Fibroblast growth factor
Fibroblast growth factor
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Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by the macrophages. They are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in their function will lead to a range of developmental defects. These growth factors typically act as a systemic or locally circulating molecules of extracellular origin that activate cell surface receptors. A defining property of FGFs is that they bind to heparin and to heparan sulfate. Thus, some are sequestered in the extracellular matrix of tissues that contains heparan sulfate proteoglycans, and released locally upon injury or tissue remodeling.[1]

Families

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In humans, 23 members of the FGF family have been identified, all of which are structurally related signaling molecules:[2][3][4]

  • Members FGF1 through FGF10 all bind fibroblast growth factor receptors (FGFRs). FGF1 is also known as acidic fibroblast growth factor, and FGF2 is also known as basic fibroblast growth factor.
  • Members FGF11, FGF12, FGF13, and FGF14, also known as FGF homologous factors 1-4 (FHF1-FHF4), have been shown to have distinct functions compared to the FGFs. Although these factors possess remarkably similar sequence homology, they do not bind FGFRs and are involved in intracellular processes unrelated to the FGFs.[5] This group is also known as the intracellular fibroblast growth factor subfamily (iFGF).[6]
  • Human FGF18 is involved in cell development and morphogenesis in various tissues including cartilage.[7]
  • Human FGF20 was identified based on its homology to Xenopus FGF-20 (XFGF-20).[8][9]
  • FGF15 through FGF23 were described later and functions are still being characterized. FGF15 is the mouse ortholog of human FGF19 (there is no human FGF15) and, where their functions are shared, they are often described as FGF15/19.[10] In contrast to the local activity of the other FGFs, FGF15/19, FGF21 and FGF23 have hormonal systemic effects.[10][11]

Receptors

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The mammalian fibroblast growth factor receptor family has 4 members, FGFR1, FGFR2, FGFR3, and FGFR4. The FGFRs consist of three extracellular immunoglobulin-type domains (D1-D3), a single-span trans-membrane domain and an intracellular split tyrosine kinase domain. FGFs interact with the D2 and D3 domains, with the D3 interactions primarily responsible for ligand-binding specificity (see below). Heparan sulfate binding is mediated through the D3 domain. A short stretch of acidic amino acids located between the D1 and D2 domains has auto-inhibitory functions. This 'acid box' motif interacts with the heparan sulfate binding site to prevent receptor activation in the absence of FGFs.[12]

Alternate mRNA splicing gives rise to 'b' and 'c' variants of FGFRs 1, 2 and 3. Through this mechanism, seven different signalling FGFR sub-types can be expressed at the cell surface. Each FGFR binds to a specific subset of the FGFs. Similarly, most FGFs can bind to several different FGFR subtypes. FGF1 is sometimes referred to as the 'universal ligand' as it is capable of activating all 7 different FGFRs. In contrast, FGF7 (keratinocyte growth factor, KGF) binds only to FGFR2b (KGFR).[13]

The signalling complex at the cell surface is believed to be a ternary complex formed between two identical FGF ligands, two identical FGFR subunits, and either one or two heparan sulfate chains.

History

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A mitogenic growth factor activity was found in pituitary extracts by Armelin in 1973[14] and further work by Gospodarowicz as reported in 1974 described a more defined isolation of proteins from cow brain extract which, when tested in a bioassay that caused fibroblasts to proliferate, led these investigators to apply the name "fibroblast growth factor."[15] In 1975, they further fractionated the extract using acidic and basic pH and isolated two slightly different forms that were named "acidic fibroblast growth factor" (FGF1) and "basic fibroblast growth factor" (FGF2). These proteins had a high degree of sequence homology among their amino acid chains, but were determined to be distinct proteins.

Not long after FGF1 and FGF2 were isolated, another group of investigators isolated a pair of heparin-binding growth factors that they named HBGF-1 and HBGF-2, while a third group isolated a pair of growth factors that caused proliferation of cells in a bioassay containing blood vessel endothelium cells, which they called ECGF1 and ECGF2. These independently discovered proteins were eventually demonstrated to be the same sets of molecules, namely FGF1, HBGF-1 and ECGF-1 were all the same acidic fibroblast growth factor described by Gospodarowicz, et al., while FGF2, HBGF-2, and ECGF-2 were all the same basic fibroblast growth factor.[1]

Functions

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FGFs are multifunctional proteins with a wide variety of effects; they are most commonly mitogens but also have regulatory, morphological, and endocrine effects. They have been alternately referred to as "pluripotent" growth factors and as "promiscuous" growth factors due to their multiple actions on multiple cell types.[16][17] Promiscuous refers to the biochemistry and pharmacology concept of how a variety of molecules can bind to and elicit a response from single receptor. In the case of FGF, four receptor subtypes can be activated by more than twenty different FGF ligands. Thus the functions of FGFs in developmental processes include mesoderm induction, anterior-posterior patterning,[8] limb development, neural induction and neural development,[18] and in mature tissues/systems angiogenesis, keratinocyte organization, and wound healing processes.

FGF is critical during normal development of both vertebrates and invertebrates and any irregularities in their function leads to a range of developmental defects.[19][20][21][22]

FGFs secreted by hypoblasts during avian gastrulation play a role in stimulating a Wnt signaling pathway that is involved in the differential movement of Koller's sickle cells during formation of the primitive streak.[23] Left, angiography of the newly formed vascular network in the region of the front wall of the left ventricle. Right, analysis quantifying the angiogenic effect.[24]

While many FGFs can be secreted by cells to act on distant targets, some FGF act locally within a tissue, and even within a cell. Human FGF2 occurs in low molecular weight (LMW) and high molecular weight (HMW) isoforms.[25] LMW FGF2 is primarily cytoplasmic and functions in an autocrine manner, whereas HMW FGF2s are nuclear and exert activities through an intracrine mechanism.

