Cellular growth regulation Flashcards
what is the difference between hyperplasia and hypertrophy
increase in cell numbers (hyperplasia)
increase is cell size (hypertrophy)
Give some examples of when apoptosis can occur in normal development
involution
(for example the uterus returning to normal size post pregnancy)
separation of the digits in embryogenesis
immune system development if the the B cells don’t receive survival signals
What 3 things promote cell division/cell growth?
- mitogens that stimulate proliferation (called mitogens) and maintain survival
(PDGF, IL-2,4) - stimulatedifferentiation and inhibit proliferation
(TGF-B) - induce apoptosis
(TNF-A)
what is the role of mitogens and give examples
stimulate proliferation and maintain survival
- EGF
- FGF
- IL4/5
- NGF
- PGDF
- IGF1
what is the role of TGF-B
stimulate differentiation and inhibits proliferation
growth inhibitor
what is the role of TNF-A
induces apoptosis
death signal
What are the three broad classes of growth factors, cytokines and interleukins?
paracrine
autocrine
endocrine
What is the difference between paracrine, autocrine and endocrine signalling?
Paracrine signalingis a form of cellsignallingor cell-to-cell communication in which a cell produces asignalto induce changes in nearby cells, altering the behaviour of those cells.
Autocrine signalingis a form of cellsignallingin which a cell secretes a hormone or chemical messenger (called theautocrineagent) that binds toautocrinereceptors on that same cell, leading to changes in the cell.
Endocrine signalingoccurs whenendocrinecells release hormones that act on distant target cells in the body.
What are the 2 fates of a daughter cell arrested in mitosis (G0)?
- These cells can re-enter the cell cycle and start dividing if stimulated by mitogen (growth factor)
OR - Receive TGF-B (growth inhibitor) which will induce differentiation of the cells. These cells can then be used to renew the cells removed by cell shedding/apoptosis.
briefly go over the phases of the cell cycle
The cell cycle consists of four stages: G1, S, G2, and M.
• G1 and G2are ‘gap’ phases in which the cell grows and prepares to divide.
• S in the synthesis phase in which the chromosomes (DNA) are copied (replicated).
• M is the mitotic phase in which the cell physically divides into two daughter cells.
Most cells are NOT actively dividing. These cells are in a resting state (G0).
what is the name of the cells that arrest in G0
Quiescent cells
state what is produced during M phase (mitosis) and what occurs directly after
Mitosis in normal cells produces TWO cells with identical genetic content. The daughter cells can:
• Re-enter the cell cycle
• Withdraws from the cell cycle (arrests). This cell is in the G0 phase of the cell cycle and is called the Quiescent cells. These cells can then take 2 different paths
1. These cells can re-enter the cell cycle and start dividing if stimulated by mitogen (growth factor)
OR
2. Receive TGF-B which will induce differentiation of the cells. These cells can then renew the cells removed by cell shedding/apoptosis .
What is one technique where you can find out the DNA content of a cell?
briefly describe what the results from this test can indicate
You can use a fluorescence-activated cell sorter to analyze the cell DNA content.
Use a fluorescent DNA stain to treat the cells. This will bind to DNA. This can then be used to measure the amount of DNA present.
High rate of cell division = more cells would be in the G2/M and S phases AND less cells in the G1 phase
As a recap, go through DNA replication.
1) DNA is replicated semi conservatively (daughter cells inherit one parental and one new strand).
2) New DNA is synthesized in the 5’ to 3’ direction from deoxynucleotide triphosphate precursors at a replication fork by a multienzyme complex (a replication machine).
3) Fidelity is determined by base pairing (A=T, G≡C) and presence of a proof reading enzyme in DNA polymerase.
4) Synthesis of the new DNA strand uses an RNA primer and occurs continuously on the leading strand and discontinuously on the trailing strand (giving rise to Okazaki fragments, which are ligated together after removal of the RNA primer).
what occurs in prophase
- Nucleus becomes less definite
- Microtubular spindle apparatus assembles
- Centrioles (yellow) migrate to poles
what occurs in prometaphase
- Nuclear membrane breaks down
- Kinetochores attach to spindle in nuclear region
what occurs in metaphase
- Chromosomes (blue) align in equatorial plane
what occurs in anaphase
- Chromatids separate and migrate to opposite poles
what occurs in telophase
- Daughter nuclei form
what occurs in cytokinesis
- Division of cytoplasm
- Chromosomes decondense
Name 2 drugs that can act during the S phase of mitosis and describe what it does
5-fluorouracil (an analogue of thymidine BLOCKS thymidylate synthase activity)
No thymidine = no DNA replication = arrests mitosis in S phase
Bromodeoxyuridine aka BrdU (an analogue that can be incorporated into DNA, it is detected by antibodies to identify the cells that have passed through the S phase)
What is tamoxifen and its role in breast cancer?
Oestrogen is required for the breast cancer cells to grow. Tamoxifen is an antagonist of oestrogen. Cells can be treated with tamoxifen to prevent growth of breast cancer cells. You can check if the growth of the cells has stopped using BrdU which will bind to any breast cancer cells that have progressed through the S phase. After tamoxifen treatment the amount of BrdU detected should decrease.
Tamoxifen = less BrdU detected = indicates less breast cancer cells progressing through S phase
Name 2 drugs that can act during the M phase of mitosis and describe what it does
Colchicine • Stabilises free tubulin = preventing microtubule polymerisation = arrests cells in mitosis therefore they cannot divide • This can be used in karyotype analysis Vinca alkaloids (similar action to colchicine)
Paclitaxel (taxol)
• Stabilises microtubules
• Prevents destabilisation
• This means the cells cannot separate
Name 4 drugs used in the treatment of cancer
- 5-flurouracil
- Paclitaxel
- Vinca alkaloids
- Tamoxifen
These all act to prevent cell division/growth of tumours
What are the controls involved in the cell cycle checkpoints?
