Decoding the Significance of FGF and FGFR in Cancer Biology

What is FGF？

FGF refers to the fibroblast growth factor (Fibroblast Growth Factor) family. FGFs are a multifunctional class of protein factors that are critical for a variety of cell biological and developmental processes. This protein family includes several different FGF proteins that are ubiquitously found in the human body.

FGF proteins transmit signals primarily by binding to fibroblast growth factor receptors (FGFRs). Once FGFs bind to FGFR, they can activate intracellular signaling pathways, thereby affecting biological functions such as cell proliferation, differentiation, migration, and survival. These processes play key roles in embryonic development, tissue repair, angiogenesis, bone growth, and nervous system development.

FGF/FGFR Family

The human FGFR family encompasses four distinct members: FGFR1–4. These receptors, characterized by their transmembrane nature and tyrosine kinase activity, belong to the immunoglobulin (Ig) superfamily and can be activated by extracellular ligands. Despite being closely related in terms of amino acid sequences, FGFR family members vary in their ligand binding capacity and tissue-specific distribution. An additional family member, FGFR5 (also known as FGFRL1), lacks the tyrosine kinase domain and was initially identified due to its interaction with FGFR-binding ligands. FGFRL1 is believed to function as a decoy receptor, negatively regulating FGFR signaling, thus inhibiting cellular proliferation and promoting differentiation[1].

Structurally, FGF receptors comprise a sizable extracellular ligand-binding domain, a single transmembrane helix, and an intracellular portion housing two split tyrosine kinase domains. The extracellular domain features three immunoglobulin-like (Ig-like) domains, with a crucial linker region between the first and second Ig-loop, harboring a highly conserved sequence rich in glutamate, aspartate, and serine, aptly named the acid-box[2]. The second and third Ig-domains play pivotal roles in FGF binding and the regulation of ligand-binding specificity. In contrast, the first Ig domain and the acid-box are involved in mediating receptor auto-inhibition[2].

The specificity of ligand binding within FGFR1–3 primarily relies on alternative splicing of the third Ig-domain, which results in three possible IgIII isoforms. IgIIIa is encoded by exon 7 alone, while IgIIIb and IgIIIc are products of alternative splicing of exon 7 in combination with one of two mutually exclusive exons, either exon 8 or exon 9, respectively[3]. Alternative splicing is absent in FGFR4, which expresses a single isoform closely related to the FGFR-IIIc isoform due to the absence of an alternative exon[4]. Since Ig-loop III resides within the core region of the ligand-binding site, alternative splicing dramatically alters the receptor's ligand spectrum, with the IIIc variant capable of binding a broader range of FGF ligands[5]. This difference in ligand-binding specificity is also associated with variations in tissue distribution, with epithelial tissues predominantly expressing the IIIb isoform and mesenchymal tissues favoring the IIIc isoform[5].

FGFs, the native ligands of FGF receptors, belong to a family comprising 22 members identified by sequence homology. These ligands are grouped into subfamilies based on their function or phylogenetic relationships. Of these, 18 ligands are known to signal through FGF tyrosine kinase receptors. Canonical secreted FGFs belong to subfamilies Fgf1, Fgf4, Fgf7, Fgf8, and Fgf9. They perform typical FGF functions, controlling cell proliferation, differentiation, and survival by binding to and activating FGFRs[4]. Members of the Fgf15/19 subfamily have evolved to serve as endocrine factors, regulating phosphate, bile acid, carbohydrate, and lipid metabolism. The Fgf11 subfamily encodes intracellular FGFs, which do not function as signaling proteins. Canonical FGFs are tightly bound to heparin/heparan sulfate (HS) proteoglycans (HSPGs), which not only limit their diffusion through the extracellular matrix (ECM) but also act as cofactors. Cell surface HPSGs stabilize the FGF ligand–receptor interaction, forming a ternary complex with FGFR that enhances receptor binding and signaling[6]. On the other hand, the endocrine FGFs have lower affinity for heparin/HS and can diffuse away from their origin cells. However, they require proteins from the Klotho family (αKlotho, βKlotho, or KLPH) for high-affinity receptor binding[7].

Upon ligand binding, FGFR undergoes dimerization, leading to a structural change that triggers trans-autophosphorylation of the intracellular kinase domain. This activation, in turn, initiates downstream signal transduction pathways. Phosphorylated tyrosines are recognized and bound by adaptor proteins containing Src homology 2 (SH2) domains or phospho-tyrosine binding (PTB) domains. An example is FGFR substrate 2 (FRS2), an adaptor protein specific to FGFR, which, upon phosphorylation, recruits other adaptor proteins (SOS, GRB2), ultimately activating the RAS and RAF/MAPK pathway, primarily responsible for cellular proliferation. Additionally, GRB2 can also bind to GAB1 and activate the PI3K/AKT pathway, promoting cell survival. Furthermore, direct phosphorylation by activated FGFR tyrosine kinase activates other pathways, including the signal transducers and activator of transcription (STAT) pathway or phospholipase Cγ (PLCγ). The latter leads to the production of phosphatidylinositol-3,4,5-triphosphate (PIP3) and diacylglycerol (DAG), activating the downstream protein kinase C (PKC) signaling[8].

