How do Cancers Grow?

Question 1

What is meant by the ‘Hayflick limit’?

Answer 1

The limit of the number of times a cell can divide before reaching senescence; 20-60 times in culture.

Question 2

What is a telomere?

Answer 2

• Repetitive region at the ends of chromosomes
• Protects the end of the chromosome from degradation/fusion w/neighbouring chromosomes
• Like the plastic bit at the end of shoelaces

Question 3

How do cancer cells have replicative immortality?

Answer 3

• Each time a cell divides, its telmomeres become shorter to the point of senescence, where the cell can no longer divide. The cell eventually dies.
• Where a normal cell has its normal cell cycle checkpoints disrupted (e.g. p53 loss), the cell keeps on dividing past senescence till they reach a ‘Crisis’ stage
• Most cells die at Crisis (chromosomes are unstable with minimal telomere protection, start to fragment), however some survive due to mutation by activating telomerase (90%) (restoring telomeres) or ATL (10%).

Question 4

What cells are telomerase normally expressed in?

Answer 4

• Germ cells (sperm/egg - chromosomes can’t be shorter than our parents)
• Stem cells
• 90% of malignant cells; achieving Hallmark of Replicative Immortality

Question 5

What is Alternative Telomere Lengthening?

Answer 5

• At crisis, the cells that haven’t mutated to express telomerase, fusion/recombination occurs between the ends of different chromosomes to create new telomeres
• Leading the chromosomal rearrangements; fusions, deletions, amplifications, duplications.
• 10% of cancer cells

Question 6

How do cells ‘know’ what to do? E.g. divide, grow, rest, die, differentiate, move?

Answer 6

• They sense the environment and respond appropriately, changing gene expression (signals transduced to nucleus)
• They receive chemical messages (growth factors, mitogens, cytokines, hormones, nutrients, AAs, O2, CO2)

> If this goes wrong, it can lead to uncontrolled proliferation and cell survival.

Question 7

What are the key features of receptor signalling?

Answer 7

• Specificity (ligand-protein, protein-protein)
• Signal amplification
• Signal convergence
• On cell surface or intracellular
• End result: change in gene expression

Question 8

What types of receptor are there?

Answer 8

• Enzyme coupled receptors e.g. receptor tyrosine kinases (RTKs, such as EGFR; epidermal growth factor receptor, vascular- VEGFR)
• Steroid hormone receptors (nuclear receptors) e.g. estrogen
• GPCRs; not currently target of many anti-cancer drugs despite being a popular drug target in other diseases

Question 9

What is EGFR, and what does it do?

Answer 9

• Epidermal growth factor receptor (a tyrosine kinase receptor)
• Senses growth signals (e.g. EGF)
• Leads to increase in proteins required for cell division (proliferation; mutation = bad)

Composed of:

• Extracellular ligand-binding domain
• Transmembrane domain
• Kinase domain

Question 10

Describe the EGFR signalling pathway WRT RAS.

Answer 10

• EGF peptide binds to EGFR
• Conformational change; exposes dimerisation domain (for another EGFR/another protein)
• EGFR auto- and trans-phosphorylates; each other (fellow EGFRs) and themselves
• RAS is activated
• Activates RAS-RAF-MEK-ERK pathway phosphorylating each other as an “on”/”off” switch; communicates signal of EGF peptide to DNA in nucleus
• ERK (extracellular signal-related kinase) is a transcription factor/inside the nucleus, switches on 100 genes for cell division = greater proliferation

Question 11

Describe the EGFR signalling pathway WRT PI3K.

Answer 11

• PI3K; kinase, of the 3’ position
• Activation of PI3K phosphorylates and activates protein AKT (localising it in the plasma membrane)
• Whole process happens outside of the nucleus; there is no change to DNA
• Pathway downregulates p53, switching off/reducing apoptosis
• Thus promoting proliferation

Question 12

What are some examples of EGFR mutations?

Give an example.

Answer 12

• Frequently mutated in cancers
  > Change in DNA
  > Change in protein sequence
  > Change in protein function

> > > E.g. EGFR permanently activated; behaving as if bound to EGF (even in its absence), promoting cell division even when no growth signal is present.

Question 13

What drugs target EGFR and what are their modes of action?

