Growth factors, which generally considered as a subset of cytokines, refer to the diffusible signaling proteins that stimulate cell growth, differentiation, survival, inflammation, and tissue repair. They are classified into different families on the basis of their target cells, functions, structures and molecular evolution, such as Fibroblast growth factor (FGF) family, Vascular endothelial growth factor (VEGF) family, Wnt family, Epidermal growth factor (EGF) family, Platelet-derived growth factor (PDGF), Insulin-like Growth Factor (IGF) family, Transforming Growth Factor-beta (TGF-β) family.
Growth factor receptors are transmembrane proteins which bind to specific growth factors and transmit the instructions conveyed by the factors to intracellular space. The growth factor receptors on cell surface are very common, and cells mainly contains receptors for several growth factors. When a growth factor and its receptor bind together, the receptor can either transiently exert its kinase activity or form a complex with an intracellular tyrosine or serine/threonine kinase. Subsequently, the activated receptors result in the activation of other proteins in the signaling pathway and the production of various second messengers. These signals are finally transmitted into the nucleus and induce the expression of specific genes.