LECTURE 5 (Cancer Biology) Flashcards
What is Interphase?
The part of the cell cycle in which cells prepare for division and duplicate DNA
Which part of the cell cycle causes duration of individual cell cycles to vary?
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
What is the difference between Labile cells, Quiescent cells and Permanent cells?
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
Describe the different phases of interphase
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
What is the average length of the cell cycle?
16 hours
What are mitogens/growth factors?
Induce cells in G0 to re-enter the cell cycle and pass a control point called the G1 restriction point
What is important to about the need for mitogens/growth factors?
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
Which pathways do the growth factor receptors that are tyrosine kinases signal?
- PI3K/AKT/mTOR pathway
- RAS/RAF/ERK pathway (MAPK pathway)
Describe what happens in the PI3K/AKT/mTOR pathway
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
What is Cowden disease?
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
What are Cyclin-dependent kinases?
A type of inactive kinase that must be activated to enable the transition from one phase of the cell cycle to the next
What are the characteristics of Cyclin-dependent kinases?
- 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
What are Cyclins?
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
What happens when a cyclin and CDK form a complex?
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
How are cyclin proteins degraded?
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
What is Cyclin A?
- 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
What is Cyclin B?
A component of MITOSIS PROMOTING FACTOR (a cyclin-CDK complex that regulates the G2 checkpoint of the cell cycle)
What is Cyclin C?
- 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
What is Cyclin D?
- 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
What in Cyclin E?
- 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
What are the two families of inhibitors that are involved in regulating cyclin-cdk activity?
- The p16ink4a (INK) family
- The p21 (Cip/Kip) family
[these regulators bind DIRECTLY to and inactivate CDK-cyclin complexes]
What is the difference between the INK family and p21 family of inhibitors?
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
What are the characteristics of the G1 checkpoint?
- 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
What is the Retinoblastoma protein (Rb protein)?
Causes cell cycle arrest at the G1 phase by inhibiting E2F transcription factor and is a member of the ‘pocket proteins’
What is the “Pocket proteins”?
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.
Describe the Phosphorylation of Rb
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
Describe the G2 checkpoint
- 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
What is P53?
A protein that senses DNA damage signals
What happens in the absence and presence of DMA damage?
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
What are the functions of p53 and p21?
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
Describe the p53 Tumour suppressor pathway
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)
What is p27?
- 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
What happens when phosphorylated p53 can no longer be ubiquitylated and degraded?
It leaves it free to act as a transcription factor
Describe Retinoblastoma
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)
What are the characteristics of HPV?
- 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 %
What happens when HPV integrates its DNA with that of the host cell?
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
What is the importance of oestrogen receptor-positive breast cancer dependent on cyclin D1 pathway?
CDK4&6 inhibitors could be more effective against these tumours
Which drugs are CDK4&6 inhibitors?
- Palbociclib
- Ribociclib
- Abemaciclib
Which drugs are Aromatase Inhibitors?
- Letrozole
- Anastrozole
- Exemestane
