Introduction to the FGF signaling pathway

The fibroblast growth factor (FGF) signaling pathway is one of the most pivotal signal transduction pathways involved in cell fate decisions like proliferation and differentiation. Fibroblast growth factors were originally discovered by Gospodarowicz in 1974, who demonstrated the proteins' ability to drive fibroblast proliferation in cow brain extracts.¹ Following decades of research, FGF proteins are considered key players in embryonic development, tissue homeostasis, angiogenesis and tissue repair, among many other processes.²

Core components of the FGF signaling pathway

Fibroblast Growth Factors (FGFs)

The FGF family contains 23 known and structurally related molecules.³ These can be divided into three categories based on where they act.

1. Paracrine FGFs act locally by binding to the fibroblast growth factor receptors (FGFRs) on cell surfaces. Most FGF subfamilies, such as FGF1, FGF2, FGF4, FGF7 and FGF8, are in this category. Paracrine FGF activity is dependent on heparin and heparan sulfate, components abundant in the extracellular matrix. Therefore, paracrine FGFs are often called heparin-binding FGFs. This binding ensures localized signaling by limiting FGF diffusion rate and stabilizing the FGF-FGFR complex.⁴
2. Endocrine FGFs, such as FGF19, FGF21 and FGF23, have low binding affinity to heparan sulfate and circulate in the body and act on distant tissues.⁵ For instance, FGFs produced in the intestine can migrate to the liver to downregulate the bile acid synthesis pathway.⁶ FGF23, mainly produced in the bone, can bind FGFRs on kidney cells for vitamin D and phosphate regulation.⁷
3. Intracrine FGFs, such as FGF11-14, act intracellularly without secretion and independently of the receptors.⁸

Fibroblast Growth Factor Receptors (FGFRs)

FGFRs are transmembrane receptor tyrosine kinases that bind FGF and initiate signaling. An FGFR receptor contains three extracellular immunoglobulin (Ig)-like domains that bind FGF, a transmembrane domain and an intracellular tyrosine kinase domain that transduces signals by phosphorylating downstream signaling pathway components.² FGFRs act as homodimers or heterodimers, i.e., macromolecular complexes formed by two FGFR molecules.⁹

Although there are four FGFR genes, they can undergo alternate splicing to generate several FGFR isoforms, each with distinct kinase domains and FGF specificities. This mechanism ensures precise regulation of FGF signaling in different cellular contexts.¹⁰

FGFR activation mechanism

FGFR is activated upon an FGF ligand binding to its extracellular Ig-like domain with the aid of heparan sulfate proteoglycans for paracrine FGFs or Klotho co-receptors for endocrine FGFs. This interaction brings two FGFRs into proximity and induces dimerization.²

A dimerized FGFR undergoes trans-autophosphorylation, where each FGFR phosphorylates the kinase domain of the other. Phosphorylated domains act as docking sites that recruit adaptor proteins, such as FGFR substrate 2 (FRS2), phospholipase C gamma (PLCγ) and growth factor receptor-bound protein 2 (Grb2).11 Adaptor proteins relay the signal to RAS/MAPK, PI3K/Akt and PLCγ/Ca2+ pathways, ultimately driving proliferation, differentiation and other cell fate decisions.¹²

FGF signaling mechanisms and downstream pathways

Signal transduction initiation

An activated FGFR contains phosphotyrosine residues on its intracellular domain, which can be coupled to adaptor proteins. The primary FGFR kinase substrate is FRS2α, which is phosphorylated by the FGFR intracellular domain upon recruitment. Subsequently, FRS2α recruits further adaptor proteins to initiate downstream signaling cascades.¹³

Major downstream signaling cascades

RAS/MAPK pathway

The RAS/MAPK pathway is critical to cell proliferation, differentiation and survival, guiding development, homeostasis and regeneration.¹⁴ FGF/FGFR signaling activates RAS/MAPK by a FRS2-dependent mechanism.²

FRS2 phosphorylation triggers the serial recruitment of Grb2 and the guanine nucleotide exchange factor SOS, which activates the RAS GTPase. This initiates the MAPK signaling cascade involving kinases RAF, MEK and ERK, where each kinase propagates the signal by phosphorylating the next one. Finally, the phosphorylated ERK translocates to the nucleus and interacts with transcription factors to promote transcription and inform cell fate decisions.¹⁵

