L15 - Checkpoint Control II Flashcards
What is anoikis?
Cell detachment-induced apoptosis
A self-defence strategy used to eliminate cells in inappropriate locations
Which cells have anoikis resistance?
Cancer cells – have strategies to grow in the absence of anchorage and become anoikis-resistant
What does the myc protein govern?
Governs decisions to proliferate or differentiate
Localised in nucleus and control a variety of cell function
70% of human tumours overexpress 1 of 3 which members of Myc?
c-Myc, N-Myc or L-Myc
Myc is a member of which family of transcription factors?
bHLH transcription factors
Basic DNA-binding domain followed by amino acid sequences forming a-Helix, a-Loop and second a-Helix
How do members of the bHLH family interact?
Members of bHLH family form dimers and associate with gene promoters
What does the myc-max complex do?
Promotes proliferation and inhibits differentiation
What does the mad-max complex do?
Inhibits proliferation and promotes differentiation
What are the 3 ways of inducing myc expression in cancer?
- Gene amplification
- Chromosomal translocation
- Pro-virus integration
Which complex regulates expression of key components of cell cycle clock?
Myc-max complex
Which components of the cell cycle clock does the myc-max complex control?
Cyclin D2 and CDK4
- Elevated expression leads to pRb hypophosphorylation
E2F transcription factors
Myc-Miz1
- Repression of transcription of CKI - liberates cyclin E/CDK2 complex from inhibition
- Promotes the degradation of p27Kip1
What impact does the myc-max complex have on progression through the R point?
All lead to progression through R point
What experiments were used to show the powers of the Myc oncoprotein?
Myc expressed as a fusion protein with the oestrogen receptor
- Oestrogen or tamoxifen addition drives Myc translocation to the nucleus – active form
Cells in G0 in absence of growth factors
- Tamoxifen addition induces entrance into G1 and S phase
Shows myc acting on its own can relieve all of the constraints on proliferation
How does TGFb control cell cycle progression? Which two molecules does it control?
It strongly increases the levels of p15INK4B
- Leads to inhibition of cyclin D-CDK4/6 complexes
- Cells can’t reach the R point
It weakly induces p21Cip1
How does TGFb block cell cycle progression?
It prevents pRb phosphorylation blocking cell cycle progression
- It counteracts the activity of Myc
How does TGFb impact on myc?
TGFb inhibits the expression of Myc and prevents Myc from binding CKI promoters
- CKI are expressed
- CDK4/6 and CDK2 are inhibited
- pRb is not phosphorylated
- Cells don’t go through the R point
Elevated levels of Myc in tumour mean it is no longer under the control of?
TGFb
How do cancers inactivated TGFb signalling?
½ pancreatic carcinoma and ¼ colon cancers have inactivated Smad 4
- Means TGFb signalling is compromised
The gene encoding its receptor is inactivated in the majority of colon cancers
How do cancer cells counteract the deregulation of the pRB pathway by TGFb?
Inactivation of the Rb gene through mutation
Expression of high levels of cyclin D1
Down-regulation of INK4 proteins (CKI inhibitors) –> inappropriate inactivation of pRb
Mutations in CDK4 gene –> no longer able to bind to INK4 family of CKIs
How is p53 linked to cancer?
p53 is a transcription factor regulating genes controlling cell growth and apoptosis
p53 tumour suppressor gene is the most frequently mutated gene in human cancers
- Most p53 mutations are in the DNA-binding domain
- Can no longer promote transcription of p53 target genes
What signals cause p53 induction?
DNA damage and in response p53:
- Blocks cell cycle progression
- Mediates DNA repair
- Induces apoptosis
What is seen following a rapid increase in p53 protein levels?
No difference in mRNA levels
Post-translational stabilisation of p53
What happens to p53 in the absence of stress?
p53 is ubiquitinated by Mdm2 and degraded by the proteasome
Low p53 levels
What happens to p53 under stress?
p53 stabilised and protected from degradation
Phosphorylation of p53 blocks Mdm2 binding - mediated by ATM/ATR/Chk2
ATM phosphorylates Mdm2 inactivating it
High p53 levels
What happens to p53 when mitogenic and cell survival signals are present?
Phosphorylation-induced activation of Mdm2 –> p53 proteasomal degradation
Low p53 levels
What is the main way p53 arrests the cell cycle?
Upregulating p21Cip1
This inactivates cyclin/CDK complexes
How is p53 changed in cancer cells?
In p53 mutants in cancer – the p53 that accumulates in the cell cannot bind to DNA
- Cell cycle will progress regardless of any DNA damage
