FGF
The fibroblast growth factors (FGF) are a family of cell signalling proteins that are involved in a wide variety of processes, most notably as crucial elements for normal development. Any irregularities in their function lead to a range of developmental defects. These growth factors generally act as systemic or locally circulating molecules of extracellular origin that activate cell surface receptors. A defining property of FGFs is that they bind to heparin and heparan sulfate, thus some of them are found to be sequestered in the extracellular matrix of tissues that contains heparan sulfate proteoglycans and they are released locally upon injury or tissue remodeling.[1]
In humans, 22 members of the FGF family have been identified, all of which are structurally related signaling molecules:[2][3][4]
Members FGF1 through FGF10 all bind fibroblast growth factor receptors (FGFRs). FGF1 is also known as acidic fibroblast growth factor, and FGF2 is also known as basic fibroblast growth factor.
Members FGF11, FGF12, FGF13, and FGF14, also known as FGF homologous factors 1-4 (FHF1-FHF4), have been shown to have distinct functions compared to the FGFs. Although these factors possess remarkably similar sequence homology, they do not bind FGFRs and are involved in intracellular processes unrelated to the FGFs.[5] This group is also known as "iFGF".[6]
Human FGF18 is involved in cell development and morphogenesis in various tissues including cartilage.[7]
Human FGF20 was identified based on its homology to Xenopus FGF-20 (XFGF-20).[8][9]
FGF15 through FGF23 were described later and functions are still being characterized. FGF15 is the mouse ortholog of human FGF19 (there is no human FGF15) and, where their functions are shared, they are often described as FGF15/19.[10] In contrast to the local activity of the other FGFs, FGF15/19, FGF21 and FGF23 have systemic effects.[10][11]
Receptors.
FGF have been alternately referred to as "pluripotent" growth factors and as "promiscuous" growth factors due to their multiple actions on multiple cell types.[14][15] Promiscuous refers to the biochemistry and pharmacology concept of how a variety of molecules can bind to and elicit a response from single receptor. In the case of FGF, four receptor subtypes can be activated by more than twenty different FGF ligands.