Fibroblast growth factors, collectively known as FGFs, are a family of signaling proteins that orchestrate an enormous range of biological processes, from building limbs in a developing embryo to healing a wound on your skin. Despite the name, they do far more than make fibroblasts grow. There are 22 known FGF family members in humans, and they influence cell division, migration, survival, and specialization across nearly every tissue in the body. Understanding what FGFs do and what happens when they malfunction sheds light on topics as varied as dwarfism, cancer, liver disease, and experimental heart therapies.
Three Ways FGFs Work
Not all FGFs operate the same way. Researchers group them into three functional classes based on how they reach their target cells. Paracrine FGFs are the largest class. They are released by one cell and act on nearby neighbors, sticking close to the tissue they came from. They rely on a sugar-rich molecule called heparan sulfate, which sits on cell surfaces and helps the FGF latch onto its receptor tightly enough to trigger a response. Endocrine FGFs, by contrast, travel through the bloodstream like hormones and act on distant organs. This group includes FGF19, FGF21, and FGF23, and instead of heparan sulfate, these circulating FGFs depend on a different set of co-receptors known as Klotho proteins. The third class, intracrine FGFs, never leave the cell that makes them. They work inside the cell and do not use the same surface receptors at all.1PubMed Central. Endocrine FGFs: Evolution, Physiology, Pathophysiology, and Pharmacotherapy
This three-part classification matters because it explains why different FGFs have such different jobs. The paracrine members are the workhorses of development and tissue repair, acting locally to shape organs and heal injuries. The endocrine members function more like metabolic regulators, adjusting how your body handles sugar, fat, bile, and minerals. The intracrine members appear to serve structural roles inside neurons and other cells. The evolutionary roots of the family trace back to a single intracrine ancestor; over hundreds of millions of years, gene duplications gave rise to the paracrine and then the endocrine branches.2The Journal of Biochemistry. Fibroblast growth factors: from molecular evolution to roles in development, metabolism and disease
How FGFs Turn On a Cell
When a paracrine FGF reaches a target cell, it binds to one of four FGF receptors (FGFR1 through FGFR4) embedded in the cell membrane. Heparan sulfate plays a dual role here: it stabilizes the bond between the FGF and the receptor, and it helps two receptor molecules pair up, a step called dimerization that is required to fire off the signal.3PubMed. Crystal structure of a ternary FGF-FGFR-heparin complex reveals a dual role for heparin in FGFR binding and dimerization Once the paired receptors activate each other, they set off a cascade of chemical changes inside the cell. Several major intracellular pathways get switched on, including pathways that drive cell growth, survival, and movement.4PubMed Central. The Fibroblast Growth Factor signaling pathway
The specific downstream pathway that matters depends on the cell type and the context. In the developing inner ear, for example, two of those pathways are required for the sensory cells that detect sound to form properly, while others are dispensable for that particular job.5PubMed Central. FGF20-FGFR1 signaling through MAPK and PI3K controls sensory progenitor differentiation in the organ of Corti This selectivity is part of what makes FGF signaling so versatile. The same receptor family can produce very different outcomes in different tissues, because each tissue filters the signal through its own combination of intracellular machinery.
Building an Embryo
FGFs are among the first signals that shape a developing vertebrate. One of their most dramatic roles is in limb formation. A strip of specialized tissue at the tip of each developing limb bud, called the apical ectodermal ridge, produces several FGFs, including FGF-4 and FGF-8. These signals keep the cells underneath dividing and prevent them from differentiating too early. If that ridge is removed in experimental animals, the limb stops growing and comes out truncated. Supplying FGF-8 externally can rescue limb growth even after the ridge is gone, confirming that FGFs are the essential ingredient the ridge provides.6Current Biology. Expression of fgf-8 completes the set of fibroblast growth factors expressed in the apical ectodermal ridge and suggests a role in promoting limb bud outgrowth
Limb development is only one example. FGFs also help pattern the brain, guide the branching of airways in the lungs, and shape the face and skull. Their influence is so widespread during embryonic life that mutations affecting even a single FGF receptor can have consequences visible at birth.
