Cancer is a disease characterized by rapid and undesirable growth of cells wherein there is a loss of regulation of the cell cycle leading to excess growth, proliferation, differentiation and migration of cells.
What does it mean to lose control over the cell cycle and what leads to a condition of cell overgrowth or rather dysregulation of which pathways or factors are responsible for giving rise to one of the scariest diseases ever??
Let us understand briefly a very important factor which is majorly involved and is responsible for most types of cancer.
KINASES
Kinases are an important class of enzymes responsible for transferring the phosphate group from ATP to the hydroxyl group of serine, threonine and tyrosine and thereby activating the downstream signal transduction which leads to a range of cellular and metabolic effects.
There are in total 538 human kinases present in the body and indeed they play an important role across different tissues, cells, and majorly in disease conditions.
KINOME FAMILY
Kinome is a very huge family comprising seven members which play an extensive, integrated and complex role in not only cancer but also in other diseases like diabetes, obesity, infection and other neurological disorders.
Following are its family members: -
- AGC: Containing protein kinase A, G and C
- CAMK: Calcium/calmodulin-dependent protein kinase
- CK1: Casein Protein kinase 1
- CMGC: Containing CDK, MAPK, GSK3, CLK families
- STE: Homologs of yeast Sterile 7, Sterile 11, Sterile 20 kinases
- Tyrosine kinase family
- Tyrosine kinase-like family
When we look at the role and the function of the kinome family as a whole it pretty much seems to be very similar with each family involved in signal transduction regulating growth, migration, differentiation, survival and proliferation. But, when we take a deeper look at the individual family’s role in different types of cancer, their mutation profile in different forms of cancer, and the resistance mechanism, each family has its peculiarity and ways of treatment.
Out of all the above members, we will briefly take a closer look at the receptor tyrosine kinases mechanism, function, oncogenic activation and resistance.
What are RTKs?
Receptor tyrosine kinases (RTK) are the advanced cell surface receptors which have a crucial role in the initiation and progression of cancer when abnormally activated due to several mutations that can happen at one location or multiple locations throughout the receptor.
Mechanism
RTK have a complex mechanism by which the downstream signalling cascade is activated and initiates a range of dynamic functions. All the growth factors such as epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), neural/ glial growth factor, and fibroblast growth factor (FGF) are the ligands that bind and activate the receptor by phosphorylating the important sites located on the receptor itself.
a) Catalytic site: The intracellular C terminal region consists of the catalytic site responsible for the kinase enzyme activity, which catalyzes the receptor autophosphorylation and tyrosine phosphorylation of RTK substrates.
b) Non-catalytic site: This site acts as the docking site for the proteins where different proteins interact such as scaffolding, allosteric regulation and DNA-protein interaction via SH2 and phosphotyrosine (PTB) domain.
An overview of the receptor activation:
RTK is activated upon ligand binding which in this case can be any growth factor, which upon ligand binding leads to receptor activation, causing receptor autophosphorylation, and ultimately receptor dimerization.
This pathway then stimulates the downstream effectors activating multiple pathways at a time but the dominance of a specific pathway depends on the stimuli, physiological or tumor microenvironment and in the case of cancer, the mutation profile largely decides the dominating pathway.
Receptor dimerization further activates Ras which in turn activates RAF, followed by the MEK and erk pathways which translocate into the nucleus activating the transcription factors responsible for post-translational modification. Another pathway activates the PI3K pathway, which stimulates the Akt pathway majorly involved in cell survival and growth by inhibiting cell death pathways such as apoptosis. Simultaneously, the activated receptor can also stimulate PLC-gamma which leads to the production of IP3 and DAG, the ones responsible for the mobilization of calcium and activation of protein kinase C respectively. Under the influence of some stressors such as UV radiation, oxidative stress, cytokine and inflammation, the MAP3K pathway gets activated and all these activated pathways ultimately lead to cell growth, survival, migration and differentiation.
It is very clear from the mechanism itself that if there is any dysregulation even in one part of the receptor or the pathway that could lead to uncontrolled cell growth, differentiation and proliferation of cells causing tumorigenesis initiation and progression.
Oncogenic activation of RTK
Various factors can be responsible for the oncogenic activation of the RTK including suppression of phosphatases- the hydrolytic enzyme which removes the phosphate group attached to the proteins or the downstream effectors thus, ceasing their action of cell proliferation and survival and preventing the oncogenic activation.
Next, the most important ways to induce tumorigenesis are by mechanisms such as gain-of-function, amplification, chromosomal rearrangements and autocrine activation.
- Gain-of-function
A gain of function mutation in the receptor tyrosine kinase leads to aberrant activation and signal transduction, not under the control of normal checks and balances of the physiological system.
