LECTURE 4 (Growth factor signalling and oncogenes) Flashcards
What is a ligand?
A molecule that binds to a specific site on another molecule
What are the different types of receptors?
- Intracellular receptor
- Ligand-gated ion channel receptor
- G-protein coupled receptor
- Enzyme-linked receptor
What is the Epidermal growth factor (EGF)?
A tyrosine kinase receptor
What is the function of tyrosine kinase receptors?
Tyrosine kinase receptors phosphorylate tyrosine residues on target proteins -> Phosphorylation results in a CONFORMATIONAL CHANGE of the protein target
Describe the mechanism of the Epidermal growth factor (EGF)
1) EGF binds to the Epidermal growth factor receptor (EGFR)
2) Growth factor binding causes a conformational change that unmasks a DIMERISATION DOMAIN required for receptor dimerisation
3) The close proximity of two receptors enables kinase domain of one receptor of the dimer to phosphorylate the other receptor of the dimer
Describe how phosphorylated tyrosine residues activate intracellular transducer RAS
1) Phosphorylated tyrosine residues create high-affinity binding sites for proteins that contain Src homology 2 (SH2) domains
2) Grb2 = an intracellular protein that contains SH2 and SH3 domains
3) The two SH3 domains of Grb2 interact with the exchange protein SOS (son of sevenless) which facilitates the activation of the pivotal intracellular transducer RAS
Describe how RAS activation leads to MAP kinase cascade
1) SOS stimulates the inactive RAS to replace its bound GDP by GTP which activates RAS to relay the signal downstream
2) Activated RAS propagates signalling further inside the cell via a kinase that goes from RAS to RAF KINASE to MEK KINASE and finally MAP KINASE
3) MAP KINASE dimerises and enters the nucleus phosphorylating proteins and activating immediate early genes (including G1 cyclins)
What is the function of the AP1 transcription factor?
It binds to DNA and regulates gene expression involved in growth, differentiation and death
What is one mechanism by which AP-1 induces cell cycle progression?
Binding to and activating the cyclin D gene, a critical regulator of the cell cycle
What are the mechanisms that induce AP-1?
- Direct phosphorylation of members of the Fos family by MAPK affects their DNA-binding activity
- MAPK phosphorylation and subsequent activation of other transcription factors increases the expression of both fos and jun genes
Describe the mechanism of Phosphatidylinositol 3-kinase
1) Phosphatidylinositol 3-kinase (PI3K), a lipid kinase, is an effector protein
2) Production of the second messenger (PIP3) recruits the serine/threonine kinase PDK-1 to the membrane -> AKT (another serine/threonine kinase) is also recruited to the membrane where it is phosphorylated and activated by PDK-1
3) Activated AKT is translocated into the nucleus where it phosphorylates nuclear substrates
What is an Oncogene?
A mutated gene that has the potential to cause cancer
[before an oncogene becomes mutated its called a PROTO-ONCOGENE and plays a role in regulating normal cell division]
What do Point mutations/deletions in coding sequences result in?
Structural and functional changes
What do point mutations and deletions in regulatory sequences result in?
Over-expression
What do Chromosomal translocations result in?
Fusion proteins with novel functions
What do insertional mutagenesis caused by viral integration result in?
Aberrant expression
What does gene amplification result in?
Increase in gene dose and protein production
Human epidermal growth factor receptor 2 (HER2) is over expressed in which cancers?
- Breast
- Gastric
- Ovary
- Prostate
- Lung
[HER2 is also a tyrosine kinase receptor]
Why must signalling through the ERbB family of receptors be tightly regulated?
Signalling through the ErbB family of receptors promotes cell proliferation and opposes apoptosis -> must be tightly regulated to prevent uncontrolled cell growth from occurring
How does Herceptin work?
1) Too much HER2 generates too many protein receptors, signalling for cancer cells to divide and multiply
2) Herceptin works to block receptors and stop signal responsible for cancer cell growth and division
Breast cancer
MONOCLONAL ANTIBODIES:
- Trastuzumab
- Pertuzumab
ANTIBODY-DRUG CONJUGATE
- Trastuzumab-emtansine
KINASE INHIBITORS
- Lapatinib
- Neratinib
Non-small cell lung cancer (NSCLC)
EGFR TKIs:
- Gefitinib
- Erlotinib
- Afatinib
- Osimertinib
EGFR TARGETED MONOCLONAL ANTIBODIES:
- Necitumumab
Colorectal cancer (CRC)
EGFR-TARGETED MONOCLONAL ANTIBODIES:
- Certuximab
- Panitumumab
EGFR is activated in which cancers?
- Non-small cell lung cancer
- Colorectal cancer
What causes Colorectal cancer (CRC)?
Amplification/overexpression of the wild-type gene
What causes Non-small cell lung cancer (NSCLC)?
Point mutations that affect the intracellular domain of the receptor resulting in constitutive catalytic activity
What are the most common EGFR mutations?
- Exon 19 deletions
- Exon 21 point mutation
What causes EGFR-mutated lung cancer?
Activating somatic mutations in exons 18-21 can lead to constant activation of EGFR kinase domain with continuous downstream signalling through PI3K/AKT and MAPK/RAF pathways irrespective of ligand binding to growth hormones -> Cell survival, proliferation and resistance to apoptosis
What does the point mutation T790M in exon 20 lead to?
Resistance to first-and-second-generation EGFR TKIs due to a higher affinity of the ATP-binding pocket in the EGFR to ATP
What are RAF proteins?
Serine/threonine kinases that become activated following membrane recruitment by RAS-GTP and phosphorylation
