LECTURE 5 (Cancer Biology)

Question 1

What is Interphase?

Answer 1

The part of the cell cycle in which cells prepare for division and duplicate DNA

Question 2

Which part of the cell cycle causes duration of individual cell cycles to vary?

Answer 2

G1 PHASE

Explanation: There are different lengths of G1 phases since some cells that are not stimulated to duplicate their DNA can enter into a specialised form of G1 phase called the G0 phase

Question 3

What is the difference between Labile cells, Quiescent cells and Permanent cells?

Answer 3

LABILE CELLS
- rapidly dividing cells
- have a short G1 phase and never enter the G0 phase
- fast cell turnover makes cells specifically vulnerable to chemotherapy

QUIESCENT CELLS
- can enter G1 phase from G0 phase when stimulated

PERMANENT CELLS
- remain in the G0 phase
regenerate via cell differentiation from stem cells

Question 4

Describe the different phases of interphase

Answer 4

G1 PHASE (several hours to months)
- synthesis of RNA, proteins and cell organelles
- cells growth
- nucleotide excision repair takes place

S PHASE (8h)
- DNA replication -> 2 sister chromatids per chromosome
- synthesis of proteins for DNA packaging
- where most mismatch repair occurs

G2 phase (2-5h)
- further synthesis of proteins for mitosis
- G2 checkpoint

Question 5

What is the average length of the cell cycle?

Answer 5

16 hours

Question 6

What are mitogens/growth factors?

Answer 6

Induce cells in G0 to re-enter the cell cycle and pass a control point called the G1 restriction point

Question 7

What is important to about the need for mitogens/growth factors?

Answer 7

Before the passage of the restriction point, cell division is dependent on MITOGENS -> after, cells are irreversibly allowed to progress through the cycle without the need for growth factors

Question 8

Which pathways do the growth factor receptors that are tyrosine kinases signal?

Answer 8

• PI3K/AKT/mTOR pathway
• RAS/RAF/ERK pathway (MAPK pathway)

Question 9

Describe what happens in the PI3K/AKT/mTOR pathway

Answer 9

1) PIP3 is a phospholipid in the plasma membrane that activates the kinase AKT
2) Phosphatase PTEN dephosphorylates PIP3 to PIP2 inhibiting signalling in the PI3K/AKT/mTOR pathway

Question 10

What is Cowden disease?

Answer 10

A heritable cancer syndrome associated with an increased likelihood of neoplasms in the thyroid gland, breast, endometrium, kidneys, colon and rectum

CAUSE:
- inherited loss of one allele of PTEN

Question 11

What are Cyclin-dependent kinases?

Answer 11

A type of inactive kinase that must be activated to enable the transition from one phase of the cell cycle to the next

Question 12

What are the characteristics of Cyclin-dependent kinases?

Answer 12

• Present throughout the entire cell cycle
• Activated via binding of cyclins to form cyclin-CDK complexes
• Inhibited by cyclin-dependent kinase inhibitor proteins (CDKIs) if any errors in the genome are detected

Question 13

What are Cyclins?

Answer 13

A family of regulatory proteins that control the progression of the cell cycle and activate Cyclin-dependent kinases (CDKs) which control cell cycle processes through phosphorylation

Question 14

What happens when a cyclin and CDK form a complex?

Answer 14

The complex will bind to a target protein and modify it via phosphorylation -> Phosphorylated target protein will trigger a specific event within the cell cycle -> After event has occurred, cyclin is degraded and CDK is rendered inactive again

Question 15

How are cyclin proteins degraded?

Answer 15

Proteasome (a complex of proteases)

EXPLANATION: the covalent addition of ubiquitin, a small polypeptide, to the lysine amino acids of the cyclin flags the protein for degradation by the proteasome

Question 16

What is Cyclin A?

Answer 16

• A protein that regulates cell cycle transition by binding to CYCLIN-DEPENDENT KINASES (CDKs) 1 and 2
• Transcription of Cyclin A begins at the G1 RESTRICTION POINT and peaks in the middle of the S phase

Question 17

What is Cyclin B?

Answer 17

A component of MITOSIS PROMOTING FACTOR (a cyclin-CDK complex that regulates the G2 checkpoint of the cell cycle)

Question 18

What is Cyclin C?

Answer 18

• Cyclin C-CDK3 complex allows the transition from the G0 to the G1 phase
• Cyclin C-CDK8 complex regulates GENE TRANSCRIPTION by phosphorylating TRANSCRIPTION FACTORS and RNA POLYMERASE II

Question 19

What is Cyclin D?

Answer 19

• A protein that regulates the transition from G1 phase to S phase in the cell cycle via activation of CYCLIN-DEPENDENT PROTEIN KINASES
• Forms a complex with CDK4 which initiates DNA REPLICATION by inactivating RETINOBLASTOMA PROTEIN

Question 20

What in Cyclin E?

Answer 20

• A protein that regulates the transition from the G1 to the S phase of the cell cycle by binding to CYCLIN-DEPENDENT KINASE 2 and activating it

Question 21

What are the two families of inhibitors that are involved in regulating cyclin-cdk activity?

Answer 21

• The p16ink4a (INK) family
• The p21 (Cip/Kip) family

[these regulators bind DIRECTLY to and inactivate CDK-cyclin complexes]

Question 22

What is the difference between the INK family and p21 family of inhibitors?