PI3K/AKT pathway

The PI3K/AKT pathway is critical to cell survival. It inhibits apoptosis and promotes growth and metabolic activity, helping cells develop resistance to environmental stressors.¹⁶ For the activation of this pathway, the Grb2 engaged by FRS2 recruits the GRB2-associated-binding-protein 1 (Gab1), which employs the phosphoinositide 3-kinase (PI3K). Then, PI3K phosphorylates a cell membrane phospholipid called PIP2 and converts it to PIP3, which phosphorylates AKT. Finally, active AKT activates downstream proteins that support cell cycle progression and inhibit pro-apoptotic protein activity.¹¹

PLCγ pathway

PLCγ pathway navigates gene expression through calcium-dependent transcription factors and facilitates cytoskeletal remodeling, angiogenesis and cell migration.¹⁷ Unlike the previous two pathways, FGFR directly recruits and phosphorylates PLCγ, which hydrolyzes the membrane lipid PIP2 and induces Ca2+ from the endoplasmic reticulum. Another product of this reaction is diacylglycerol (DAG), which activates protein kinase C (PKC). Increased levels of Ca2+ and PKC activate calcium-dependent proteins and genes.¹⁸

Biological functions of FGF signaling

Due to its connection to several key signal transduction pathways, FGF signaling impacts cell fate decisions underlying development, homeostasis and regeneration.²

Cellular outcomes

FGFs determine phenotypic outcomes by controlling proliferation, differentiation, apoptosis and survival. FGF signaling ensures the timely division of epithelial, quiescent and stem cells while establishing distinct functions in various tissues.² Furthermore, they activate survival pathways, including PI3K/AKT, to protect cells from stress and injury.¹⁹

Developmental processes

The importance of FGF signaling pathways in embryonic development cannot be understated. They guide embryogenesis by mediating gastrulation, neural tube formation, limb bud outgrowth and organ development. Signaling pathways activated by FGF are integral to cell division, migration and specialization during embryonic development. Furthermore, FGF plays a critical role in body plan formation by defining anterior-posterior and dorsal-ventral axes.²⁰

Skeletal development and morphogenesis

FGF signaling is vital in skeletal development, driving chondrogenesis (cartilage formation), osteogenesis (bone formation), and the Wnt and BMP signaling pathways.²¹ FGFs contribute to the proliferation and differentiation of chondrocytes in cartilage, facilitating their transition to osteocytes during endochondral ossification.²² Additionally, FGF ligands provide cues for these cells during limb bud initiation and craniofacial development.²³

Tissue repair and regeneration

Several FGF ligands are upregulated during wound healing and tissue repair. During wound healing, these FGFs stimulate fibroblast proliferation, angiogenesis and epithelial cell migration. Furthermore, FGFs establish tissue homeostasis by maintaining the pluripotency of stem cells and regulating extracellular matrix structure and composition.¹⁹

Angiogenesis

FGF ligands, such as FGF2, are potent pro-angiogenic factors. They coordinate endothelial cell proliferation, adhesion and migration to promote vascular sprouting and control vascular permeability.²⁴ Thus, FGF signaling ensures a sufficient supply of nutrients and oxygen to tissues during organogenesis and wound healing.²⁵ Nevertheless, FGF signaling can be hijacked in cancer to occasion angiogenesis around the tumor site and favor tumor growth.²⁶

Stem cell pluripotency

One key component of tissue homeostasis is stem cell pluripotency, which preserves and controls stem cell self-renewal potential. FGF ligands activate the RAS/MAPK and PI3K/AKT pathways and work in crosstalk with the Activin/nodal and BMP pathways to balance pluripotency and differentiation in the stem cell niche. These pathways could activate transcription factors that convert somatic cells into pluripotent stem cells. From this perspective, FGF ligands are widely used in regenerative medicine applications to generate embryonic or induced pluripotent stem cell cultures.²⁷

Dysregulation and therapeutic targeting

The wide influence of FGF and FGFR signaling on cellular processes means that dysregulations can have far-reaching consequences.

Dysregulation in disease

Cancer is characterized by uncontrolled cell growth, aberrant metabolism, cell survival and migration, due to disruptions in cell cycle regulation. Dysregulated FGF signaling is strongly associated with cancer.

Tumorigenesis mechanisms may involve FGFs, FGFRs and their binding kinetics. FGFR genes are amplified in many cancers, including breast, gastric and bladder cancer.²⁸ Simultaneously, overexpression of both paracrine and endocrine FGFs is observed.²⁹ In addition, mutations to the alternative splicing mechanism can alter the ligand-binding capabilities of FGFRs and prolong their activation.³⁰

Constitutive activation of the FGF downstream pathways, especially MAPK and PI3K/AKT, strongly correlates with tumorigenic outcomes and chemotherapy resistance.³¹,³²

Targeted therapy approaches

Due to the frequency of mutations in the FGF/FGFR axis, FGFRs have become attractive targets for cancer therapy. Research emphasizes the potential of FGFR inhibitors to prevent aberrant FGFR activity from triggering downstream pathways that contribute to tumor growth.³³

There are different types of FGFR inhibitors, each acting via a distinct mechanism.

