Protein Tyrosine Kinase (PTK) Inhibitors
Phosphorylation of tyrosine residues modulate enzymatic activity and create binding sites for recruitment of downstream signaling proteins. Protein tyrosine kinases (PTKs) play a key role in the regulation of cell proliferation, differentiation, metabolism, migration, and survival. PTKs catalyze the transfer of γ-phosphoryl groups from ATP to tyrosine hydroxyls of proteins. They are classified as receptor PTKs and non-receptor PTKs. Receptor PTKs contain a single polypeptide chain with a transmembrane segment. The extracellular end of this segment contains a high affinity ligand-binding domain, while the cytoplasmic end comprises the catalytic core and the regulatory sequences. The cytosolic end also contains tyrosine residues, which become substrates or targets for the tyrosine kinase portion of the receptor. PTK remains inactive until a ligand binds to the receptor, which leads to the dimerization of two ligand-bound receptors (exception: the tetrametric insulin receptor). Once activated, receptors are able to autophosphorylate tyrosine residues outside the catalytic domain. This stabilizes the active receptor conformation and creates phosphotyrosine-docking sites for proteins that transduce signals within the cell. The cytosolic portion of the phosphorylated receptor recruits a number of cytosolic adapter proteins via interactions between phosphorylated tyrosine residues on the receptor and the SH2 domain on the adapter molecule. Different proteins have different SH2 domains that recognize specific phosphotyrosine residues. An SH2-containing protein, Grb2, acts as a common adapter protein in a majority of growth factor related signaling events.
Grb2 binding to phosphotyrosine residues changes its conformation and allows it to bind to proline-rich sequences in the carboxy terminal tail of Sos, a GDP-GTP exchange protein. This binding displaces an inhibitory domain in Sos and allows the activation of Sos, which then translocates to the plasma membrane to cause an exchange of GDP for GTP and activates Ras. A wide variety of effectors of Ras activation have been reported; however, activation of Raf, a cytoplasmic protein kinase, is one of the best studied examples. Ras binds to the N-terminus of Raf and recruits it to the inner surface of the plasma membrane, where it is phosphorylated by protein kinase C. Translocation of Raf to the membrane positions it in direct proximity to MAP kinase kinase (MEK). Raf phosphorylates MEK, which in turn phosphorylates MAP kinase (MAPK). In a resting cell, MAPK remains inactive because its phosphorylation lip excludes ATP access to the binding pocket. However, MEK binding destabilizes the lip and exposes the buried tyrosine residues. Phosphorylation of the exposed tyrosine and the nearby threonine residues cause the lip to alter its conformation allowing ATP binding.
Non-receptor tyrosine kinases include members of the Src, Tec, JAK, Fes, Abl, FAK, Csk, and Syk families. They are located in the cytoplasm as well as in the nucleus. They are activated by a large number of stimuli including hormones, neurotransmitters, growth factors, and cytokines. They exhibit distinct kinase regulation, substrate phosphorylation, and function. Deregulation of these kinases has also been linked to several human diseases. In most cases, their activation also begins with the phosphorylation of a tyrosine residue present in an activation loop. The best studied enzymes in this group include Src kinases. Src is believed to be negatively regulated by phosphorylation at Tyr527 present at the C-terminus by Csk and other cellular kinases. The enzyme assumes an inactive conformation when this phosphotyrosine is bound by the Src SH2 domain in an intramolecular fashion. In this structure, the Src SH3 domain interacts with a single proline, Pro250, in the linker region between the SH2 and catalytic domain. In contrast to Src, c-Abl kinase activity is stimulated by phosphorylation of a catalytic domain tyrosine residue, Tyr412, either via autophosphorylation or transphosphorylation by c-Src. Recent studies have indicated that dimerization or oligomerization of c-Abl might also be sufficient to activate Abl kinase activity in vivo. Due to their involvement in various forms of cancers, PTKs have become prominent targets for therapeutic intervention. Selective receptor and non-receptor PTK inhibitors represent a promising class of anti-tumor agents. These agents are shown to inhibit multiple features of cancer cells, including proliferation, survival, invasion, and angiogenesis.
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Highlighted below are inhibitors included in InhibitorSelect™ EGFR Signaling Pathway Inhibitor Panel (Cat. No. 324839)