One important function of FGF1 and FGF2 is the promotion of endothelial cell proliferation and the physical organization of endothelial cells into tube-like structures. They thus promote angiogenesis, the growth of new blood vessels from the pre-existing vasculature. FGF1 and FGF2 are more potent angiogenic factors than vascular endothelial growth factor (VEGF) or platelet-derived growth factor (PDGF).[26] FGF1 has been shown in clinical experimental studies to induce angiogenesis in the heart.[24]

As well as stimulating blood vessel growth, FGFs are important players in wound healing. FGF1 and FGF2 stimulate angiogenesis and the proliferation of fibroblasts that give rise to granulation tissue, which fills up a wound space/cavity early in the wound-healing process. FGF7 and FGF10 (also known as keratinocyte growth factors KGF and KGF2, respectively) stimulate the repair of injured skin and mucosal tissues by stimulating the proliferation, migration and differentiation of epithelial cells, and they have direct chemotactic effects on tissue remodelling.

During the development of the central nervous system, FGFs play important roles in neural stem cell proliferation, neurogenesis, axon growth, and differentiation. FGF signaling is important in promoting surface area growth of the developing cerebral cortex by reducing neuronal differentiation and hence permitting the self-renewal of cortical progenitor cells, known as radial glial cells,[27] and FGF2 has been used to induce artificial gyrification of the mouse brain.[28] Another FGF family member, FGF8, regulates the size and positioning of the functional areas of the cerebral cortex (Brodmann areas).[29][30]

FGFs are also important for maintenance of the adult brain. Thus, FGFs are major determinants of neuronal survival both during development and during adulthood.[31] Adult neurogenesis within the hippocampus e.g. depends greatly on FGF2. In addition, FGF1 and FGF2 seem to be involved in the regulation of synaptic plasticity and processes attributed to learning and memory, at least in the hippocampus.[31]

The 15 exparacrine FGFs are secreted proteins that bind heparan sulfate and can, therefore, be bound to the extracellular matrix of tissues that contain heparan sulfate proteoglycans. This local action of FGF proteins is classified as paracrine signalling, most commonly through the JAK-STAT signalling pathway or the receptor tyrosine kinase (RTK) pathway.

Members of the FGF19 subfamily (FGF15, FGF19, FGF21, and FGF23) bind less tightly to heparan sulfates, and so can act in an endocrine fashion on far-away tissues, such as intestine, liver, kidney, adipose, and bone.[10] For example:

  • FGF15 and FGF19 (FGF15/19) are produced by intestinal cells but act on FGFR4-expressing liver cells to downregulate the key gene (CYP7A1) in the bile acid synthesis pathway.[32]
  • FGF23 is produced by bone but acts on FGFR1-expressing kidney cells to regulate the synthesis of vitamin D and phosphate homeostasis.[33]

Structure

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The crystal structures of FGF1 have been solved and found to be related to interleukin 1-beta. Both families have the same beta trefoil fold consisting of 12-stranded beta-sheet structure, with the beta-sheets are arranged in 3 similar lobes around a central axis, 6 strands forming an anti-parallel beta-barrel.[34][35][36] In general, the beta-sheets are well-preserved and the crystal structures superimpose in these areas. The intervening loops are less well-conserved - the loop between beta-strands 6 and 7 is slightly longer in interleukin-1 beta.

Clinical applications

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Dysregulation of the FGF signalling system underlies a range of diseases associated with the increased FGF expression. Inhibitors of FGF signalling have shown clinical efficacy.[37] Some FGF ligands (particularly FGF2) have been demonstrated to enhance tissue repair (e.g. skin burns, grafts, and ulcers) in a range of clinical settings.[38]

See also

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References

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Fibroblast growth factor

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Fibroblast growth factors (FGFs) are a family of 22 structurally related polypeptide growth factors that function as key signaling molecules in mammals, regulating essential cellular processes such as proliferation, differentiation, migration, survival, and apoptosis.[1] These proteins are evolutionarily conserved and play pivotal roles in embryonic development, tissue homeostasis, and repair by binding to specific receptors on target cells.[2] The FGF family is divided into seven subfamilies based on sequence homology and function: FGF1 (FGF1 and FGF2), FGF4 (FGF4, FGF5, FGF6), FGF7 (FGF3, FGF7, FGF10, FGF22), FGF8 (FGF8, FGF17, FGF18), FGF9 (FGF9, FGF16, FGF20), FGF15/19 (FGF19, FGF21, FGF23), and FGF11 (FGF11–FGF14, which are intracellular and non-secretory).[2] Most FGFs (the canonical or paracrine subtypes) require heparan sulfate proteoglycans as co-factors to bind and activate their receptors, while the endocrine FGFs (FGF19, FGF21, FGF23) depend on klotho proteins for specificity and activity.[1] Signaling occurs primarily through four tyrosine kinase receptors (FGFR1–FGFR4), each with alternative splice variants (e.g., IIIb and IIIc isoforms) that confer ligand specificity, leading to activation of downstream pathways including MAPK/ERK, PI3K/AKT, and PLCγ.[3] In health, FGFs are indispensable for organogenesis—such as limb, lung, and skeletal development—and adult physiological processes like angiogenesis, wound healing, metabolic regulation (e.g., glucose and phosphate homeostasis via FGF21 and FGF23), and tissue regeneration.[1] Dysregulation of FGF signaling contributes to numerous pathologies, including congenital disorders like achondroplasia and craniosynostosis (due to FGFR mutations), metabolic diseases such as obesity and chronic kidney disease, and cancers where amplified FGFRs or overexpressed FGFs promote tumorigenesis in organs like the bladder, breast, and prostate.[3] Ongoing research explores FGF-targeted therapies, including receptor inhibitors for oncology and recombinant FGFs for regenerative medicine.[1]