Wound Healing and New Blood Vessels
After an injury, your body needs to rebuild damaged tissue, and FGFs play several roles in that process. FGF-2 (sometimes called basic FGF) is one of the most studied members here. It stimulates the growth of new blood vessels, a process called angiogenesis, by encouraging the cells lining existing vessels to divide and migrate into the wound. Part of how FGF-2 does this is indirect: it triggers those endothelial cells to produce another growth factor, VEGF, which then acts as a secondary signal that amplifies the blood-vessel response.7PubMed Central. Fibroblast growth factor-2 (FGF-2) induces vascular endothelial growth factor (VEGF) expression in the endothelial cells of forming capillaries: an autocrine mechanism contributing to angiogenesis Blocking VEGF dramatically reduced the vessel growth that FGF-2 had started, showing how intertwined these two factors are.
FGF-2 preferentially drives the growth of small vessels rather than large ones, which is exactly what a healing wound needs. Tiny capillaries are what deliver oxygen and nutrients to the new tissue.8PubMed. Fibroblast growth factor-2 selectively stimulates angiogenesis of small vessels in arterial tree In skin wound models, delivering FGF-2 in a controlled-release format accelerated healing by boosting cell proliferation, stimulating VEGF secretion, and promoting collagen deposition and blood vessel maturation.9PubMed. Heparin-Based Coacervate of FGF2 Improves Dermal Regeneration by Asserting a Synergistic Role with Cell Proliferation and Endogenous Facilitated VEGF for Cutaneous Wound Healing A related family member, acidic FGF (FGF-1), similarly spurs angiogenesis and the growth of new epithelium and granulation tissue in wounds, though it does not promote wound contraction.10PubMed. Acidic fibroblast growth factor accelerates dermal wound healing
FGFs also interact with the immune system during repair. FGF-2 does not attract immune cells on its own, but when inflammatory signals are already present, pretreating tissue with FGF-2 significantly amplifies the recruitment of monocytes, boosting their influx by roughly 1.5 to 2 times compared to untreated tissue.11The American Journal of Pathology. Basic Fibroblast Growth Factor Synergistically Enhances Leukocyte Recruitment to Inflammation by Promoting Adhesion Molecule Expression in the Microvasculature This synergy between FGFs and immune signals helps ensure that the healing response is robust without FGFs triggering inflammation by themselves.
The Hormone Branch of the Family
Three FGFs break the local-signaling mold entirely. FGF19, FGF21, and FGF23 circulate in the blood and regulate metabolism in organs far from where they were made. They are sometimes called the endocrine FGFs, and their biology is distinct enough that they deserve a closer look individually.
FGF19 and Bile Acid Control
FGF19 is produced by cells lining the small intestine, mainly after meals. Its job is to travel to the liver and put the brakes on bile acid production. When bile acids activate a receptor in the gut wall, the intestinal cells release FGF19 into the portal circulation. Once it reaches the liver, FGF19 binds to FGFR4 together with the co-receptor β-Klotho, and this suppresses the enzyme that controls how fast the liver makes new bile acids.12PubMed Central. A variant of FGF19 for treatment of disorders of cholestasis and bile acid metabolism Without this feedback loop, bile acid levels can climb to toxic concentrations. FGF19 also influences glucose and fat metabolism, making it a target of interest for metabolic liver diseases.
FGF21 and Metabolic Flexibility
FGF21 is primarily made by the liver and acts as a nutrient sensor. Its levels rise during fasting, protein restriction, and other states of nutritional stress, and it orchestrates a broad metabolic response to help the body adapt. FGF21 promotes fat breakdown, enhances insulin sensitivity, and influences energy expenditure.13PubMed Central. Fibroblast Growth Factor 21: A Versatile Regulator of Metabolic Homeostasis Like FGF19, it requires β-Klotho to activate its receptor, which explains why it only affects tissues that express that co-receptor, such as fat tissue and the brain.14PubMed Central. Structures of β-klotho reveal a ‘zip code’-like mechanism for endocrine FGF signalling The dependence on Klotho proteins essentially creates an address system: only cells displaying the right co-receptor respond to the hormonal FGF, no matter how much of it is circulating.