Answer 13

Cetuximab:
- Stops EGF binding to EGFR; an antibody that binds to the extracellular domain of EGFR, blocking the receptor

Erlotinib, Gefitinib:

• Stops EGFR activation; drugs look v. similar to ATP (which is required for activation), but doesn’t behave like ATP, blocking activation
• Binds to kinase domain, inhibiting auto/trans-phosphorylation.

Question 14

What is the ‘silver lining’ to an overexpressed, mutated EGFR WRT treatment?

Answer 14

• Same EGFR mutations that promote cancer also make EGFR more sensitive to erlotinib and gefitinib

Question 15

What is a cancer patient’s DNA sequence determined?

Answer 15

• To decide which drug to use; e.g. if EGFR mutated, then cetuximab may be ineffective
• Personalised medicine

Question 16

What are the issues surrounding evolution & resistance of EGFR WRT erlotinib anti-cancer therapy?

Answer 16

• Mutations can cause sensitivity or resistance to drugs
• Mutations in EGFR can cause resistance to drug (e.g. erlotinib) treatment; e.g. if the kinase domain of EGFR (drug target of erlotinib) were to mutate, the drug would no longer be able to bind to EGFR and block activation (competitive inhibition over ATP)
• There is a good chance of mutation at the drug target

Question 17

Would a mutation downstream in the RAS-RAF-MEK-ERK pathway yield resistance to erlotinib treatment?

Answer 17

• If RAF (downstream of RAS) was mutated and constantly switched on (phosphorylated) for example, this would lead to uncontrolled division/proliferation
• Binding of erlotinib at EGFR would hence be futile if a mutation occurred downstream
• RAS is over-activated in smokers

Question 18

Why is combination therapy used?

Answer 18

Combination therapy means it is more difficult for cancers to become resistant

Question 19

What are steroid hormone receptors? (derived from? which superfamily? examples?)

Answer 19

• Derived from cholesterol (a membrane component)
• Can diffuse into cells; no need for cell surface receptors
• Part of the nuclear receptor superfamily
• Transcription factors which become activated by steroid hormones (proteins that bind to DNA to switch on/off genes)
• E.g. estrogen receptor (ER), androgen receptor (AR), progesterone receptor (PR), retinoic acid receptor

Question 20

What is the estrogen receptor an example of, and what occurs upon ligand binding?

Answer 20

• Type I steroid hormone receptor (activated in cytoplasm, not nucleus)
• Ligand (estradiol/estrogen) diffuses into cell
• Binds to (monomer) receptor, displacing associated chaperone proteins e.g. HSP90 (heat shock protein 90)
• ER is phosphorylated
• ER dimerises (w/another ER) and migrates to nucleus
• ER dimer then associates w/co-activators (AF1 or AF2) or co-repressors, modifying transcription of target genes

Question 21

What is Tamoxifen, why does it need to be metabolised, and what is its mechanism of action?

Answer 21

• Selective ER modifier (SERM; it can be estrogenic)
• Estrogen antagonist (recruits co-repressor to domain AF), but with some agonism (upregulation of ER/estrogenic in uterus & bone = cancer risk, as ER has 2 activator domains; still recruits co-activator to AF1)
• Mostly anti-estrogenic effect though (AF2)
• Is a pro-drug; metabolised to active metabolites by CYP2D6 (of CYP450 superfamily)

Question 22

What is Fulvestrant and how does it differ to Tamoxifen?

Answer 22

• Selective ER down-regulator (SERD)
• “Pure” anti-estrogen (no agonism; unlike agonism w/Tamoxifen of uterus & bone)
• Prevents dimerisation, thus no activation (ER dimer dissociation means ER never reaches nucleus)
• Increases degradation of ER
• Thus no transcription, no activation of estrogen-sensitive gene

Question 23

What are the general principles of signalling in cancer?

Answer 23

• Phosphorylation cascades
• Pathways interact
• Gene expression is altered
• Abnormal signalling seen in cancers = abnormal proliferation/survival
• Signalling can be target for therapy

Question 24

What is meant by constitutive activation?

Answer 24

• A constantly activated protein irrespective of whether there is a ligand stimulus present
• Can be EGFR or downstream protein e.g. RAF, or both