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Fibroblast growth factors (FGFs) comprise a large family of signaling polypeptides. In the adult organism, FGFs are homeostatic factors and function in tissue repair and response to injury. In the embryo, FGFs regulate cell proliferation, migration, differentiation and survival. When inappropriately expressed, some FGFs can contribute to developmental defects and to the pathogenesis of cancer and other diseases. Mutations in FGF receptor tyrosine kinases (FGFRs) in humans causes skeletal diseases such as Achondroplasia, the most common form of dwarfism in humans, and Craniosynostosis syndromes, a cluster of diseases that involves premature fusion of cranial bones. In vertebrates, 22 Fgf genes encode polypeptides that range in molecular mass from 17 to 34 kDa. Eighteen of the 22 FGFs bind and activate a subset of seven distinct high affinity FGFRs that are derived by alternative splicing of four genes. Affinity and specificity of FGFs for their receptors is regulated by alternative mRNA splicing in three of the four known Fgfr tyrosine kinase genes. FGF activity and specificity is further regulated by heparin sulfate proteoglycans, which serve as co-receptors for FGFs.

FGF receptors (FGFR) contain three extracellular immunoglobulin-like (Ig) domains and a heparin binding sequence. Alternative mRNA splicing in the carboxy-terminal half of Ig-domain III creates b and c isoforms of Fgfrs 1-3Fgfr alternative splicing is regulated in a tissue-specific manner and dramatically affects ligand-receptor binding specificity. The b splice form is utilized in epithelial lineages and the c splice is utilized in mesenchymal lineages. Ligands specific for these receptor splice forms are expressed in adjacent tissues resulting in directional epithelial-mesenchymal signaling. For example, FGFR2b is expressed in many epithelial tissues and can be activated by FGF7, FGF10 and FGF22, ligands produced in mesenchymal tissue. These ligands show no activity towards mesenchymally expressed FGFR2c. Conversely, ligands such as FGFs 4, 8 and 9 are expressed in epithelial-like tissue and activate c splice forms of Fgfrs.

An important feature of FGF biology involves the interaction between FGF and heparin and heparan sulfate proteoglycans (HSPG). Heparin and heparan sulfate stabilizes FGFs to thermal denaturation and proteolysis, limits the diffusion and release of FGFs into interstitial spaces, and regulates the binding affinity and specificity of FGFs for their receptors. Heparin or heparan sulfate is required for FGF to effectively activate an FGFR in cells that are deficient in or unable to synthesize HSPG or in cells pretreated with heparin degrading enzymes or inhibitors of sulfation. Mutations in enzymes involved in heparan biosynthesis also affect FGF signaling pathways during development.

FGFRs are receptor tyrosine kinases that are activated by ligand-induced dimerization and subsequent auto-phosphorylation. FGFs bind to FGFRs in a 1:1 complex that is facilitated by heparin/heparan sulfate, which makes numerous contacts with both the FGF and FGFR molecule. Heparin/heparan sulfate also interacts with an adjoining FGFR to promote FGFR dimerization. The activated FGFR interacts with intracellular signaling molecules allowing it to couple to several signal transduction pathways. The two primary pathways activated by the FGF receptor are the ras-raf-map kinase pathway and the phospholipase C-gamma (PLC-γ) pathway. An adapter protein, FRS2, couples the FGFR to MAP kinase and phosphatidylinositol-3 (PI-3) kinase activation, chemotactic response, and cell proliferation. The mitogenic response to FGF is likely to require activation of map kinase and possibly p38; however, the ability of FGFs to regulate differentiation and embryonic patterning may involve the ras, PLC-γ or novel pathways. For example, the anti-proliferative effects of FGF on chondrocytes are likely to be mediated through STAT1. Feedback inhibition of FRS2 is regulated by MAP kinase phosphorylation of multiple threonine residues in response to FGF, insulin, EGF, and PDGF extracellular signals. Differential signaling in diverse cell types is also a consequence of sequence differences between the four FGF receptors and possible alternative splicing within the cytoplasmic domain.

Two main classes of small molecule inhibitors have been identified. The SU compounds contain a substituted oxindole core (indolinone) while the PD compounds have a substituted pyrido[2,3-d]pyrimidine core. Crystallographic studies show that both SU5402 and PD173074 bind in the ATP-binding cleft of the FGFR1 tyrosine kinase domain between the two lobes of the kinase. It is likely that these molecules inhibit all FGFRs.

The Table below contains accepted modulators and additional information. For a list of additional products, see the Materials section below.

Abbreviations

PD161570: 1-Tert-butyl-3-[6-(2,6-dichloro-phenyl)-2-(4-diethylamino-butylamino)-pyrido[2,3-d]pyrimidin-7-yl]urea
PD166285: 6-(2, 6-dichlorophenyl)-2-[[4-[2-(diethylamino)ethoxy]phenyl]amino]-8-methyl-Pyrido[2, 3-d]pyrimidin-7(8H)-one dihydrochloride
PD166866: 1-[2-Amino-6-(3,5-dimethyoxy-phenyl)-pyrido[2,3-d]pyrimidin-7-yl]-3-tert-butyl-urea
PD173074: N-[2-[[4-(diethylamino)butyl]amino-6-(3, 5-dimethoxyphenyl)pyrido[2, 3-d]pyrimidin-7-yl]-N'-(1,1-dimethylethyl)-urea
SU5402: 3-[4-Methyl-2-(2-oxo-1,2-dihydro-indol-3-ylidenemethyl)-1H-pyrrol-3-yl]-propionic acid

Materials
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References

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