Mapping Ligand Binding Domains in Chimeric Fibroblast Growth Factor Receptor Molecules

Abstract

Fibroblast growth factors (FGFs) 1 are essential signaling molecules that regulate embryonic growth, development, cell proliferation, differentiation, and angiogenesis (1). At least 19 related members of the FGF family have been identified thus far, and all but four are known to activate one of four high affinity cell surface FGF receptors (FGFR) (2-9).2 The FGFRs consist of an extracellular region containing three immunoglobulin-like (Ig) domains, a stretch of seven conserved acidic amino acids, and a heparin binding domain (10, 11). The FGFRs share an overall 48 -69% amino acid identity (pairwise comparison of the "c" splice form of mouse FGFRs); however, certain sub-regions are nearly 100% conserved. Individual FGFRs are activated by a subset of ligands, and alternative splicing in Ig domain III can dramatically change the specificity for certain .FGFs range in molecular mass from 18 to 29 kDa and show 13-71% amino acid identity (pairwise comparisons of mouse or rat FGFs). Most members of the family share 28 highly conserved amino acid residues, and all FGFs have six identical positions. All members of the FGF family have a high affinity for heparin. Significantly, heparin or heparan sulfate can enhance the high affinity binding of FGFs to their receptors by forming a trimolecular complex with FGF and the FGFR and by decreasing the rate of ligand dissociation (22-28). The formation of this complex results in receptor dimerization, transphosphorylation, and downstream signaling.During development, FGFs are involved in several and sometimes opposing functions requiring tight regulation of their activity and specificity. This may be achieved by regulating spatial and temporal expression of FGFs and FGFRs and by regulating binding specificity (21, 29 -34). A major determinant of ligand binding specificity is alternative splicing in the C terminus of Ig domain III of FGFRs 1-3. This splicing event is tissue-specific and is likely to regulate important signaling events across epithelial/mesenchymal boundaries (35-38). The "b" (epithelial) splice form of FGFR2 (FGFR2b) can be activated by FGF7 and FGF10, ligands produced in mesenchymal tissue. These ligands show no activity toward c (mesenchymal) spliced receptors. Conversely, FGF8 activates c-spliced FGFR2 (FGFR2c) but shows no activity toward FGFR2b (21, 39).2 Notably, FGF8 expression is often restricted to epithelial tissue such as the apical ectodermal ridge of the developing limb bud (40, 41).The C-terminal region of Ig domain III is clearly important for ligand binding and shows specificity toward different ligands. For example, specific mutations in this region in FGFR2 can decrease the binding of FGF2 without affecting the binding of FGF1 or FGF7 (42). The b splice form of FGFR3 (FGFR3b) also has unique properties in that it can only be activated by FGF1, which shows little specificity toward any receptor, and FGF9, which shows no activity toward FGFR1b and FGFR2b (21,43,44). Alternative splicing of Ig domain III can also lead to truncation of the transme...

Novel cross talk between MEK and S6K2 in FGF-2 induced proliferation of SCLC cells

“…FGFs elicit their biological activities by binding to high a nity FGF receptor (FGFR) tyrosine kinases in the presence of heparin or heparan sulfate. There are four FGFRs (FGFR1 ± 4), with multiple additional isoforms due to alternative mRNA splicing, but only FGFR1 and FGFR2 isoforms appear to have high a nity for FGF-2 (Chellaiah et al, 1994;Mansukhani et al, 1992;Ornitz et al, 1996;Ron et al, 1993;Werner et al, 1992). FGFR1 has often been used as a model to understand the activation of FGFRs as well as the early signalling events induced by FGF-2.…”

Reciprocal relationship in gene expression between FGFR1 and FGFR3: implication for tumorigenesis

“…Therefore, FGFR1 may confer a selectable advantage on clones of cells in colorectal tumorigenesis, favoring proliferation, whereas FGFR3 may have the effect of an unfavorable negative regulation of progression of the carcinomas to malignancy, probably promoting differentiation. In support, several studies using BaF3 cells system have indicated that FGFR1 is much better at producing mitogenic signals than FGFR3 (Chellaiah et al, 1994;Wang et al, 1994;Ornitz et al, 1996).…”

Increased expression of fibroblast growth factor 8 in human breast cancer

“…One kind of splice variant involves the use of alternative exons encoding the carboxyl-terminal half of the third immunoglobulin domain. This form of splice variation occurs for FGFR1, FGFR2 and FGFR3 so that IIIb and IIIc forms of these receptors have very di erent ligand binding properties (Miki et al, 1992;Werner et al, 1992;Chellaiah et al, 1994). These two isoforms appear to be expressed in a mutually exclusive fashion with cells of mesenchymal origin expressing IIIc variants of FGFR2 and FGFR3 whereas epithelial cells express the IIIb isoform (Pekonen et al, 1993;Savagner et al, 1994).…”