8 - Control of Cell Proliferation Flashcards
Cell Proliferation
- Cell division
- Regulated / controlled by signals outside (e.g growth factors, nutrient availability) and inside (e.g. signalling pathway activity) the cell
- Loss or abnormalities of control of cell proliferation
Phases of cell cycle
-G1
- S
- G2
- M
- G0
G0
Non-proliferating state of cells. (resting cells, cells that have withdrawn from the cell cycle)
Cell cycle checkpoints
Places where the cell cycle can be arrested to ensure faithful cell replication
Restriction point
- Part of (late) G1 where cells commit to cell division
- During G1, cells are responsive to external factors (eg. growth factors) which can either
promote progression through G1 or promote cell cycle exit (G0)
Regulation of cell cycle progression
Regulated by cyclin dependent kinases (CDKs)
CDKs
Serine/threonine kinases (phosphorylate other proteins on serines/threonines)
- Function (and appropriate target recognition) is dependent on cyclins
- Cell cycle progression occurs via sequential activation / inactivation of CDK/cyclin complexes
CDK / cyclin complexes in G1 phase
CDK4/6 : Cyclin D1, D2 or D3
CDK / cyclin complexes in late G1 phase (after R point)
CDK2 : Cyclin E1 or E2
CDK / cyclin complexes in S phase entry
CDK2 : Cyclin A1 or A2
CDK / cyclin complexes in late S phase
CDK1 : Cyclin A1 or A2
CDK / cyclin complexes in G2 / onset of mitosis
CDK1 : Cyclin B1 or B2
Regulation of activity of cyclin - CDK complexes
- Regulated by CDK inhibitors (CDKIs)
- Two classes are INK4 and Cip/Kip inhibitors
Ink4 Inhibitors
Inhibitors of CDK4 and CDK6 by:
- Distorting the cyclin D binding site, reducing affinity for cyclin D binding.
- Distort ATP binding site, reducing enzymatic activity
Examples of Ink4 inhibitors
- p15
- p16
- p18
- p19
Cip/Kip
- Bind all cyclin-CDK complexes
- Promote the formation of CDK4/6-cyclin D complexes
- Function by blocking the ATP binding site in the catalytic cleft of the CDK
Examples of Cip/Kip inhibitors
- p21
- p27
- p57
Early G1
- Low levels of cyclins
- High levels of p21/p27
Early/mid G1
- Cyclin D levels increase (due to mitogenic stimuli)
- p21/p27 bind and promote formation of Cyclin D/CDK4/6 complexes (enhances G1 progression)
- Cyclin E/CDK2 start to increase but p21/p27 bind and inhibit CDK2 complex activity
Mid late G1
- Cyclin E/CDK2 start to increase but p21/p27 bind and inhibit CDK2 complex activity
- Prevents premature S phase entry
Late G1
- Cyclin E/CDK2 levels continue to increase and there isn’t enough p21/p27 to inhibit activity
- CDK2 becomes activated & phosphorylates p27,
targeting it for ubiquitin-mediated degradation - Lack of CDK inhibitors promotes S phase entry
Retinoblastoma protein (RB)
- Negative regulator of G1-S transition
- RB, p107, p130
- Hypophosphorylated in
early/middle G1 - Becomes progressively
phosphorylated by activated CDKs during G1 and throughout the remainder of the cell cycle - Hyperphosphorylation of RB inactivates it allowing cell cycle progression
Hypophosphorylated RB
- Binds E2F complexes which are bound to the promotors of target genes
- Recruits histone deacetylases (HDACs)
- HDACs remove acetyl groups from histones
- Deacetylated histones are associated with
closed chromatin = repressed transcription
Hyperphosphorylated RB
- Does NOT bind E2F complexes
- Lack of RB enables histone acetylases
(HATs) to bind E2F complexes - Histones become acetylated, chromatin
opens = activation of target gene transcription
RB in late G1
- In late G1, Cyclin E/CDK2 activity increases and phosphorylates RB
- RB becomes hyperphosphorylated
(inactive) - RB releases E2F’s
- E2F transcriptionally activate genes required for G1/S phase progression (eg. cyclin E)
Regulation of cell cycle regulators
- MYC regulation of cell cycle regulators
- Regulation of cyclin D1 levels by signalling pathways
- Regulation of p21 by p53
Regulation of G1-S transition pathways
- MYC regulation of cell cycle regulators
- Regulation of cyclin D1 levels
- Regulation of p21 expression by p53
MYC/MIZ1
Represses expression of p15, p21 and p27
MYC/MAX
- Increases expression of cyclin D2
- Increases expression of CDK4
- Increases expression of CUL1 (which degrades p27)
- Increases expression of E2F1, E2F2, E2F3
MYC transcription factor
- Family of bHLH TFs
- Activates transcription in partnership with MAX
- MAD/MAX complexes
repress MYC target genes
Effect of too much MYC (MYC gene amplication)
Promotes uncontrolled cell cycle progression
Three examples of signalling pathways that drive cell proliferation
- MAPK
- PI3K
- JAK/STAT
Regulation of cyclin D1 levels
- Many cell signalling pathways activate expression of cyclin D1
- Some cancers have abnormally increased activity in signalling pathways which drives cancer proliferation (e.g. HER2 gene amplification in breast cancer)
p53
- Transcription factor
- Functions as a homotetramer
- Is activated following DNA damage
- Upregulates p21 expression to arrest cell proliferation until
DNA damage is repaired, or to induce cell death
Loss of p53
- Most common events in human cancer
- CAn occur via gene deletion, or by acquisition of inactivating mutations
Control of proliferation in cancer cells
- Cancer cells proliferate more rapidly than non malignant cells due to defects in cell cycle control
- Less responsive to the external environment (growth factors, other cells)
Oncogenes
Genes/proteins that activate normal cell proliferation, but are mutated/activated in cancer resulting in uncontrolled proliferation
Tumour suppressor genes
Genes/proteins that normally prevent cell proliferation, but are mutated/inactivated in cancer resulting in uncontrolled proliferation
Examples of oncogenes
- MYC
- RAS
- EGFR
- HER2
Examples of tumour suppressor genes
- TP53 (encodes p53)
- RB
- CDKN2A
D type cyclin mutations
- Overexpression &/or gene amplification
- Hyperactivity of signaling pathways
- Reduced degradation
D type cyclins consequence
Increased G1-S phase progression in the absence of real mitogenic stimuli
E type cyclin mutations
- Gene amplification &/or overexpression
- Reduced degradation
E type cyclin consequence
Increased G1-S phase progression
Targeting cell cycle for cancer therapy
- Some cancer treatments target rapidly proliferating cells BUT these treatments also affect non malignant cells (e.g. cells lining gut, hair follicle cells)
Drugs that target G1 - S phase transition
- Antimetabolites
- Topoisomerase II inhibitors
Drug names ending in ‘ciclib’
Indicate they are cyclin dependent kinase inhibtors
Abnormal cell proliferation
- Hallmark of cancer
- May be due to mutations in cell cycle regulatory proteins or abnormal regulation of cell cycle regulators (e.g. cell death regulation, signalling pathway activity, DNA repair mechanisms)
What is cell cycle progression / inhibitions regulated by
- Control of cell death
- DNA repair
- Activation / inhibition of signalling pathways
- Telomerase activity
-Function of tumour suppressor genes / oncogenes
