP) as a protease that inactivates human FGF21 in circulation . This post-translational regulation adds complexity to FGF21 biology and has several implications for researchers:
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Total vs. active FGF21: Standard immunoassays may not distinguish between active and inactive forms
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Tissue-specific activity: FAP activity may vary between tissues and metabolic states
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Pharmacokinetic considerations: FAP-mediated degradation influences the half-life of both endogenous and exogenous FGF21
Researchers should consider measuring FAP activity alongside total FGF21 levels and potentially develop assays that specifically detect the active form of FGF21 for more accurate physiological assessment.
What explains the differential effects of FGF21 between rodent models and humans?
Despite promising results in rodent models, FGF21-based interventions in humans have shown discrepancies in metabolic outcomes, particularly regarding glucose metabolism . When translating findings between species, researchers should consider:
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Expression patterns: Nearly exclusive liver expression in humans vs. broader tissue expression in mice
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Fasting response: Marked differences in the threshold for fasting-induced FGF21 elevation
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Receptor distribution: Potential differences in tissue-specific expression of receptors and co-factors
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Dose-response relationships: Potential differences in sensitivity to FGF21 signaling
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Compensatory mechanisms: Species-specific counter-regulatory pathways
Translational studies should acknowledge these differences and incorporate comparative analyses of signaling pathways and gene expression networks.
What are the critical considerations for designing FGF21 intervention studies in humans?
When designing interventional studies involving FGF21 or manipulations that might affect FGF21, researchers should consider:
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Sampling schedule: Given the high variability of FGF21, multiple baseline and post-intervention measurements are recommended
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Control for confounders: Standardize or record dietary intake (especially protein and fructose), physical activity, and sleep patterns
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Sex stratification: Consider analyzing male and female participants separately given potential sex differences in FGF21 regulation
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Time of day standardization: Control for circadian rhythms by consistent sampling times
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Metabolic phenotyping: Comprehensive assessment of metabolic parameters to contextualize FGF21 changes
These considerations will strengthen study designs and improve reproducibility across research groups.
How should researchers interpret contradictory findings in FGF21 research?
The FGF21 literature contains numerous apparently contradictory findings. When evaluating such discrepancies, researchers should systematically consider:
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Methodological differences: Assay sensitivity, specificity, and sample handling variations
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Population heterogeneity: Baseline metabolic status, age, sex, and ethnicity differences
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Contextual factors: Differences in dietary status, physical activity levels, and sleep patterns
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Study design: Cross-sectional vs. longitudinal approaches, sample size limitations
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Biological complexity: FGF21's integration into complex physiological networks with feedback mechanisms
Systematic reviews and meta-analyses should carefully address these factors when synthesizing existing literature.
What are the most promising methodological advances for FGF21 research?
Emerging approaches that may advance FGF21 research include:
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Active vs. total FGF21 assays: Development of assays that specifically detect the biologically active form
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Tissue-specific FGF21 action: Novel techniques to assess tissue-specific FGF21 signaling in humans
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Genetic approaches: Mendelian randomization studies using FGF21 pathway genetic variants
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Multi-omics integration: Combining FGF21 measurements with metabolomics, proteomics, and transcriptomics
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Novel imaging techniques: Methods to visualize FGF21 receptor engagement in vivo
Researchers should consider incorporating these approaches to address current limitations in the field.
What are the key unresolved questions regarding FGF21 in human metabolism?
Despite significant advances, several fundamental questions remain unresolved:
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Tissue-specific contributions: The relative importance of different tissues as sources and targets of FGF21
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Regulatory hierarchy: How FGF21 integrates with other metabolic hormones in humans
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Circadian regulation: Detailed understanding of temporal regulation and its metabolic significance
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Developmental programming: Effects of early life exposures on FGF21 signaling pathways
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Therapeutic window: Identification of specific populations most likely to benefit from FGF21-based interventions
These questions represent high-priority areas for future investigation with significant translational potential.
Product Science Overview
FGF21 is a protein consisting of 209 amino acids, with a signal peptide of 28 amino acids at the N-terminus, resulting in a mature FGF21 polypeptide of 181 amino acids . The protein also contains a disulfide bond (Cys75-Cys93) within its core domain, which contributes to its stability .
FGF21 is expressed in various tissues, including the liver, adipose tissue, and pancreas. It is secreted into the bloodstream, where it acts on distant target tissues, making it an endocrine hormone .
FGF21 has been extensively studied for its role in metabolic regulation. It has several key functions:
- Glucose Homeostasis: FGF21 enhances glucose uptake in peripheral tissues, thereby improving insulin sensitivity and reducing blood glucose levels .
- Lipid Metabolism: It promotes the oxidation of fatty acids and reduces lipid accumulation in the liver, which can help prevent conditions like fatty liver disease .
- Energy Expenditure: FGF21 increases energy expenditure by stimulating thermogenesis in brown adipose tissue .
- Anti-inflammatory Effects: It has been shown to reduce inflammation, which is beneficial in conditions like obesity and type 2 diabetes .
Recombinant human FGF21 (rhFGF21) has been developed to harness its therapeutic potential. It has shown promise in treating various metabolic disorders, including:
- Type 2 Diabetes: rhFGF21 improves insulin sensitivity and glycemic control in diabetic patients .
- Obesity: It aids in weight loss by enhancing lipid metabolism and energy expenditure .
- Cardiovascular Diseases: FGF21’s anti-inflammatory properties can help reduce the risk of cardiovascular complications associated with metabolic disorders .
Producing rhFGF21 in a biologically active form poses several challenges. When expressed in bacterial systems, rhFGF21 tends to form inclusion bodies, making the purification process labor-intensive and time-consuming . Researchers have developed various strategies to improve the soluble expression and secretion of rhFGF21, such as optimizing codon usage and using specific signal peptides .
The potential of FGF21 as a therapeutic agent continues to be explored. Ongoing research aims to better understand its mechanisms of action and develop more efficient production methods. Additionally, clinical trials are being conducted to evaluate the efficacy and safety of rhFGF21 in treating a broader range of metabolic and inflammatory diseases.
In conclusion, Fibroblast Growth Factor-21 (Human Recombinant) represents a promising avenue for the treatment of metabolic disorders. Its multifaceted roles in glucose and lipid metabolism, energy expenditure, and inflammation make it a valuable target for therapeutic intervention.
