Fibroblast growth factors (FGFs) [, ]are a family of multifunctional proteins, often referred to as 'promiscuous growth factors' due to their diverse actions on multiple cell types [, ]. FGFs are mitogens, which stimulate growth or differentiation of cells of mesodermal or neuroectodermal origin. The function of FGFs in developmental processes include mesoderm induction, anterior-posterior patterning, limb development, and neural induction and development. In mature tissues, they are involved in diverse processes including keratinocyte organisation and wound healing [, , , , , ]. FGF involvement is critical during normal development of both vertebrates and invertebrates, and irregularities in their function leads to a range of developmental defects [, , , ]. Fibroblast growth factors are heparin-binding proteins and interactions with cell-surface-associated heparan sulfate proteoglycans have been shown to be essential for FGF signal transduction. FGFs have internal pseudo-threefold symmetry (β-trefoil topology) []. There are currently over 20 different FGF family members that have been identified in mammals, all of which are structurally related signaling molecules [, ]. They exert their effects through four distinct membrane fibroblast growth factor receptors (FGFRs), FGFR1 to FGFR4 [], which belong to the tyrosine kinase superfamily. Upon binding to FGF, the receptors dimerize and their intracellular tyrosine kinase domains become active [].This entry represents fibroblast growth factor 21 (FGF21), which stimulates glucose uptake in differentiated adipocytes via the induction of glucose transporter SLC2A1/GLUT1 expression []. FGF21 has been shown to protect animals from diet-induced obesity when overexpressed in transgenic mice. It also lowers blood glucose and triglyceride levels when administered to diabetic rodents [], suggesting it may exhibit the therapeutic characteristics necessary for effective treatment of diabetes. Treatment of animals with FGF21 results in increased energy expenditure, fat utilisation and lipid excretion []. FGF21 is most abundantly expressed in the liver, and also expressed in the thymus at lower levels [].
cAMP-responsive element-binding protein 3-like protein 3 (also known as CREBH protein) belongs to the OASIS family, which is a group of ER stress transducers, including OASIS, BBF2H7, CREBH, CREB4 and Luman []. Mammalian CREBH protein plays a role in activating expression of inflammatory response in the liver []. This protein also regulates FGF21 transcription [].Members of the OASIS family are transcription factors that transduce signals from the ER to the cytoplasm and nucleus when unfolded proteins are accumulated in the ER. They contains a basic leucine zipper (bZIP) domain that has regulatory function and a transmembrane domain that associates with the ER. Under ER stress the protein is cleaved at the membrane, and their cleaved N-terminal cytoplasmic domain (contains the bZIP domain) translocates into the nucleus where it activates the transcription of target genes that are mediated by ER stress-responsive and cyclic AMP-responsive elements [].
This entry includes fibroblast growth factor 15/19/21 (FGF15/19/21).Fibroblast growth factor 15 (FGF15) plays a key role in enterohepatic signaling, regulation of liver bile acid biosynthesis, gallbladder motility and metabolic homeostasis [, , ]. Mouse FGF15 has been shown to be stimulated when bile acids bind to farnesoid X receptor (FXR) [], and is therefore thought to a factor in chronic bile acid diarrhoea and in certain metabolic disorders [].FGF15 has been experimentally characterised in mouse, but has not been found in other species. However, there is an orthologous human protein, FGF19, and together they share about 50% amino acid identity and display similar endocrine functions, so are often referred to as FGF15/19 [, ]. FGF15 and FGF19 differ from other FGFs due to subtle changes in their tertiary structure, they have low heparin binding affinity enabling them to diffuse away from their site of secretion and signal to distant cells. FGF signaling through the FGF receptors is also different, as they require klotho protein cofactors rather than heparin sulfate proteoglycan [].Fibroblast growth factor 19 (FGF19) plays a key role in enterohepatic signaling, regulation of liver bile acid biosynthesis, gallbladder motility and metabolic homeostasis [, , ]. Human FGF19 expression has been shown to be stimulated approximately 300-fold by physiological concentrations of bile acids including chenodeoxycholic acid, glycochenodeoxycholic acid and obeticholic acid in explants of ileal mucosa []. The protein is thought to be a factor in chronic bile acid diarrhoea and in certain metabolic disorders [, ]. FGF19 has been experimentally characterised in humans and other species, but has not been found in mouse. However there is an orthologous mouse protein, FGF15, and together they share about 50% amino acid identity and display similar endocrine functions, so are often referred to as FGF15/19 [, ]. FGF15 and FGF19 differ from other FGFs due to subtle changes in their tertiary structure. They have low heparin binding affinity, enabling them to diffuse away from their site of secretion and signal to distant cells. FGF signaling through the FGF receptors is also different, as they require klotho protein cofactors rather than heparin sulfate proteoglycan []. Unlike other members of the family that can bind several FGF receptors, FGF19 is specific for FGFR4 [].FGF21 stimulates glucose uptake in differentiated adipocytes via the induction of glucose transporter SLC2A1/GLUT1 expression []. FGF21 has been shown to protect animals from diet-induced obesity when overexpressed in transgenic mice. It also lowers blood glucoseand triglyceride levels when administered to diabetic rodents [], suggesting it may exhibit the therapeutic characteristics necessary for effective treatment of diabetes. Treatment of animals with FGF21 results in increased energy expenditure, fat utilisation and lipid excretion []. FGF21 is most abundantly expressed in the liver, and also expressed in the thymus at lower levels [].