Dysregulation of FGF signaling can result from mutations that drive ligand-independent receptor signaling or alterations that support ligand-dependent receptor activation. Such dysregulation is implicated in tumor development and progression or resistance to therapies in various tumor types.

Enhanced FGF signaling in tumors can be attributed to receptor amplification, gene fusion, mutation, or ligand-dependent activation through autocrine or paracrine signaling. For instance, FGFR1 amplification is observed in less than 20% of squamous non-small-cell lung carcinoma (NSCLC) and breast cancer cases. In contrast, somatic activating mutations are more common in FGFR2 and FGFR3, with FGFR2 mutations occurring in 10–12% of endometrial carcinomas and approximately 4% of NSCLCs and gastric cancers. FGFR3 mutations are highly recurrent in urothelial carcinomas. Additionally, oncogenic gene fusions have been frequently identified in several cancers. For instance, FGFR3 fusions are relatively common in glioblastoma and bladder cancer, while FGFR2 chimeric transcripts are prevalent in lung squamous cell carcinomas and intrahepatic cholangiocarcinomas[9]. Numerous studies also suggest that aberrant FGF and/or FGFR signaling has a multifaceted impact on tumor cells and the surrounding stroma, encompassing induction of proliferation, resistance to cell death, increased motility and invasiveness, enhanced metastasis, and resistance to chemotherapy[10].

A Pharmacological Overview on the Anti-FGFR Agents

FGF, a growth protein secreted by fibroblasts and stored near the basal membrane of endothelial cells, plays a pivotal role in endothelial cell proliferation and differentiation—critical processes in embryonic development, wound healing, and intra-tumoral angiogenesis. In the realm of cancer treatment, the modulation of endothelial biology and angiogenesis has become common practice, primarily with the use of vascular endothelial growth factor blockers (VEGFs). Despite their success in managing gastrointestinal malignancies, results in gastric cancer (GC) patients have fallen short of expectations[12]. As a result, the tumor microenvironment has garnered increasing attention as an active player in tumorigenesis and metastasis, leading to the identification of new molecules as potential targets for novel drugs. Notably, the interaction between FGF-2 and FGFR-2 has emerged as a significant contributor to angiogenesis and proliferation, particularly in GC[12].

Against this backdrop, several FGFR inhibitors are in development. Tyrosine kinase inhibitors (TKIs) stand out as the most prevalent FGFR antagonists, especially for cancers that exhibit intrinsic resistance to chemotherapy and targeted therapies[13]. The initial generation of TKIs was characterized by their multi-target inhibition, which extended to all four main isoforms of FGFR and other signaling proteins within the tumor microenvironment, including VEGFR, KIT, and RET. However, the broad inhibition profile was associated with severe adverse effects, limiting their clinical utility. Subsequent advancements in molecular techniques for designing target-specific molecules and the meticulous selection of patients have unveiled the potential benefits of FGFR blockade, particularly in cancers marked by poor survival, such as cholangiocarcinoma. Recent research has shown that 76% of patients with FGFR-driven tumors would be deemed ineligible for targeted therapy under current approved indications, encompassing 15 different tumor types potentially responsive to TKIs.

Among the newer agents, erdafitinib and pemigatinib have secured accelerated regulatory approval. They exert their effects by inhibiting FGFR phosphorylation and blocking signaling, contingent upon the presence of FGFR alterations[14]. Erdafitinib, a small, orally active TKI, targets FGFR1–4. In vivo studies have demonstrated its potency and selectivity as a pan-FGFR inhibitor, inhibiting downstream signaling and exerting potent anti-proliferative activity. Its intracellular lysosomal localization results in sustained pathway inhibition, with Growth Inhibition 50% (GI50 or IC50) values of 1.2, 2.5, 3.0, and 5.7 nM/L for FGFR1-4, respectively. These results led to accelerated approval for advanced urothelial carcinoma cases exhibiting FGFR2 and FGFR3 genetic alterations[15].

Pemigatinib is another orally active agent that targets FGFR1, 2, and 3, with IC50 values of less than 2 nM. While pemigatinib also inhibits FGFR4 in vitro, it does so at significantly higher concentrations than those required for FGFR1, 2, and 3 inhibition[17]. Following the promising results of the Fight-202 trial, pemigatinib received accelerated approval from the U.S. Food and Drug Administration (FDA) and conditional marketing authorization from the European Medicines Agency (EMA) for cholangiocarcinoma characterized by FGFR2 rearrangements or fusions.

In addition to TKIs, monoclonal antibodies have emerged as a significant advancement in the design of protocols for advanced esophagogastric and GC treatment. Bemarituzumab, an IgG1 antibody targeting the ligand-binding domain of FGFR2b, hinders ligand-dependent activation of FGFR2b by disrupting its union with FGF. Additionally, it facilitates antibody-dependent cytotoxicity[18].

FGF and FGFR Protein

Recombinant Human FGF1 Protein

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Recombinant Human FGFR1 Protein

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References:

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