Definition
Protein tyrosine kinases (PTKs) are enzymes that catalyze the phosphorylation of tyrosyl residues. They are important in physiological and pathophysiological processes.
Types of PTKs:
Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs. Receptor tyrosine kinases (RTKs) are transmembrane glycoproteins that are activated by the binding of their cognate 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 1. The RTK family includes the receptors for insulin and for many growth factors, such as epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and nerve growth factor (NGF).
In addition to the RTKs, there exists a large family of nonreceptor tyrosine kinases (NRTKs), which includes Src, the Janus kinases (Jaks), and Abl, among others. The NRTKs are integral components of the signaling cascades triggered by RTKs and by other cell surface receptors such as G protein-coupled receptors and receptors of the immune system 2.
Structural Characteristics
RTKs exist as a single polypeptide chain and are monomeric in the absence of ligand. Exceptions include Met and its family members, which comprise a short a chain disulfide-linked to a membrane-spanning ß chain, and the insulin receptor and its family members, which consist of two extracellular a chains disulfide-linked to two membrane-spanning ß chains. The a chains are also disulfide-linked to one another, forming an a2ß2 heterotetramer. The extracellular portion of RTKs typically contains a diverse array of discrete globular domains such as immunoglobulin (Ig)-like domains, fibronectin type III–like domains, cysteine-rich domains, and EGF-like domains. In contrast, the domain organization in the cytoplasmic portion of RTKs is simpler, consisting of a juxtamembrane region (just after the transmembrane helix), followed by the tyrosine kinase catalytic domain and a carboxy-terminal region. Some receptors, most notably members of the PDGF receptor family, contain a large insertion of ~ 100 residues in the tyrosine kinase domain. The juxtamembrane and carboxy-terminal regions vary in length among RTKs. Along with the tyrosine kinase insert; these regions contain tyrosine residues that are autophosphorylated upon ligand binding 3.
NRTKs lack receptor-like features such as an extracellular ligand-binding domain and a transmembrane-spanning region, and most NRTKs are localized in the cytoplasm. Some NRTKs are anchored to the cell membrane through amino terminal modification, such as myristoylation or palmitoylation. In addition to a tyrosine kinase domain, NRTKs possess domains that mediate protein-protein, protein-lipid, and protein-DNA interactions. The most commonly found protein-protein interaction domains in NRTKs are the Src homology 2 (SH2) and 3(SH3) domains (18). The SH2 domain is a compact domain of ~ 100 residues that binds phosphotyrosine residues in a sequence-specific manner. The smaller SH3 domain (~ 60 residues) binds proline-containing sequences capable of forming a polyproline type II helix. Some NRTKs lack SH2 and SH3 domains but possess subfamily-specific domains used for protein-protein interactions. For example, members of the Jak family contain specific domains that target them to the cytoplasmic portion of cytokine receptors 3.
Mode of Action
Transmembrane receptor PTKs: Autophosphorylation of tyrosine residues on the PTK is triggered by binding of the appropriate extracellular ligand (e.g., growth factor or hormone) to the transmembrane receptor binding site. The binding results in activation of the intrinsic PTK activity of the receptor. This in turn causes additional protein-protein interactions to occur that modulate the activity and location of a variety of intracellular signaling molecules, thereby determining the appropriate physiological response.
Non-PTK receptor: Binding of an extracellular ligand to a non-PTK receptor (e.g., cytokine binding to a cytokine receptor) or activation of the T cell receptor leads to association and activation of a series of intracellular and soluble PTK molecules (e.g., p53/p56lck, p60src, p70zap). Moreover, the balance between protein phosphorylation and dephosphorylation events determines the final response of the cell to the stimulus.
Functions
PTKs acts as a prognostic parameter in breast cancer: Its activity was assayed in cytosolic extracts from normal breast tissue, benign tumors, and 84 T1-T2, N0-N1 M0, breast carcinomas. Normal breast tissue extracts yielded an average value of 1.9 ± 1.1 pmol 32p incorporated/min/mg protein, whereas a mean of 12.5 ± 6.1 was obtained for cancer samples. With a median follow-up of 34 months, in the series of 40 patients classified N-, PTK positive patients presented a significantly smaller 3-year disease free survival than the PTK negative ones. Multivariate analysis shows that PTK activity emerges as a potential prognostic factor in breast cancer (p = 0.02). These preliminary results will be updated on a bigger cohort of patients 4.
Tyrosine kinase inhibition - an approach to drug development: PTKs regulate cell proliferation, cell differentiation, and signaling processes in the cells of the immune system. Uncontrolled signaling from receptor tyrosine kinases and intracellular tyrosine kinases can lead to inflammatory responses and to diseases such as cancer, atherosclerosis, and psoriasis. Thus, inhibitors that block the activity of tyrosine kinases and the signaling pathways they activate may provide a useful basis for drug development 5.
References
1. Schlessinger J, Ullrich A (1992). Growth factor signaling by receptor tyrosine kinases. Neuron, 9(3):383–391.
2. Rodrigues GA, Park M (1994). Oncogenic activation of tyrosine kinases. Curr. Opin. Genet. Dev., 4(1):15-24.
3. Hubbard SR and Till JH (2000). Protein tyrosine kinase structure and function. Annu. Rev. Biochem., 69:373-98.
4. Bolla M, Rostaing-Puissant BR, Chedin M, Souvignet C, Marron-Charriere J, Colonna M, Berland E, Chambaz EM (1993). Protein tyrosine kinase activity as a prognostic parameter in breast cancer, a pilot study. Breast Cancer Research and Treatment, 26(3):283-287.
5. Levitzki A, Gazit A (1995). Tyrosine kinase inhibition: an approach to drug development. Science,267(5205):1782-1788.