Molecular Structure and Families

Protein Structure

Fibroblast growth factors (FGFs) are a family of secreted polypeptides typically comprising 150–300 amino acids, with mature proteins exhibiting molecular weights in the range of 17–34 kDa.[3] These proteins lack signal peptides in some cases, such as FGF1 and FGF2, leading to non-classical secretion mechanisms, while others possess cleavable N-terminal extensions.[3] The defining structural feature of FGFs is a conserved central core domain of approximately 120–130 amino acids that adopts a β-trefoil fold, characterized by 12 antiparallel β-strands (β1–β12) arranged into three β-sheet subdomains with approximate threefold rotational symmetry.[3] This compact fold is stabilized by hydrophobic interactions and includes heparin-binding sites primarily located in the β1–β2 loop and the β10–β12 region, which facilitate interactions with extracellular matrix components.[3] Divergent N- and C-terminal extensions flank this core, varying in length and sequence across family members to influence solubility, proteolytic stability, and biological specificity.[3] Most FGFs contain a conserved cysteine residue at position 83 (numbered relative to the core domain), which remains free in canonical members like FGF1 and FGF2, contributing to thermodynamic stability without forming intramolecular disulfides. In contrast, certain subfamilies, such as FGF8 and FGF19, feature an additional cysteine at position 66 that forms a disulfide bond with Cys83, enhancing thermostability by approximately 14 kJ/mol. For example, human FGF1 (UniProt P05230) has cysteines at positions 16, 83, and 117, none of which participate in disulfide bridges, preserving a monomeric structure essential for function.[4] Post-translational modifications further modulate FGF stability and activity. Phosphorylation sites, such as Ser116 in human FGF1, can alter protein half-life and signaling potency when mutated, as demonstrated by the S116R variant that enhances extracellular signal-regulated kinase phosphorylation.[5] Glycation, a non-enzymatic modification occurring under hyperglycemic conditions, targets lysine and arginine residues in FGF2, reducing angiogenic activity and signal transduction in endothelial cells by impairing heparin binding.[6] High-resolution crystal structures have elucidated these features; for instance, the X-ray structure of human FGF2 was determined at 1.9 Å resolution, revealing the β-trefoil core and solvent-exposed loops critical for ligand interactions.[7] These intrinsic structural motifs, particularly the β-trefoil fold and heparin-binding regions, underpin the capacity of FGFs to engage fibroblast growth factor receptors.[3]

Classification into Families

In humans, the fibroblast growth factor (FGF) family consists of 22 ligands, encoded by Fgf genes and designated FGF1 through FGF14 and FGF16 through FGF23, with FGF15 being the ortholog of human FGF19 in mice. These ligands are classified into three main functional groups based on their modes of action and expression patterns: paracrine-acting FGFs, which primarily include FGF1–10, FGF16, FGF17, FGF18, FGF20, and FGF22 and function locally by binding cell-surface receptors; intracrine/autocrine-acting FGFs (FGF11–14), which lack signal peptides and exert effects intracellularly without secretion; and endocrine-acting FGFs (FGF19, FGF21, and FGF23), which circulate systemically and require co-receptors such as β-klotho for binding to fibroblast growth factor receptors (FGFRs).[1][8] Phylogenetically, the FGF family is divided into seven subfamilies based on sequence homology, evolutionary relationships, and conserved chromosomal synteny, reflecting their divergence from a common ancestor.[9] These subfamilies are: FGF1/2 (canonical paracrine); FGF4/5/6 (canonical paracrine, sharing over 40% amino acid sequence identity); FGF3/7/10/22 (paracrine, involved in epithelial-mesenchymal interactions); FGF8/17/18; FGF9/16/20 (paracrine, expressed in mesenchymal tissues); FGF11/12/13/14 (intracrine); and FGF19/21/23 (endocrine, with lower sequence similarity to others but distinct hormone-like functions).[10] This classification highlights evolutionary expansions, such as duplications within subfamilies, and underscores functional diversification, with paracrine members typically sharing a conserved β-trefoil core domain for receptor binding.[11] The Fgf genes are distributed across multiple chromosomes, often in clusters that preserve synteny across vertebrates, indicating coordinated regulation. For example, FGF1 is located on chromosome 5q31.3, FGF2 on 4q26, FGF3 and FGF4 on 11q13.3, and FGF23 on 12p13.32, with some subfamilies showing proximity such as the FGF3/4/19 cluster on 11q13.[12] Many FGF genes produce multiple isoforms through alternative splicing or other mechanisms, enhancing functional versatility; for instance, FGF2 generates four isoforms (18, 21, 22, and 24 kDa) via alternative translation initiation from a single mRNA, influencing subcellular localization and activity.[13]

Receptors and Binding Mechanisms

Fibroblast Growth Factor Receptors (FGFRs)