Controls (involving specific protein kinases and phosphatases) ensure the strict alternation of mitosis and DNA replication
What are the 3 cell cycle checkpoints?
- G1 checkpoint checks for:
• DNA damage
• Cell size
• Metabolites/nutrient store - G2 checkpoint checks for:
• Complete DNA replication
• No DNA damage - M checkpoint checks for:
Chromosomes aligned on the spindle
At what point in the cell cycle are the cells responsive to growth factors?
G1 phase is where the cells are responsive to growth factors
This is the main site of control for cell growth
What is progression through the cell cycle controlled by?
Cyclin-dependent kinase activity
Cyclin-CDK complex can go on to phosphorylate specific substrates
Describe the regulation of cyclin-CDK activity
Cyclical synthesis = gene expression = more RNA = more protein
Destruction by proteasome
Post translational modification by phosphorylation
Depending on modification site may result in activation, inhibition or destruction
Dephosphorylation
Binding of cyclin-dependent kinase inhibitor
What is the name of the key substrate of G1 and G1/S cyclin-dependent kinases?
Retinoblastoma protein
Describe the mechanism of action of retinoblastoma protein
• Unphosphorylated RB binds to E2F = prevents stimulation of S phase protein expression
• Cyclin D-CDK4 & Cyclin E-CDK2 can phosphorylate RB = RB unbinds from E2F
E2F can then stimulate the expression of more cyclin E AND S phase proteins = allowing DNA replication
What are S phase proteins?
DNA polymerase
Thymidine kinase
PCNA
What are the 2 families of CDKIs (cyclin dependent kinase inhibitors)?
- CDKN1 (CDK inhibitory protein/kinase inhibitory protein) aka CIP/KIP)
• Expression of CDKN1 members stimulates WEAKLY (by TGF-B) and STRONGLY (by DNA damage- involving TP53)
• Inhibits all other CDK-cyclin complexes (late G1, G2 and M)
• Gradually sequestered by G1 CDKs = allowing activation of CDKs - CDKN2 (inhibitor of kinase 4 family)
• Expression stimulated by TGF-B
Specifically inhibit G1 CDKs (eg- CDK4 the kinase activated by growth factors)
Briefly explain how growth factors induce cyclin expression
Growth factor binds to growth factor receptor
Activates intracellular signalling pathway = kinase cascade
Effects nucleus = activates expression of P21, cyclin or CDK4 etc = formation of protein
describe the progression of mitotic daughter cells through the cell cycle
- Cyclin D-CDK4/6 = progression through G1 phase
- Cyclin E-CDK2 = progression from G1 —> S phase
- Cyclin A-CDK2 = progression through S phase
- Cyclin A-CDK1 & Cyclin B-CDK1 = progression through G2 phase & G2 —> M phase
which cyclin-CDK complexes are secreted in an inactive form and how do they become active and what is their function
Cyclin A-CDK2
Cyclin A-CDK1
Cyclin B-CDK1
Secreted in inactive form and activated by:
- Post translational modification
- Removal of inhibitors
Activation of these drives the S phase and M phase of the cycle
what does growth factor signalling activate and give some examples
activates early gene expression which results in the production of transcription factors:
- FOS
- JUN
- MYC
growth factor activates the production of early gene expression (transcription factors). What do these then go on to do?
early gene products stimulate delayed gene expression which results in the production of:
- Cyclin D-CDK4/6
- E2F
describe the progression of the cell cycle from G1–> M in terms of:
- cyclin D-CDK4/6
- cyclin E-CDK2
- E2F and retinoblastoma
- cyclin A-CDK2
- cyclin A/B-CDK1
E2F binds to unphosphorylated RB
cyclin D-CDK4/6 hypophosphorylates RB
cyclin E-CDK2 hyperphosphorylates RB
this results in RB unbinding from E2F
E2F stimulates the production of more cyclin E-CDK2 AND S phase proteins
cyclin A-CDK1 allows the progression through G2 phase
cyclin B-CDK1 allows the progression through M phase
name the 3 S phase proteins and their function
DNA polymerase
thymidine kinase
proliferating cell nuclear antigen (PCNA)
function:
progression of cells through S phase
G1 and G2 checkpoints can detect DNA damage. What happens if DNA damage is detected?
If DNA damage is detected at the checkpoints, it triggers cell cycle arrest or apoptosis. 2 things can happen:
1. Stops the cycle using:
• Cyclin dependent kinase inhibitors
• CHEK2
THEN attempt to DNA repair using:
• Nucleotide or base excision enzymes
• Mismatch repair
If the DNA is repaired then it can re enter the cell cycle
- Programmed cell death is repair is impossible using:
• BCL2 family
• Caspases
name 2 things that can be used to stop the cell cycle from progressing once DNA damage is spotted at the checkpoints
- Cyclin dependent kinase inhibitors
* CHEK2
How is DNA repaired
Nucleotide or base excision enzymes
Mismatch repair
What occurs if DNA repair cannot occur
Programmed cell death if repair is impossible using:
• BCL2 family
• Caspases
What is the role of TP53 in response to DNA damage?
• DNA damage activates kinases. These kinases can activates substrates by phosphorylation. Unphosphorylated TP53 is degraded by proteasomes. Kinases can phosphorylate TP53 to activate TP53 (preventing its degradation)
• TP53 is a transcription factor and can activate the expression of different genes:
1. Activates expression of CDKN1 (inhibitor) = cell cycle arrest
- Activates expression of DNA repair proteins = attempts to repair damaged DNA and if the DNA is repairs then it can continue into the cell cycle
- If DNA repair is NOT possible then TP53 activates genes required for apoptosis/cell death