Few examples of such mutation:
EGFR TKD mutations: exon 19 or L858R pm 21 exons: Hyperactivate Kinase
EGFR ECD: P596L, G598V, A289V: Increased expression of EGFR protein, which undergoes phosphorylation in the absence of ligand stimulation: Glioblastoma
A point mutation in FGF3-ECD: Uterine cervix cancer
The star mark in the above diagram indicates that mutation can occur at any location throughout the receptor.
2. Amplification
Amplification refers to the overexpression and increased basal concentration of the receptor which overwhelms the antagonizing effects of the regulatory effects. Gene amplification is the major mechanism leading to overexpression pf the receptors in addition to overexpression of transcriptional/translational enhancement, oncogenic viruses, and derailment of normal regulatory mechanisms such as loss of phosphatase or other negative regulators. Mutations in the following effectors could lead to amplification and the subsequent type of cancer.
FGFR1: Lung & bladder cancer
ERBB4: Breast & gastric cancer
FLT3: Colon cancer
KIT: Melanoma
PDGFRA: GBM
RTK amplification confers resistance to the treatment.
3. Chromosomal rearrangements
Genomic studies have revealed numerous types of chromosomal rearrangement that lead to the formation of novel tyrosine kinase fusion oncoproteins.
BCR-ABL: Chronic myeloid leukemia- The first type of tyrosine kinase fusion oncoprotein identified.
NPM-ALK fusion oncoprotein: 50% of anaplastic large cell lymphoma ALCL
TRKA, TRKB, TKBC: 9 types of tumours — sarcoma, melanoma, head& neck, glioma, breast, colon, lung, thyroid
Fusion occurs in the N and C-terminal of the receptor
The target for adults and children is tyrosine kinase +ve cancers.
4. Autocrine activation
Cell-to-cell communication between cells occurs through messengers such as cytokine, growth factors or any such ligand having a property to activate the receptor which is released by secretary cells and acts at distant targets. In the case of autocrine activation, the target cell itself secretes the ligand that leads to undesirable activation of receptors over time leading to clonal expansion and tumor formation. Constitutive activation refers to the state wherein the target cell is activated even in the absence of the ligand.
RTK autocrine loop may work synergistically with other autocrine growth pathways and drive tumour development.
SCF-KIT loop partially synergizes with IGF-1 & bombesin: small cell lung cancer
EGRF mutants: less sensitive to EFGR tyrosine kinase inhibitor.
Resistance to tyrosine kinase inhibitors
The emergence of resistance to tyrosine kinase inhibitors is developing at a rapid pace leading to failure of the therapy and thus it is important to understand the methods involved in such resistance and way to overcome these treatment failures.
Resistance to tyrosine kinase inhibitors could be mainly of 2 types: Primary/ Innate or Acquired resistance.
Intrinsic resistance: This type of resistance is present even before the therapy is initiated and leads to the failure of therapy at the therapeutic concentration of the drug. Clonal population harbour cells which are refractory to treatment.
Acquired resistance: This refers to a state wherein the clonal cells are sensitive to the drug when administered for the first time and for the subsequent months or years that follow, but gradually the tumour cells find a way to escape the drug by employing different strategies which confer the cells resistant to the treatment and thus, leading to failure of the therapeutic regimen. Acquired resistance usually develops in patients with advanced metastatic disease. The complex and diverse mechanism is involved in the development of such resistance wherein the ultimate goal of cancer cells is to find a route to maintain continued proliferative and survival signalling.
Acquired resistance could be caused by on-target secondary mutations by the acquisition of bypass signaling, histological transformation, alteration in the biology of tumor microenvironment or metastatic cells find a sanctuary that the inhibitor cannot gain access to and by the activation of drug efflux transporter leading to multidrug resistance.
Following are the tactics employed by the clonal cells to acquire mutations:
- Gatekeeper mutation
The gatekeeper is a conserved amino acid-bound residue near the ATP binding site occupied by an amino acid with a small side chain which has a small hydrophobic pocket which is occupied by the kinase inhibitor. In the case of gatekeeper mutation, the small side chain is replaced by a larger bulkier hydrophobic pocket which is still accessible to the ATP but not to the drug.
2. Solvent-front mutation
The solvent front mutation is the ATP binding pocket in the catalytic domain that has relatively high solvent exposure, and where kinase inhibitors contact and show their therapeutic action. Mutation in this pocket frequently causes drug resistance.
3. By-pass signalling
4. Mutations in the downstream effectors
This is just a brief overview of receptor tyrosine kinases. There is a lot of research being going on in this domain which we will see in more detail in the coming blogs.