Fibroblast growth factor receptors (FGFRs) are a family of receptor tyrosine kinases that transduce signals from fibroblast growth factors (FGFs) to regulate cellular processes such as proliferation and differentiation. The four principal members, FGFR1 through FGFR4, share a conserved structural architecture consisting of an extracellular ligand-binding domain, a single transmembrane helix, and an intracellular split tyrosine kinase domain. The extracellular domain comprises three immunoglobulin-like (Ig-like) loops, designated D1, D2, and D3, with D2 featuring a cysteine-rich region that contributes to receptor folding and stability through disulfide bond formation.[14] Mutations in the cysteine-rich domain of D2, such as those altering glycosylation sites, can enhance receptor stability and promote aberrant signaling, as observed in certain developmental disorders and cancers.[1] Alternative splicing primarily affects the third Ig-like domain (D3) in FGFR1, FGFR2, and FGFR3, generating two major isoforms: IIIb and IIIc. The IIIb isoform, encoded by exon 8 in FGFR1, FGFR2, and FGFR3, predominates in epithelial tissues and exhibits specificity for mesenchymal-derived FGFs, such as FGF7 (also known as keratinocyte growth factor) binding to FGFR2-IIIb. In contrast, the IIIc isoform, encoded by exon 9 in FGFR1, FGFR2, and FGFR3, is prevalent in mesenchymal cells and binds paracrine FGFs like FGF2. FGFR4 lacks this splicing variability and expresses only the IIIc-like form. The transmembrane domains of FGFRs harbor motifs that facilitate ligand-induced dimerization and oligomerization, enabling cooperative signaling.[14][1][15] FGFRs display distinct tissue-specific expression patterns that correlate with their physiological roles. FGFR1 is ubiquitously expressed across multiple tissues, including brain, kidney, and vascular endothelium, supporting broad developmental functions. FGFR2 is enriched in epithelial structures during embryogenesis, while FGFR3 is prominently expressed in cartilage and skeletal tissues, influencing bone growth. FGFR4 shows restricted distribution, such as in liver and skeletal muscle. The genes encoding these receptors are located on specific chromosomal sites: FGFR1 at 8p11.23, FGFR2 at 10q26.13, FGFR3 at 4p16.3, and FGFR4 at 5q35.2.[1][14] A fifth member, FGFR5 (also termed FGFRL1), diverges from the tyrosine kinase family by lacking an intracellular kinase domain and instead possessing a short histidine-rich tail; it functions as a modulator of FGFR1 signaling through interactions via its extracellular Ig-like domains. Heparan sulfate proteoglycans serve as essential co-receptors that enhance FGFR-FGF interactions.[16][14][17]
ReceptorGene LocationMajor IsoformsKey Tissue Expression
FGFR18p11.23IIIb, IIIcUbiquitous (e.g., brain, kidney)
FGFR210q26.13IIIb, IIIcEpithelial tissues
FGFR34p16.3IIIb, IIIcCartilage, bone
FGFR45q35.2IIIc onlyLiver, skeletal muscle
FGFR54p16.3None (single)Variable, regulatory

Ligand-Receptor Interactions

Fibroblast growth factors (FGFs) interact with fibroblast growth factor receptors (FGFRs) primarily through high-affinity binding between the core domain of the FGF ligand and the D2 immunoglobulin-like (Ig) loop of the FGFR extracellular domain, with dissociation constants (Kd) typically in the range of 10-100 nM for the binary complex. This interaction forms the initial basis for signal initiation, where the D2 domain serves as the primary contact site, facilitating subsequent complex assembly.[18] The formation of a stable signaling complex requires heparan sulfate proteoglycans (HSPGs), such as syndecans and glypicans, which act as co-receptors to bridge and stabilize a ternary FGF-FGFR-HSPG complex through electrostatic interactions with clusters of basic residues (e.g., arginine and lysine) on the FGF surface.[18] These HSPGs enhance binding affinity, reducing the Kd to the low nanomolar range (e.g., from ~62 nM to ~2.7 nM for FGF2-FGFR1), and promote the organization of a 2:2:2 stoichiometric dimer essential for activation. In contrast, endocrine FGFs exhibit reduced HSPG affinity, relying instead on alternative co-receptors. Ligand-receptor specificity is governed by structural features in both FGFs and FGFR isoforms, with FGF1 capable of binding all FGFRs (1-4) due to its versatile interface, while FGF7 shows high specificity for FGFR2-IIIb, restricting its activity to epithelial tissues.[19] For endocrine FGFs, such as FGF23, binding to FGFR1 (particularly the IIIc isoform) is facilitated by α-klotho as a co-receptor, enabling phosphate regulation, whereas β-klotho co-operates with FGFR1c for FGF19 and FGF21 in metabolic control.[1] Upon ligand binding, FGFs induce conformational changes in the FGFR extracellular domains, particularly involving rotation and juxtaposition of the D2 and D3 loops, which drive receptor dimerization and align the transmembrane helices for signal propagation.[20] These interactions are optimized under physiological conditions, with binding affinity peaking at pH 7.4 and moderate ionic strength (~150 mM NaCl), where electrostatic contributions from HSPGs are most effective.[21]

