Protein-tyrosine kinases (PTKs) catalyze the transfer of the γ-phosphate of ATP to tyrosine residues of protein substrates, are critical components of signaling pathways that control cellular proliferation and differentiation. Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs.
The RTK family includes the receptors for insulin and for many growth factors, such as EGF, FGF, PDGF, VEGF, and NGF. RTKs are transmembrane glycoproteins that are activated by the binding of their ligands, and they transduce the extracellular signal to the cytoplasm by phosphorylating tyrosine residues on the receptors themselves (autophosphorylation) and on downstream signaling proteins. RTKs activate numerous signaling pathways within cells, leading to cell proliferation, differentiation, migration, or metabolic changes. In addition, nonreceptor tyrosine kinases (NRTKs), which include Src, JAKs, and Abl, among others, are integral components of the signaling cascades triggered by RTKs and by other cell surface receptors such as GPCRs and receptors of the immune system. NRTKs are critical components in the regulation of the immune system.
RTKs and NRTKs have been implicated in the progression of diseases such as cancer, diabetic retinopathy, atherosclerosis, and psoriasis. Protein kinases, including RTKs, are one of the most frequently mutated gene families implicated in cancer, which has prompted numerous studies on their role in cancer pathogenesis. There are four main mechanisms of RTK dysregulation in human cancers: genomic rearrangements, autocrine activation, overexpression and gain- or loss-of-function mutations. Currently, there are several clinically available small molecule inhibitors and monoclonal antibodies against specific RTKs.