FGF23 and Phosphate Balance
FGF23 is made by bone cells and acts on the kidneys to control how much phosphate and vitamin D are in the blood. It binds to FGF receptors together with the co-receptor α-Klotho, promoting the excretion of phosphate in urine and lowering the production of active vitamin D.15PubMed. Regulation of FGF23 production and phosphate metabolism by bone-kidney interactions This bone-to-kidney signaling axis is critical for mineral balance. When FGF23 levels are too high, as happens in chronic kidney disease, the result is persistent phosphate wasting and a deficiency of active vitamin D, both of which weaken the skeleton and contribute to cardiovascular calcification.16PubMed Central. Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease
Achondroplasia and What Overactive FGF Signaling Does to Bone
One of the clearest demonstrations that FGF signaling can be harmful when it is too strong comes from achondroplasia, the most common form of short-limbed dwarfism. Achondroplasia is caused by a single mutation in the gene for FGFR3, the third fibroblast growth factor receptor. The mutation locks the receptor in a permanently active state, so it fires continuously even when no FGF is binding to it.17PubMed. Delayed bone age due to a dual effect of FGFR3 mutation in Achondroplasia
Counterintuitively, this overactivation suppresses bone growth rather than accelerating it. FGFR3 normally acts as a brake on the growth plate, the cartilage zone at the ends of long bones where new bone forms during childhood. When that brake is stuck on, the cartilage cells in the growth plate fail to multiply and mature properly, resulting in shorter bones and a smaller growth plate overall.18PubMed Central. Achondroplasia: Development, pathogenesis, and therapy Laboratory studies confirmed this mechanism directly: treating cartilage cells with FGF-2 or introducing the achondroplasia mutation into cell lines reduced the rate of cartilage cell division.19Genes & Diseases. Advances in the mechanism and therapies of achondroplasia The approved drug vosoritide, which recently changed treatment options for children with achondroplasia, works by counteracting the overactive FGFR3 signal. It does not target FGF directly but instead boosts a competing pathway that the runaway receptor suppresses.
FGF Receptor Alterations in Cancer
Because FGF signaling tells cells to grow, survive, and build new blood vessels, it is not surprising that the system gets hijacked in cancer. Tumors can amplify FGF receptor genes, acquire mutations that make the receptors overactive, or develop chromosomal rearrangements that fuse an FGFR gene to another gene, creating a hybrid protein that signals nonstop.20PubMed Central. Fibroblast growth factor receptor fusions in cancer: opportunities and challenges These alterations have been found across many tumor types, and the growing availability of genomic sequencing is revealing FGFR changes in cancers where they were not previously suspected.21PubMed Central. Genomic aberrations in the FGFR pathway: opportunities for targeted therapies in solid tumors
Drugs that block FGFR activity have already reached the clinic. Several selective FGFR inhibitors are approved for cancers harboring specific FGFR alterations, including certain bladder cancers and bile duct cancers. The challenge, as with many targeted therapies, is resistance. In one study of patients with bile duct cancers driven by FGFR2 fusions, about 60% developed secondary mutations in the FGFR2 gene after their initial response to an FGFR inhibitor. The most common escape routes were mutations at two key sites within the receptor’s enzyme domain.22PubMed Central. Landscape of Clinical Resistance Mechanisms to FGFR Inhibitors in FGFR2-Altered Cholangiocarcinoma Cancer cells can also bypass the blocked receptor entirely by activating alternative growth pathways or by remodeling the surrounding tissue microenvironment.23PubMed. FGFR-driven lung cancer: Dissecting resistance and exploring therapeutic avenues Ongoing clinical trials are testing next-generation inhibitors designed to overcome these resistance mechanisms, as well as combination approaches that block both the primary and escape pathways simultaneously.24PubMed Central. Targeting FGFR for cancer therapy
Keeping Stem Cells in an Undifferentiated State
Anyone who works with human embryonic stem cells or induced pluripotent stem cells in the lab knows that FGF-2 is an essential ingredient in the culture medium. Without it, these cells tend to spontaneously differentiate and lose their ability to become any cell type.25PubMed Central. FGF Signaling Pathway: A Key Regulator of Stem Cell Pluripotency FGF-2 activates signaling pathways that maintain the expression of key pluripotency markers, and blocking the FGF receptor causes those markers to drop, nudging the cells toward differentiation.26PubMed. FGF-2 modulates Wnt signaling in undifferentiated hESC and iPS cells through activated PI3-K/GSK3beta signaling
A practical problem in stem cell labs is that FGF-2 degrades quickly at body temperature, so daily feeding of fresh growth factor is standard practice. One solution is to attach FGF-2 to slow-release beads, which sustain effective levels over several days. Cultures fed this way performed just as well, or better, at staying undifferentiated compared to cultures given fresh FGF-2 every day.27PLOS ONE. Sustained Levels of FGF2 Maintain Undifferentiated Stem Cell Cultures with Biweekly Feeding Seemingly mundane, this kind of engineering matters enormously for scaling up stem cell production for regenerative medicine. If you eventually need millions of identical stem cells for a therapy, small improvements in culture stability translate into big differences in cost and reliability.
