Growth factors promote cell growth by acting on-

Growth factors can act on specific cell surface receptors that subsequently transmit their growth signals to other intracellular components and eventually result in altered gene expression. The general process of transmitting an external molecular signal to a cell to evoke a cellular response is called signal transduction . The receptor binding site for peptide/protein growth factors is on the outer cell membrane surface, in other words, the extracellular domain. Most cell surface receptors for growth factors show tyrosine kinase activity, meaning they put a phosphate group on a downstream protein tyrosine residue. An exception is the surface receptors for the transforming growth factor-beta (TGF-beta) family of cytokines. When activated by the binding of TGF-beta cytokines, this receptor can phosphorylate downstream proteins on serine and threonine residues. "Downstream" here means an event triggered after TGF-beta binding to its receptor. Some growth factors are small peptides called cytokines. While all cytokines affect signal transduction pathways, only those cytokines affecting cell growth/differentiation signaling pathways are considered growth factors. Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an example of a cytokine growth factor since it promotes the production of white blood cells by stem cells. Examples of protein growth factors are vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Ref - Harrison 17/e p 2190 , <a href="