1. Small-molecule tyrosine kinase inhibitors inhibit FGFR activity by blocking its ATP-binding site. Inhibitors in this category include FDA-approved drugs, such as erdafitinib and pemigatinib, respectively used for the treatment of urothelial carcinoma and bile duct cancer.³⁴
2. Decoy receptors mimic FGFR and bind FGFs to sequester them from the endogenous FGFRs.³⁵
3. Monoclonal antibodies are currently being investigated for their potential to target the FGFR extracellular domain to prevent FGF binding.³⁶

More research must be conducted to optimize specificity, ensuring the continuity of normal FGF signaling for development and homeostasis while abrogating aberrant signaling.

Conclusion

The FGF signaling pathway plays a multifaceted role in regulating key biological processes such as cell proliferation, differentiation, tissue repair and organogenesis. Due to its diverse ligands and the alternative splicing mechanisms of FGFRs, the pathway orchestrates general and tissue-specific functions. Simultaneously, its dysregulation is linked to a broad spectrum of diseases, including cancer and skeletal disorders.

A growing understanding of FGF biology will allow therapeutic exploitation, particularly in targeted cancer therapies and regenerative medicine. Advances in FGFR inhibitors and targeting strategies for other pathway components offer potential for effective clinical interventions.

Nevertheless, the convoluted interaction network surrounding FGF signaling calls for a deeper investigation into the signal transduction dynamics, splicing variations and crosstalk with other pathways. Continued research will refine therapeutic strategies and expand our ability to manipulate FGF signaling for treatment and tissue engineering applications.