Signaling Pathways

Activation and Intracellular Signaling

Upon ligand binding, fibroblast growth factor receptors (FGFRs) undergo dimerization, typically forming a 2:2:2 complex with FGF and heparan sulfate proteoglycans, which positions the intracellular tyrosine kinase domains for trans-autophosphorylation. This process activates the kinase, with sequential phosphorylation of specific tyrosine residues in the activation loop, such as Y653 and Y654 in FGFR1, leading to a 50- to 100-fold increase in activity at Y653 and a further enhancement to 500- to 1,000-fold upon Y654 phosphorylation. Half-maximal receptor activation occurs at ligand concentrations of approximately 5–10 nM, depending on the FGF isoform and cellular context, reflecting the high-affinity binding (K_d ≈ 10–60 nM for FGF2-FGFR1 interactions).[22][23] Autophosphorylation creates docking sites for adaptor proteins, notably FRS2α, which constitutively associates with the juxtamembrane region of FGFR via its phosphotyrosine-binding domain and becomes phosphorylated at residues like Y463 upon activation. FRS2α then recruits the GRB2-SOS complex, initiating the Ras-MAPK pathway through guanine nucleotide exchange on Ras, followed by sequential activation of Raf, MEK, and ERK kinases to promote cell proliferation. Parallel recruitment of GRB2-GAB1 activates the PI3K-Akt pathway, generating PIP3 to recruit and phosphorylate Akt, which inhibits pro-apoptotic factors like FOXO1 and TSC2 for cell survival. Additionally, phosphorylation of Y766 in FGFR1 directly binds and activates PLCγ, leading to hydrolysis of PIP2 into IP3 and DAG, which mobilize intracellular calcium and activate PKC. Negative regulation attenuates signaling to prevent excessive activation; for instance, Sprouty proteins (e.g., Spry1/2) are transcriptionally induced and inhibit the Ras-MAPK cascade by binding GRB2 or Raf, forming a feedback loop. FGFR-specific phosphatases, such as SHP2, dephosphorylate receptor tyrosines, while MAPK phosphatases (e.g., MKPs) target ERK to terminate the cascade. Cross-talk with other receptor tyrosine kinases enhances pathway integration; notably, FGFR signaling intersects with VEGF receptor (VEGFR) pathways in angiogenesis, where FGF2 upregulates VEGFR2 expression and cooperates to amplify endothelial cell responses via shared PI3K-Akt activation.[24]

Downstream Effects

Upon activation of fibroblast growth factor receptors (FGFRs), the downstream signaling primarily engages the MAPK/ERK pathway, leading to the phosphorylation and activation of ETS transcription factors such as Etv4 (Pea3) and Etv5 (Erm). These factors drive the expression of specific transcriptional targets that regulate cellular responses. For instance, FGF signaling upregulates Dusp6 (dual specificity phosphatase 6, also known as MKP3) and Spry1 (sprouty 1), which function as negative feedback regulators to attenuate ERK activity and prevent excessive signaling.[25][26] This feedback loop ensures temporal control of the response, as demonstrated in studies showing Dusp6 dephosphorylates ERK to limit pathway duration.[27] FGF signaling promotes cell proliferation by inducing the expression of Cyclin D1, a key regulator that facilitates the G1/S phase transition in the cell cycle. This occurs through ERK-mediated activation of transcription factors that enhance CCND1 transcription, driving cyclin-dependent kinase 4/6 (CDK4/6) activity and retinoblastoma protein (Rb) phosphorylation to release E2F transcription factors.[28][29] Additionally, FGF exerts anti-apoptotic effects by modulating Bcl-2 family members, upregulating pro-survival proteins like Bcl-2 and Bcl-xL while suppressing pro-apoptotic ones such as Bax. This modulation, often via ERK and PI3K/Akt pathways, protects cells from stress-induced apoptosis, as evidenced in neuronal and chondrocyte models.[30][31] In endocrine FGFs, such as FGF21, signaling induces metabolic shifts toward enhanced glycolysis, partly through stabilization and activation of hypoxia-inducible factor 1α (HIF-1α), which transcriptionally upregulates glycolytic enzymes like hexokinase and phosphofructokinase.[32] This adaptation supports energy demands in metabolic tissues. Feedback inhibition further refines signaling, with ERK phosphorylating FGFR1 at Ser777 in the C-terminal tail, thereby inhibiting receptor tyrosine phosphorylation and downstream signaling as a negative feedback mechanism.[33] Such mechanisms maintain signaling homeostasis. FGF signaling also influences epigenetic landscapes by promoting histone acetylation, particularly through recruitment of histone acetyltransferases such as p300, which mediates acetylation of H3K27 at target enhancers to facilitate gene activation. This is critical for selective expression of developmental and proliferative genes.[34][35]

