Receptor tyrosine kinases are the enzyme-linked cell surface receptors
Tyrosine kinases are the enzymes that catalyze the transfer of a phosphate group from ATP to the hydroxyl group of a tyrosine residue that means they catalyze the phosphorylation of the tyrosine residue of the target proteins. There are 90 identified genes that encode tyrosine kinases and 58 of them are receptor tyrosine kinases while others are non-receptor tyrosine kinases.
Receptor tyrosine kinases are those tyrosine kinases that are embedded in the membrane and act as receptors while non-receptor tyrosine kinases are those that are present in the cytosol. All the receptor tyrosine kinases are grouped into 20 subfamilies among them EGFR/ErbB, insulin receptor, PDGFR, FGFR, VEGL, and HGF are associated with the cancerous disease.
Receptor tyrosine kinases have a single polypeptide chain with three domains; an extracellular ligand-binding domain, a transmembrane domain, and a cytoplasmic tyrosine kinase domain. The extracellular domains of the RTKs are involved in the receptor dimerization when bound to their respective ligands. While cytoplasmic domains have intrinsic tyrosine kinase activity that catalyzes the ATP-dependent autophosphorylation of the key tyrosine residue to provide the binding site for the downstream signaling molecules.
The tyrosine kinase domains contain different parts; an activation loop to make the kinase domain active or inactive, a nucleotide-binding lobe (N-lobe) to provide ATP binding site and a catalytic lobe (C-lobe) with a key tyrosine residue for autophosphorylation. Tyrosine kinase domains of all the RTKs are similar in their structure and mechanism of activation while a segment between the transmembrane domain and the tyrosine kinase domain (called as juxtamembrane domain) vary significantly among the RTKs.
Activation and dimerization of the receptor tyrosine kinase
When a dimeric ligand binds to the receptor in the cytoplasmic ligand-binding domain, conformational changes occur that lead to the receptor dimerization. Once the receptor forms a dimer, the key tyrosine residue in the C-lobe is autophosphorylated leading to the activation of the RTK. However, the activation loop also plays an important role in the activation of the RTK.
In an inactive form of the receptor tyrosine kinase, the activation loop is projected towards the catalytic site (C-lobe) in such a way that prevents the access of the ATP of the N-lobe for the autophosphorylation (this is called as cis-autoinhibition). Therefore, it is necessary to remove the activation loop from the C-lobe. Ligand binding causes a conformational change that leads to the conformational change in the activation loop in such a way that it removes the inhibitory effect of the activation loop.
Once the activation loop is removed from the C-lobe, the key tyrosine residue in the C-lobe is autophosphorylated and causes trans-phosphorylation of other tyrosine residues in the C-lobe of the partner receptor making the RTK active. Activated tyrosine kinase domain trans-phosphorylates different key tyrosine residues in the partner receptors and creates binding sites for downstream cytoplasmic signaling molecules.
Signaling cascade of the receptor tyrosine kinase
These cytoplasmic signaling proteins contain Src Homology-2 (SH2) or phosphotyrosine binding domains (PTB) that recognize the phosphotyrosine residues of the tyrosine kinase domains of the RTKs. These proteins have an intrinsic catalytic activity and their example includes Src or PLCγ. These proteins may also serve as adapter proteins to recruit other downstream signaling proteins such as Grb2 linked to the mitogen-activated protein kinase (MAPK) activation pathway.
Proteins that are recruited to phosphotyrosine residue of the tyrosine kinase domain by their SH2 domains are adaptor proteins while proteins that are directly recruited to the phosphotyrosine residue of the tyrosine kinase domain or to the adaptor proteins attached to the tyrosine kinase domain are anchoring proteins. Once adaptor proteins and anchoring proteins form a signaling cascade with the activated tyrosine kinase domain of the RTKs, they activate different intracellular enzymes leading to specific cellular process activation.
The main downstream signaling pathway of the RTK includes MAPK, PI3K, Src, and some other signaling pathways that involve the PLCγ, JAK/STAT signaling pathways. The MAPK signaling pathway plays an important role in the various cellular processes such as cell proliferation, cell death, cell differentiation, migration, and angiogenesis.
MAPK pathway of the receptor tyrosine kinase signaling
In the MAPK signaling pathway, an adaptor protein Grb2 binds to the tyrosine kinase domain through its SH2 domain and provides binding sites for another downstream adaptor protein SOS. SOS proteins are attached to the membrane-embedded PIP2 and it binds to the Grb2 through its SH3 domain. SOS binding to the Grb2 allows the activation of different downstream signaling pathways including Ras via SOS.