References

1. Gospodarowicz D. Localisation of a fibroblast growth factor and its effect alone and with hydrocortisone on 3T3 cell growth. Nature 1974;249(5453):123-127.
2. Xie Y, Su N, Yang J, Tan Q, Huang S, Jin M, et al. FGF/FGFR signaling in health and disease. Signal Transduct Target Ther 2020;5(1):181.
3. Ornitz DM, Itoh N. The fibroblast growth factor signaling pathway. Wiley Interdiscip Rev Dev Biol 2015;4(3):215-266.
4. Li Y, Sun C, Yates EA, Jiang C, Wilkinson MC, Fernig DG. Heparin binding preference and structures in the fibroblast growth factor family parallel their evolutionary diversification. Open Biol 2016;6(3):150275.
5. Phan P, Saikia BB, Sonnaila S, Agrawal S, Alraawi Z, Kumar TKS, et al. The saga of endocrine FGFs. Cells 2021;10(9):2418.
6. Wang LX, Frey MR, Kohli R. The role of FGF19 and MALRD1 in enterohepatic bile acid signaling. Front Endocrinol (Lausanne) 2022;12:799648.
7. Agoro R, Ni P, Noonan ML, White KE. Osteocytic FGF23 and its kidney function. Front Endocrinol (Lausanne) 2020;11:592.
8. Lee KW, An YJ, Lee J, Jung Y-E, Ko IY, Jin J, et al. Expression and purification of intracrine human FGF 11 and study of its FGFR-dependent biological activity. J Microbiol 2022;60(11):1086-1094.
9. Roskoski Jr R. The role of fibroblast growth factor receptor (FGFR) protein-tyrosine kinase inhibitors in the treatment of cancers including those of the urinary bladder. Pharmacol Res 2020;151:104567.
10. Al-Obaidy KI, Cheng L. Fibroblast growth factor receptor (FGFR) gene: pathogenesis and treatment implications in urothelial carcinoma of the bladder. J Clin Pathol 2021;74(8):491-495.
11. Liu Q, Huang J, Yan W, Liu Z, Liu S, Fang W. FGFR families: biological functions and therapeutic interventions in tumors. MedComm 2023;4(5):e367.
12. Ferguson HR, Smith MP, Francavilla C. Fibroblast growth factor receptors (FGFRs) and noncanonical partners in cancer signaling. Cells 2021;10(5):1201.
13. Furusho M, Ishii A, Hebert JM, Bansal R. Developmental stage‐specific role of Frs adapters as mediators of FGF receptor signaling in the oligodendrocyte lineage cells. Glia 2020;68(3):617-630.
14. Seger R. MAPK Signaling Cascades in Human Health and Diseases. MDPI; 2024:11226.
15. Zheng J, Zhang W, Li L, He Y, Wei Y, Dang Y, et al. Signaling pathway and small-molecule drug discovery of FGFR: a comprehensive review. Front Chem 2022;10:860985.
16. Xu J, Li Y, Kang M, Chang C, Wei H, Zhang C, et al. Multiple forms of cell death: A focus on the PI3K/AKT pathway. J Cell Physiol 2023;238(9):2026-2038.
17. Chen D, Simons M. Emerging roles of PLCγ1 in endothelial biology. Sci Signal 2021;14(694):eabc6612.
18. Tomlinson DC, Baxter EW, Loadman PM, Hull MA, Knowles MA. FGFR1-induced epithelial to mesenchymal transition through MAPK/PLCγ/COX-2-mediated mechanisms. PLoS One 2012;7(6):e38972.
19. Farooq M, Khan AW, Kim MS, Choi S. The role of fibroblast growth factor (FGF) signaling in tissue repair and regeneration. Cells 2021;10(11):3242.
20. Kumar V, Goutam RS, Park S, Lee U, Kim J. Functional roles of FGF signaling in early development of vertebrate embryos. Cells 2021;10(8):2148.
21. Toosi S, Behravan J. Osteogenesis and bone remodeling: A focus on growth factors and bioactive peptides. BioFactors 2020;46(3):326-340.
22. Julien A, Perrin S, de Lageneste OD, Carvalho C, Bensidhoum M, Legeai-Mallet L, et al. FGFR3 in periosteal cells drives cartilage-to-bone transformation in bone repair. Stem Cell Rep 2020;15(4):955-967.
23. Pereur R, Dambroise E. Insights into Craniofacial Development and Anomalies: Exploring Fgf Signaling in Zebrafish Models. Curr Osteoporos Rep 2024;22(3):340-352.
24. Jin S, Yang C, Huang J, Liu L, Zhang Y, Li S, et al. Conditioned medium derived from FGF-2-modified GMSCs enhances migration and angiogenesis of human umbilical vein endothelial cells. Stem Cell Res Ther 2020;11:1-12.
25. Wang X, Fan W, Li N, Ma Y, Yao M, Wang G, et al. YY1 lactylation in microglia promotes angiogenesis through transcription activation-mediated upregulation of FGF2. Genome Biol 2023;24(1):87.
26. Ichikawa K, Watanabe Miyano S, Minoshima Y, Matsui J, Funahashi Y. Activated FGF2 signaling pathway in tumor vasculature is essential for acquired resistance to anti-VEGF therapy. Sci Rep 2020;10(1):2939.
27. Mossahebi-Mohammadi M, Quan M, Zhang J-S, Li X. FGF signaling pathway: a key regulator of stem cell pluripotency. Front Cell Dev Biol 2020;8:79.
28. Brown LM, Ekert PG, Fleuren ED. Biological and clinical implications of FGFR aberrations in paediatric and young adult cancers. Oncogene 2023;42(23):1875-1888.
29. Szymczyk J, Sluzalska KD, Materla I, Opalinski L, Otlewski J, Zakrzewska M. FGF/FGFR-dependent molecular mechanisms underlying anti-cancer drug resistance. Cancers (Basel) 2021;13(22):5796.
30. Chioni A-M, Grose RP. Biological significance and targeting of the FGFR axis in cancer. Cancers (Basel) 2021;13(22):5681.
31. Bahar ME, Kim HJ, Kim DR. Targeting the RAS/RAF/MAPK pathway for cancer therapy: from mechanism to clinical studies. Signal Transduct Target Ther 2023;8(1):455.
32. He Y, Sun MM, Zhang GG, Yang J, Chen KS, Xu WW, et al. Targeting PI3K/Akt signal transduction for cancer therapy. Signal Transduct Target Ther 2021;6(1):425.
33. Kommalapati A, Tella SH, Borad M, Javle M, Mahipal A. FGFR inhibitors in oncology: insight on the management of toxicities in clinical practice. Cancers (Basel) 2021;13(12):2968.
34. Weaver A, Bossaer JB. Fibroblast growth factor receptor (FGFR) inhibitors: A review of a novel therapeutic class. J Oncol Pharm Pract 2021;27(3):702-710.
35. An X, Paoloni J, Oh Y, Spangler JB. Engineering growth factor ligands and receptors for therapeutic innovation. Trends Cancer 2024.
36. Du S, Zhang Y, Xu J. Current progress in cancer treatment by targeting FGFR signaling. Cancer Biol Med 2023;20(7):490.

recent-articles