Biological Functions

Roles in Development and Growth

Fibroblast growth factors (FGFs) play pivotal roles in embryogenesis and organogenesis, orchestrating key processes such as cell proliferation, migration, and differentiation through paracrine signaling, as classified in their subfamily structures. These factors are essential for establishing body axes, inducing germ layers, and patterning tissues during early vertebrate development. For instance, FGF signaling integrates with pathways like MAPK to regulate spatiotemporal gene expression, ensuring coordinated embryonic growth.[36] During gastrulation and mesoderm induction, FGF4 and FGF8 are critical for primitive streak formation and mesodermal specification. FGF4/8 activate MAPK signaling to promote cell ingression and maintain paraxial mesoderm markers such as MyoD and Myf5, while inhibiting primitive blood formation. In Fgf4^{-/-} mouse embryos, mesoderm maintenance is disrupted, leading to early lethality post-implantation. Similarly, Fgfr1 null mutants exhibit defective cell migration during gastrulation, highlighting the dosage sensitivity of FGF signaling in these early stages.[36][37][38] In neural tube patterning, FGFs establish the anterior-posterior axis via concentration gradients that regulate Hox gene expression in neuroectodermal progenitors. FGF3 and FGF8, expressed in caudal regions, posteriorize the neural tube and support axis extension by maintaining neuromesodermal progenitors. For example, FGF8 acts as a diffusible morphogen from posterior mesoderm, shifting rostrocaudal domains of genes like Hoxb6. Knockout studies show that double Fgf4; Fgf8 conditional mutants in mice result in truncated axes and reduced progenitor markers such as Sox2 and Tbx6. FGF3 null mutants further demonstrate premature neural crest specification and axis truncation due to altered BMP signaling in the neuroepithelium.[39][40][41] FGFs also drive limb bud development through AER-FGF signaling, where FGF8 and FGF10 from the apical ectodermal ridge (AER) and underlying mesenchyme promote proximal-distal outgrowth. FGF8 induces Shh expression in the zone of polarizing activity (ZPA) for anterior-posterior patterning, while FGF10 initiates ectodermal Fgf8 expression in a feedback loop that sustains AER integrity. In Fgf8^{-/-} mice, severe limb defects occur, including loss of distal skeletal elements and truncated appendages, underscoring the precise dosage requirements for limb morphogenesis. AER-specific Fgf8 inactivation at embryonic day 10.5 leads to digit loss and skeletal abnormalities.[42] Angiogenesis during embryogenesis relies on FGF2 to stimulate endothelial cell proliferation and vessel formation. FGF2, expressed in embryonic mesenchyme, promotes vascular endothelial growth by activating FGFR1 and upregulating genes for migration and tube assembly, essential for establishing the embryonic circulatory system.[43] In organogenesis, FGF9 exemplifies stage-specific roles, particularly in lung branching morphogenesis. FGF9 from distal lung mesenchyme signals to epithelial progenitors via FGFRs, driving iterative branching and alveolar specification through epithelial-mesenchymal interactions. Disruptions in FGF9 signaling impair lung bud outgrowth, as seen in mutant models with reduced branching complexity.[44]

Physiological Roles in Adults

In adult organisms, fibroblast growth factors (FGFs) play essential roles in maintaining tissue homeostasis, repair, and adaptive responses through paracrine and endocrine signaling. These functions are distinct from their developmental roles, focusing instead on steady-state regulation and injury response in mature tissues. Key members such as FGF2 and FGF7 contribute to wound healing by stimulating fibroblast migration and collagen synthesis, while endocrine FGFs like FGF19, FGF21, and FGF23 regulate metabolic and mineral balance. FGF2 and FGF7 are critical for wound healing and preventing excessive fibrosis in adult skin and other tissues. FGF2 promotes fibroblast proliferation, migration, and extracellular matrix remodeling by activating RAS/MAPK and PI3K-AKT pathways, which upregulate matrix metalloproteinases (MMP2 and MMP9) to facilitate tissue repair without promoting scar formation.[45] Similarly, FGF7 enhances keratinocyte migration and epithelialization through FGFR2 IIIb activation and AKT-NF-κB signaling, supporting re-epithelialization during cutaneous repair.[45] These actions help maintain skin integrity and limit fibrotic responses in response to minor injuries. Angiogenesis and vascular remodeling in adults rely on FGF1 and FGF2 to sustain endothelial function and vessel stability. FGF1 and FGF2 induce vascular endothelial growth factor (VEGF) expression in endothelial cells, promoting capillary formation and maturation via synergistic interactions with PDGF-BB, which increases pericyte coverage and reduces vascular permeability.[46] This process supports tissue perfusion during physiological adaptations, such as exercise-induced vascular changes, by enhancing arteriogenesis through MCP-1 induction in mesenchymal cells.[46] FGF23 maintains bone remodeling homeostasis by regulating phosphate levels through the kidney. Secreted from osteocytes, FGF23 binds FGFR1/3/4 in complex with klotho on renal proximal tubules, downregulating sodium-phosphate cotransporters (e.g., NPT2a) to promote phosphaturia and prevent hyperphosphatemia, thereby supporting mineral balance for bone maintenance.[47] This endocrine axis coordinates bone-derived signals with renal function to sustain skeletal integrity in adults.[47] Metabolic regulation involves FGF21 and FGF19 in the liver-adipose and enterohepatic axes, respectively. FGF21, primarily produced in the liver during fasting, enhances glucose uptake and lipid oxidation in adipocytes via PPARγ activation and β-Klotho/FGFR1c signaling, improving insulin sensitivity and maintaining energy homeostasis.[48] FGF21 also inhibits hepatic lipogenesis, promoting fatty acid β-oxidation to balance lipid metabolism.[48] Complementing this, FGF19, secreted from ileal enterocytes in response to bile acids via FXR activation, suppresses hepatic CYP7A1 expression through FGFR4/β-Klotho, inhibiting bile acid synthesis and preventing cholestatic overload.[49] FGFs contribute to hematopoiesis in the adult bone marrow niche, particularly during recovery from stress or injury. FGF2 stimulates osteoblastic niche cells via FGFR1, FGFR2, and FGFR3, upregulating CXCL12 (SDF-1), SCF, and Jagged-1 to support HSC self-renewal and mobilization after bone marrow damage. This signaling expands endosteal osteoblasts to aid hematopoietic recovery.[50]