FGF Therapies for Heart Disease
The ability of FGFs to stimulate new blood vessel growth made them an obvious candidate for treating ischemic heart disease, where clogged arteries starve parts of the heart muscle of blood. The logic was appealing: deliver an FGF directly to the heart, grow new vessels around the blockage, and restore blood flow without surgery. Early clinical trials explored this idea through two main routes: injecting FGF protein into the coronary arteries, and delivering an FGF gene using a viral vector so the heart cells would make the protein themselves.
Results have been mixed. A gene therapy trial using an adenovirus carrying FGF-4 showed a meaningful reduction in the size of the heart’s oxygen-starved zone at eight weeks, with the treated group improving while the placebo group did not.28PubMed. A randomized, double-blind, placebo-controlled trial of Ad5FGF-4 gene therapy and its effect on myocardial perfusion in patients with stable angina In another trial, a single infusion of recombinant FGF-2 protein into the coronary artery did not improve exercise capacity or perfusion measurements, though it did modestly reduce angina symptoms at 90 days in the most symptomatic patients. By 180 days, even that difference had faded as the placebo group continued to improve.29PubMed. Pharmacological treatment of coronary artery disease with recombinant fibroblast growth factor-2: double-blind, randomized, controlled clinical trial The broader takeaway from these cardiac trials is that growth-factor-based angiogenesis therapy is safe, but producing durable clinical benefit has proven harder than the lab results suggested.30PubMed Central. Gene therapy for ischemic heart disease The powerful placebo effect in angina trials, where patients who think they received treatment often feel substantially better, has made it especially difficult to prove a clear advantage for FGF-based approaches.
FGF21 Analogs for Fatty Liver Disease
While cardiac applications have stalled somewhat, the endocrine branch of the FGF family is generating fresh therapeutic excitement. FGF21, the metabolic hormone that helps the body adapt to fasting and nutrient stress, has become a leading target for treating fatty liver disease. Multiple pharmaceutical companies have developed FGF21 analogs, engineered versions of the natural hormone with longer half-lives so they can be given as periodic injections rather than wearing off in hours.
Three analogs, efruxifermin, pegbelfermin, and pegozafermin, have reached mid-to-late-stage clinical trials for metabolic dysfunction-associated steatohepatitis, the progressive inflammatory form of fatty liver disease that can lead to cirrhosis.31PubMed. Efficacy and Safety of Fibroblast Growth Factor-21 Analogs for the Treatment of Metabolic Dysfunction-Associated Steatohepatitis: A Systematic Review and Meta-Analysis The interest is not hard to understand: FGF21 simultaneously improves insulin sensitivity, promotes fat burning, and reduces liver inflammation, all of which directly address the core problems in fatty liver disease. These trials are still ongoing, but FGF21 analogs represent one of the most active areas of metabolic drug development right now.
An Ancient Signaling System
FGFs and their receptors are not unique to humans. The signaling system is conserved across animals, present in creatures from simple invertebrates through fish, amphibians, birds, and mammals.32PubMed. Evolution of the Fgf and Fgfr gene families The endocrine branch appears to be a vertebrate innovation, however. Invertebrates have paracrine and intracrine FGFs, but the hormone-like FGFs that circulate in the blood and regulate metabolism seem to have evolved only after vertebrates appeared, coinciding with the emergence of Klotho co-receptors.33PubMed Central. Endocrine FGFs: Evolution, Physiology, Pathophysiology, and Pharmacotherapy The endocrine FGFs appear to have lost the ability to grip heparan sulfate, which freed them from staying close to the cells that made them and allowed them to enter the bloodstream and reach distant targets. That evolutionary trade-off, giving up local stickiness for systemic reach, is what turned a local tissue signal into a hormone.