Receptor specificity of the fibroblast growth factor family. The complete mammalian FGF family

Abstract

In mammals, fibroblast growth factors (FGFs) are encoded by 22 genes. FGFs bind and activate alternatively spliced forms of four tyrosine kinase FGF receptors (FGFRs 1-4). The spatial and temporal expression patterns of FGFs and FGFRs and the ability of specific ligand-receptor pairs to actively signal are important factors regulating FGF activity in a variety of biological processes. FGF signaling activity is regulated by the binding specificity of ligands and receptors and is modulated by extrinsic cofactors such as heparan sulfate proteoglycans. In previous studies, we have engineered BaF3 cell lines to express the seven principal FGFRs and used these cell lines to determine the receptor binding specificity of FGFs 1-9 by using relative mitogenic activity as the readout. Here we have extended these semiquantitative studies to assess the receptor binding specificity of the remaining FGFs 10-23. This study completes the mitogenesis-based comparison of receptor specificity of the entire FGF family under standard conditions and should help in interpreting and predicting in vivo biological activity.

FIGURE 1. Receptor activation by FGFs 7, 10, and 22

A, FGF22 expression in BHK cells stably transfected with expression plasmid APtag5-FGF22 (BHK/ATtag5-FGF22) or empty vector APtag5 (BHK/APtag5). Individual Zeocin-resistant clones were selected and screened by reverse transcription-PCR for Fgf22 expression. Lane 1, wild-type BHK cells do not express FGF22; lane 2, BHK cells transfected with the APtag5 plasmid do not express FGF22; lane 3, BHK cells transfected with APtag5-FGF22 express high levels of FGF22. B, BaF3 cell mitogenic assay for c-spliced FGFRs. C, BaF3 cell mitogenic assay for b-spliced FGFRs and FGFR4Δ. CM, conditioned medium.

FIGURE 2. Receptor activation by FGFs 8, -17, and -18

A, BaF3 cell mitogenic assay for c-spliced FGFRs. B, BaF3 cell mitogenic assay for b-spliced FGFRs and FGFR4Δ.

FIGURE 3. Receptor activation by FGFs 9, 16, and 20

A, BaF3 cell mitogenic assay for c-spliced FGFRs. B, BaF3 cell mitogenic assay for b-spliced FGFRs and FGFR4 Δ.

FIGURE 4. Receptor activation by FGFs 19, 21, and 23

A, heparin dose response for FGFs 19, 21, and 23 measured on FGFR1c-expressing BaF3 cells. Activity increases with increasing heparin but appears to plateau at concentrations >10 μg/ml. B, FGF1 control activity used to normalize the activity of FGFs 19, 21, and 23. C, BaF3 cell mitogenic assay for c-spliced FGFRs. D, BaF3 cell mitogenic assay for b-spliced FGFRs and FGFR4Δ.

FIGURE 5. Diagram showing relative activity of FGFs grouped by subfamilies

*, data from Ornitz et al. (12); #, not tested.