Clinical Significance

Dysregulation in Diseases

Dysregulation of fibroblast growth factors (FGFs) and their receptors (FGFRs) contributes to multiple pathologies through mechanisms such as activating mutations, overexpression, and aberrant signaling activation. In oncology, FGFR3 mutations are prevalent in bladder cancer, affecting approximately 70% of non-muscle-invasive cases and 15% of muscle-invasive cases, where they drive ligand-independent receptor dimerization and oncogenic signaling.[51] Similarly, FGF2 overexpression in gliomas promotes angiogenesis by enhancing endothelial cell proliferation and vascular permeability via heparan sulfate-mediated interactions with FGFRs.[52] The FGF19-FGFR4 axis has emerged as a key driver in hepatocellular carcinoma (HCC), particularly in the 2020s, where amplified signaling fosters tumor proliferation, metastasis, and resistance to therapies like sorafenib through downstream activation of pathways such as STAT3 and ERK.[53][54] Skeletal dysplasias arise from gain-of-function alterations in FGFR3, most notably the p.G380R mutation responsible for over 97% of achondroplasia cases, which constitutively activates the receptor and inhibits chondrocyte differentiation, leading to impaired endochondral ossification and disproportionate short stature.[55] In metabolic disorders, excessive FGF23 production by mesenchymal tumors causes tumor-induced osteomalacia, resulting in severe hypophosphatemia due to enhanced renal phosphate wasting and suppressed 1,25-dihydroxyvitamin D synthesis, often presenting with profound muscle weakness and fractures.[56] Recent studies also implicate FGF21 resistance in obesity, where elevated circulating levels fail to elicit beneficial metabolic responses, exacerbating insulin resistance and adipose tissue dysfunction through impaired FGFR1c-beta-klotho signaling.[57][58] Cardiovascular diseases involve FGF dysregulation in plaque pathogenesis; loss of endothelial FGFR1 signaling triggers endothelial-to-mesenchymal transition, promoting vascular smooth muscle cell accumulation and fibrous cap formation in atherosclerotic lesions.[59] In chronic kidney disease (CKD), progressive elevation of FGF23 precedes phosphate retention and correlates with faster glomerular filtration rate decline, left ventricular hypertrophy, and higher mortality risk, serving as an independent predictor of end-stage renal disease.[60][61]

Therapeutic Applications

Fibroblast growth factors (FGFs) have been harnessed in recombinant forms to promote wound healing, particularly in chronic ulcers. Trafermin, a recombinant human basic FGF (FGF2), was approved in Japan in 2001 for the treatment of pressure ulcers and skin ulcers associated with diabetes or collagen diseases, demonstrating accelerated granulation tissue formation and epithelialization in clinical studies.[62] Becaplermin, a recombinant platelet-derived growth factor (PDGF-BB) gel approved by the FDA in 1997 for diabetic neuropathic foot ulcers, exhibits synergistic effects with FGFs in preclinical models of impaired healing, where combined PDGF and FGF administration reverses deficits in angiogenesis and collagen synthesis in protein-malnourished diabetic mice.[63] These therapies leverage FGFs' roles in stimulating fibroblast proliferation and vascularization to enhance tissue repair in non-healing wounds.[64] In oncology, inhibitors targeting FGF receptors (FGFRs) have emerged as precision therapies for cancers driven by FGFR alterations. Erdafitinib, a selective FGFR tyrosine kinase inhibitor, received FDA accelerated approval in 2019 for adults with locally advanced or metastatic urothelial carcinoma harboring susceptible FGFR3 or FGFR2 alterations who progressed after platinum-containing chemotherapy, showing an objective response rate of 40% in the pivotal phase II trial.[65] Futibatinib, another irreversible FGFR inhibitor, was granted FDA accelerated approval in 2022 for previously treated, unresectable, locally advanced, or metastatic intrahepatic cholangiocarcinoma with FGFR2 fusions or other rearrangements, based on a 42% objective response rate and median duration of 9.7 months in the FOENIX-CCA2 phase II study.[66] These approvals highlight FGFR inhibitors' efficacy in FGFR-altered tumors, though responses are limited to genetically selected patients.[67] Targeting dysregulated FGF signaling has also yielded therapies for metabolic bone disorders. Burosumab, a fully human monoclonal antibody against FGF23, was approved by the FDA in 2018 for the treatment of X-linked hypophosphatemia (XLH) in adults and children aged 1 year and older, neutralizing excess FGF23 to normalize phosphate levels and improve rickets severity, as evidenced by increased serum phosphorus and healing of rachitic lesions in phase II/III trials.[68] This approach addresses FGF23-mediated renal phosphate wasting, offering a targeted alternative to traditional phosphate supplementation.[69] Gene therapy strategies delivering FGFs aim to induce therapeutic angiogenesis in ischemic conditions. NV1FGF, a non-viral plasmid encoding acidic FGF (FGF1), has been evaluated in phase II trials for critical limb ischemia, showing improved amputation-free survival rates compared to placebo in patients unsuitable for revascularization, with intramuscular injections promoting collateral vessel formation without significant safety concerns.[70] Although a subsequent phase III trial (TAMARIS) in 2011 did not meet its primary endpoint for reducing amputation or death, earlier phase II data supported its potential in peripheral artery disease.[71] Despite these advances, therapeutic applications of FGF modulators face significant challenges, including off-target effects and acquired resistance. FGFR inhibitors often cause hyperphosphatemia, ocular toxicity, and fatigue due to inhibition of wild-type FGFRs in non-tumor tissues, limiting tolerability in long-term use.[16] Resistance frequently arises from secondary FGFR kinase domain mutations, such as the gatekeeper T670M variant, which sterically hinders inhibitor binding and reactivates downstream signaling, as observed in post-treatment biopsies from responsive patients.[72] These mechanisms underscore the need for combination strategies or next-generation inhibitors to overcome relapse. As of 2025, the FGF21 analog pipeline shows promise for metabolic disorders like nonalcoholic steatohepatitis (NASH). Pegozafermin (formerly pegbelfermin), a PEGylated FGF21 variant from 89bio, is in phase III trials (e.g., MAESTRO-NASH) for NASH with fibrosis, demonstrating reductions in hepatic fat content by up to 26% and fibrosis improvement without worsening of NASH in phase II studies, with ongoing data supporting progression to advanced stages. In September 2025, Roche acquired 89bio, which will support the continued development of pegozafermin.[73] This reflects broader efforts to exploit FGF21's insulin-sensitizing and anti-fibrotic properties for liver disease therapy.