Ras protein is a small cytosolic G-protein that interacts with the SOS protein. It is also called a guanidine nucleotide exchange factor (GEF) with GTPase activity that catalyzes the hydrolysis of GTP to GDP. Therefore, it acts as a switch for intracellular effector molecules. Activated Ras protein recruits the next downstream signaling molecule Raf that further activates the MEK1/2 on their key serine residues. Activated MEK1/2 activates the Erk1 and Erk2 or MAPK1/2 by phosphorylating their key threonine and tyrosine residues.
Phosphorylated Erk1/2 is then translocated into the nucleus where it activates the transcription factors responsible for the regulation of the genes responsible for the survival and growth of the cells or activates another cytoplasmic protein such as RSK1/2. Other target transcriptional regulators include STAT, Elk-1, and CREB that activate the transcription of early response gene products such as c-Fos, c-Jun or c-Mys.
These early gene products stimulate the expression of the late response genes such as cyclin D1 and CDK6 involved in the control of the progression of the G1 phase and G1/S transition. Sustained activation of the Erk1/2 causes phosphorylation of the proteins such as c-Fos stabilizing them. Thus, stabilized c-Fos activates the transcription factors for the late response gene products.
MAPK pathway can also activate three additional pathways such as p38, JNK, and ERK5. Activation of the p38 pathway leads to the expression of the genes responsible for cell proliferation, angiogenesis and inflammation activation of the JNK pathway leads to the expression of the genes responsible for the cell apoptosis or the development of the immune system. Similarly, translocation of ERK5 into the nucleus leads to the expression of the genes for cyclin D1 required for the progression of the G1/S transition.
PI3K/Akt/mTOP pathway of the receptor tyrosine kinase signaling
However, PI3K/Akt/mTOP pathway activated by the SOS proteins leads to the expression of the proteins essential for the cell cycle progression, and to balance the cell survival and apoptosis. PI3K is an adaptor protein with lipid kinase activity. It interacts with the SOS via its SH2 domain and catalyzes the phosphorylation of the membrane lipid phosphatidylinositol bisphosphate (PIP2) to form phosphatidylinositol triphosphate (PIP3).
Then phosphatidylinositol-dependent kinase-1(PDK1) catalyzes the transfer of a phosphate group from PIP3 to Akt that further activates the mTOR. This pathway leads to the ubiquitination-dependent degradation of the pro-apoptotic proteins such as BAD and p53 and induces the expression of the anti-apoptotic proteins such as Bcl-2.
Regulation of the receptor tyrosine kinase signaling
Receptor tyrosine kinase signaling cascade is regulated in different ways such as positive feedback or negative feedback loops. Positive feedback loop prolongs the auto-activation of the receptors and signals amplification by producing ligands and leads to the sustained signaling pathway while negative feedback loop decreases the signal molecules and activates different phosphotyrosine phosphatases that cause inactivation of the receptor.
Receptor tyrosine kinase and cancer
Receptor tyrosine kinases are also associated with different types of molecular diseases such as cancer. It has been found that more than 30% of the RTKs are either mutated or overexpressed in almost all human cancers. Mutations in the key tyrosine residues either in the activation loop or in the C-lobe or N-lobe leads to the alteration of the RTK activation and RTKs are activated even in the absence of the ligand molecules. Either mutation or overexpression of the RTKs, both lead to the uncontrolled cell proliferation, the hallmark of cancer.
Therefore, targeting the inhibition of the RTKs in the RTK-induced cancerous cells can be the best strategy to treat cancer. Researchers have identified two families of the RTK inhibitors in the mouse MC3T3-E1 osteoprogenitor cell lines. The first family of the ETK inhibitors includes lapatinib, erlotinib, and sunitinib. The second family of the RTK inhibitors includes GSK1838705A, PF-04691502, masitinib, XL880, nilotinib, and CEP-751. Other RTKs inhibitors include imatinib mesylate and dasatinib that inhibit the RTKs in the bone-related cancers.
Reference: Journal of Bone Oncology (Receptor tyrosine kinases: Characterisation, mechanism of action and therapeutic interests for bone cancers)
Article DOI: 10.1016/j.jbo.2015.01.001