Answer 22

INK = proteins that bind CDKs 4 & 6 and interfere with their binding to Cyclin D

p21 = interact with cyclins 1, 2 & 3 and the associated CDKs and block the ATP-binding site thus disabling kinase activity

Question 23

What are the characteristics of the G1 checkpoint?

Answer 23

• Control the cell’s NUCLEAR-CYTOPLASMIC RATIO, sufficient nutrient levels and DNA damage
• Growth signals lift the checkpoint
• Cells that pass this checkpoint become committed to division
• Dysfunction of this checkpoint leads to unregulated cell division

Question 24

What is the Retinoblastoma protein (Rb protein)?

Answer 24

Causes cell cycle arrest at the G1 phase by inhibiting E2F transcription factor and is a member of the ‘pocket proteins’

Question 25

What is the “Pocket proteins”?

Answer 25

Comprises of the A domain and the B domain joined by a linker region (Rb protein). HISTONE DEACETYLASE (HDAC) and the E2F TRANSCRIPTION FACTOR binds to it.

Question 26

Describe the Phosphorylation of Rb

Answer 26

1) CYCLIN D-CDK4 phosphorylates carboxy-terminal residues of Rb upon growth factor stimulation -> Increase in -ve charge causes intramolecular interactions with LYSINE RESIDUES and releases HDAC, but not E2F
2) CYCLIN E is expressed upon the release of HDAC from Rb -> Cyclin E-cdk2 phosphorylates additional amino acid residues of Rb including SER567 close to the linker region
3) CONFORMATIONAL CHANGE of the Rb pocket domain causing the release of E2F and subsequent expression of its target genes (e.g cyclin A, Thymidylate synthase Dihydrofolate reductase) that are important for S phase

Question 27

Describe the G2 checkpoint

Answer 27

• Blocks entry into M phase in cells that have DNA damage in previous phases or have not correctly completed S phase
• Activates kinases either ATM or ATR -> phosphorylate and activate Chk1 or Chk2 kinases
• CHK1 & CHK2 inhibit CDC25 (important for activation of CDK) -> CDC25 is unable to remove inhibitory phosphate groups and activate CDKS

Question 28

What is P53?

Answer 28

A protein that senses DNA damage signals

Question 29

What happens in the absence and presence of DMA damage?

Answer 29

ABSENCE OF DNA DAMAGE = enzyme MDM2 ubiquitylates p53 so that p53 is degraded inside proteasomes

PRESENCE OF DNA DAMAGE = protein kinases ATM and ATR phosphorylate p53 and MDM2 -> increase in concentration of p53

Question 30

What are the functions of p53 and p21?

Answer 30

p53 = acts as a transcription factor and increases the concentration of the CKI p21

p21 = inhibits CDK 4, 6 & 2 leading to cell cycle arrest

Question 31

Describe the p53 Tumour suppressor pathway

Answer 31

DNA damage -> activation of protein kinases -> phosphorylation of p53 -> activation of p21 -> inhibition of CDKs -> inhibition of CDK-mediated phosphorylation of pRb -> pRb activation and binding of transcription factor E2F -> cell arrest in the G1 phase (no entry into S phase)

Question 32

What is p27?

Answer 32

• A phosphoprotein that prevents cell cycle progression by inhibiting cyclin/cyclin-dependent kinase complexes
• Leads to the same sequence of events as p21 activation
• Present in every cell but undergoes continuous ubiquitylation and degradation

Question 33

What happens when phosphorylated p53 can no longer be ubiquitylated and degraded?

Answer 33

It leaves it free to act as a transcription factor

Question 34

Describe Retinoblastoma

Answer 34

The most common PRIMARY INTRAOCULAR MALIGNANCY in children caused by sporadic and inherited mutations in the RETINOBLASTOMA GENE (Rb)

Sporadic occur UNILATERALLY and hereditary retinoblastomas occur BILATERALLY and may be associated with other malignancies

CLINICAL FEAUTURES:
- Leukocoria “cat’s eye pupil”
- Strabismus
- A painful, red eye
- Loss of vision
- Retinal detachment (later stages)

Question 35

What are the characteristics of HPV?

Answer 35

• Double-stranded DNA virus with a capsid consisting of 72 capsomeres
• Genome codes for SIX EARLY PROTEINS (E1, E2, E4, E5, E6 and E7) -> responsible for viral replication and for two LATE structural proteins (L1 and L2) then assembled into CAPSOMERES in different %

Question 36

What happens when HPV integrates its DNA with that of the host cell?

Answer 36

E2 stops inhibiting E6&E7 with consequent p553-retinoblastoma protein (pRb) suppression and morphological/functional cellular alterations -> When persistently repeated, process leads to neoplastic transformation and progression within 15 years or less

Question 37

What is the importance of oestrogen receptor-positive breast cancer dependent on cyclin D1 pathway?

Answer 37

CDK4&6 inhibitors could be more effective against these tumours

Question 38

Which drugs are CDK4&6 inhibitors?

Answer 38

• Palbociclib
• Ribociclib
• Abemaciclib

Question 39

Which drugs are Aromatase Inhibitors?

Answer 39

• Letrozole
• Anastrozole
• Exemestane