History and Research Developments

Discovery and Early Studies

The discovery of fibroblast growth factors (FGFs) began in the early 1970s with efforts to identify mitogenic agents in mammalian tissues. In 1973, Hugo A. Armelin reported that extracts from bovine pituitary glands stimulated the growth of 3T3 mouse fibroblasts, marking the initial observation of a potent mitogen later identified as part of the FGF family.[74] This activity was further characterized in 1974 by David Gospodarowicz, who purified a fraction from bovine pituitary that promoted proliferation of fibroblasts and other cell types, establishing the mitogenic properties of what would become known as acidic FGF (FGF1). The isolation of basic FGF (FGF2) followed in 1984, when Peter Bohlen, Andrew Baird, and David Gospodarowicz purified a heparin-binding growth factor from bovine brain and pituitary extracts, noting its strong affinity for heparin-Sepharose chromatography, which facilitated its enrichment. This protein demonstrated enhanced mitogenic and angiogenic effects compared to FGF1, particularly in endothelial cells. During the mid-1980s, molecular cloning efforts advanced the field: cDNA for human FGF1 was isolated from a brain library in 1986, revealing its sequence and chromosomal localization, while bovine FGF2 cDNA was cloned the same year, mapping to chromosome 4q26 in humans. These sequences confirmed the structural similarities between FGF1 and FGF2, both lacking signal peptides yet exhibiting extracellular functions. Early functional studies in the 1980s highlighted FGF2's role in angiogenesis; for instance, in vitro assays using collagen gels demonstrated that FGF2 induced capillary sprouting and tube formation in endothelial cells, providing evidence for its involvement in vascular development. The identification of the first FGF receptor (FGFR), termed flg (now FGFR1), occurred in 1989 when its cDNA was cloned from a human endothelial cell library, revealing it as a transmembrane tyrosine kinase receptor that binds FGFs with high affinity. By the early 1990s, the FGF family expanded with the characterization of FGF3 (previously int-2), an oncogene activated by mouse mammary tumor virus insertions, which shared sequence homology with FGF1 and FGF2, linking it to developmental and tumorigenic processes.[75] These milestones laid the foundation for understanding FGF signaling as a key regulator of cell growth and differentiation.

Recent Advances

The completion of the human genome sequence in 2001 enabled the comprehensive annotation of the fibroblast growth factor (FGF) family, identifying 22 distinct members in humans and highlighting polymorphisms in fibroblast growth factor receptor (FGFR) genes that influence signaling specificity and disease susceptibility.[76] This genomic insight expanded understanding of FGF diversity beyond earlier biochemical isolations, revealing evolutionary conservation and paralogous gene clusters across chromosomes.[77] Advances in structural biology during the 2000s elucidated the molecular basis of FGF-FGFR interactions, with the determination of the crystal structure of a ternary FGF-FGFR-heparin complex in 2000 demonstrating heparin's dual role in facilitating receptor binding and dimerization.[78] Subsequent studies, including analyses of FGFR2 mutations in 2007, further clarified how extracellular and intracellular domain alterations disrupt signaling, providing a framework for interpreting disease-associated variants.[79] The discovery of endocrine FGFs marked a paradigm shift in the 2000s, exemplified by the cloning of FGF23 in 2000 as the causative gene in autosomal dominant hypophosphatemic rickets, which established its role as a hormone regulating phosphate homeostasis. By 2004, targeted ablation experiments confirmed FGF23's central function in suppressing renal phosphate reabsorption and 1,25-dihydroxyvitamin D synthesis, integrating bone-kidney axes in mineral metabolism. In the 2010s, CRISPR/Cas9 genome editing facilitated precise FGF knockout models, with FGF21-null mice revealing its protective effects against diet-induced obesity, hepatic steatosis, and insulin resistance through enhanced energy expenditure and lipid metabolism.[80] These studies underscored FGF21's therapeutic potential in metabolic disorders, showing that its absence exacerbates high-fat diet-induced pathologies while highlighting compensatory mechanisms in adipose and liver tissues.[81] Single-cell RNA sequencing in the 2020s has uncovered spatial and functional heterogeneity in FGF expression within tumor microenvironments, particularly among cancer-associated fibroblasts in breast and prostate cancers as of 2023.[82] These analyses identified distinct FGF ligand subtypes driving pro-tumorigenic inflammation and extracellular matrix remodeling, with implications for subtype-specific targeting in heterogeneous solid tumors.[83] Recent innovations in 2024-2025 have leveraged artificial intelligence for virtual screening in drug design, enabling the identification of potent, selective FGFR1 inhibitors with nanomolar affinity and reduced off-target effects from millions of compounds.[84] Concurrently, phase III trials of FGFR inhibitors, such as the ongoing FIRST-308 trial of tinengotinib in FGFR-altered cholangiocarcinoma (as of 2025), aim to demonstrate improved progression-free survival over standard chemotherapy in refractory cases.[85]

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