Receptor tyrosine kinases (RTKs): Introduction, Structure, Tyrosine Kinase Inhibitors (TKIs), RTKs dysregulation and cancer

Introduction:

Receptor tyrosine kinases (RTKs) are plasma membrane receptor glycoprotein in nature with enzymatic activity.

They are a group of 90 enzymes capable of selectively phosphorylating amino acid tyrosine residue in various substrates which leads to conformational alteration of that protein and typically activates that protein.

 They are important mediators of the signaling that mediate cell growth, differentiation, cell division, metabolism and apoptosis in response to various extracellular signaling molecules such as peptide hormones, insulin, EDGF (Endothelium derived growth factor), Platelets derived growth factor (PDGF), Fibroblast Growth factor (FGF), Nerve Growth factor (NGF) etc.,

Structure of  Receptor Tyrosine kinases (RTKs)

Receptor tyrosine kinases (RTKs) are enzyme-linked receptors characterized by ligand binding ectoderm localized at the plasma membrane. It is a single transmembrane α-helix, containing an extracellular ligand binding domain, a cytosolic region comprising juxta membrane (Regulates kinase activity and receptor dimerization) and kinase domain followed by a flexible C -terminal tail.

Their structure comprises three key domains

• Extracellular Domain (N-terminal): Ligand binding ectoderm that is located at the plasma membrane.
• Transmembrane Domain: It is a single hydrophobic transmembrane α-helix that anchor the receptor in the plasma membrane
• Intracellular (Cytoplasmic) Domain: It is a tyrosine kinase domain composed of a flexible C-terminal that is responsible for catalysis of autophosphorylation and serves as a docking site for all regulatory protein for signal transduction.

RTK Signaling:

Ligand binding

The different types of ligands bind to the extracellular domain of the receptor causing the conformational change in the receptor. The monomeric structure of the receptor would be altered.

Dimerization

As the result of the ligand binding in the extracellular domain both of the receptors come together resulting in dimerization to form an active dimer. In turn, the intracellular tyrosine kinase domain is activated accordingly.

Dimer phosphorylation

The activated tyrosine kinase phosphorylates the tyrosine residues which are on the intracellular domain of the receptor. Phosphorylation is the simple addition of phosphate that converts ADP to ATP and the tyrosine kinase phosphorylates the hydroxyl group of the tyrosine residue, i.e., it is known as receptor kinase receptor.

Activation of relay proteins

The relay protein is activated by the phosphorylation and these molecules further phosphorylate many other molecules and signaling molecules in a sequential manner finally causing the activation of the target molecule that leads to change in the gene expression that results in cellular response.

The target molecules are often transcription factors that can directly bind to DNA and cause gene expression thereby changing cellular function thereby causing cellular response like cell division, cell differentiation, cell growth, etc.

Tyrosine Kinase Inhibitors (TKIs):

• Many human cancers have a defect in RTKS either they are over expressed or they contain point mutation or small deletion, dysregulation, either way. Many human cancer cells have these RTKs that are over active and they are constantly phosphorylating their dimer partner and informing the cell to go from G1 phase to S-phase.
• So, many drugs are enzyme inhibitors so it is fairly straightforward to design a drug that could inhibit an enzyme because enzymes typically have pockets that they fit into and these pockets can be used for target as drugs bind the enzyme and block the activity of enzyme. The inhibition can either be competitive and non-competitive.
• RTKs are the enzymes that are typically tyrosine kinases, so scientists have designed drugs that will bind to the RTKs and inhibit the kinase activities by binding the ATP binding pocket of the kinase.
• Thus, Tyrosine kinase inhibitors (TKI) are a group of pharmacologic agents, membrane permeable, that disrupt the signal transduction pathways of protein kinases by several modes of inhibition.
• Small-molecule inhibitors and monoclonal antibodies are among the medications that have been discovered and authorized to treat malignancies by activating RTKs. Approved TKIs can reduce tumors and/or prolong survival, but they can also cause side effects due to drug resistance and lack of specificity to a certain target.

RTKs dysregulation and cancer connections:

(Oncogenic activation receptor tyrosine kinases)

In normal situation, when the growth factor binds to the extracellular domain of the receptor, the receptor monomer dimerize the cytoplasmic tyrosine kinase domains of the receptor monomer are activated and tyrosine residues of the cytoplasmic domain are phosphorylated. The phosphorylated tyrosine residue lies within amino acid sequence motifs which are recognized by the SH2 domain of several cytoplasmic proteins. The receptor’s tyrosine kinase may phosphorylate the bound protein creating adding docking sites for SH2 domains containing proteins which themselves regulate the activation of other proteins. In this way, a signal cascade is initiated.

• Autocrine or paracrine stimulated by an excess of growth factors (e.g., EGF). Receptors are expressed to an increased concentration of growth factors that are produced either by the tumor cell itself or by the adjacent cells. Since a higher number of receptors occupy the cytoplasm, signals are more intense when compared to the situation in normal tissue.
•  Due to the amplification or dysregulation of the receptor genes, the density of receptor monomers in the plasma membrane increases. The receptors dimerize spontaneously, independent of the ligand binding leading to in appropriate signaling.
• Mutation in the transmembrane domain (e.g., Neu in rat neuroblastoma). Because of the point mutation in the transmembrane domain of the receptor. The receptor dimerizes spontaneously independent of ligand binding leading to in appropriate signaling.
• Activation of tyrosine kinase domain by mutations (e.g., EGF receptors). Due to the mutation in the cytoplasmic kinase domain of the receptor, the kinase is constitutively active signaling is inappropriate.
• Dimerization of tyrosine kinase domain through fusion with dimerization motifs of other proteins (e.g., NGF and PDGF receptors). Oncogene may code for fusion proteins that contain tyrosine kinase domain and dimerization motifs from unrelated proteins, the dimers are constitutively active.

The dysregulation of RTKs may occur by a gain-of -function mutation, an amplification, chromosomal rearrangements, TK domain duplication or by an autocrine or paracrine activation The dysregulations are generating abnormal activation of the RTKs which will be translated into enhanced proliferation, differentiation or angiogenesis, same as into a dysregulated cell cycle and metabolism.

The dysregulations are generating abnormal activation of the RTKs which will be translated into enhanced proliferation, differentiation or angiogenesis, same as into a dysregulated cell cycle and metabolism.

RTK dysregulation leads to uncontrolled signaling, contributing to diseases like cancer. Key mechanisms include:

• Gene Amplification – Excess RTK expression (e.g., HER2 in breast cancer).
• Activating Mutations – Ligand-independent activation (e.g., EGFR mutations in lung cancer).
• Chromosomal Translocations – Fusion proteins with constitutive activity (e.g., BCR-ABL in CML).
• Autocrine/Paracrine Loops – Overproduction of ligands sustaining RTK activation (e.g., VEGF in tumors).