Cell Growth and Differentiation Flashcards
What important protein regulates the progression of cells through the G1 checkpoint?
How does it do this?
Retinoblastoma Protein
Binds to E2F -> stimulates S-phase protein expression.
What happens when Rb after it is phosphorylated by ___ and ___?
When Rb is phosphorylated by cyclin D-CDK4 and Cyclin E-CDK2, it can no longer bind to E2F and the cell is consequently allowed to progress to the S phase. DNA replication will consequently start.
What proteins do the target genes of E2F include?
Cyclin E and S phase proteins including DNA polymerase, thymidine kinase and PCNA (protein involved in DNA replication).
What are G1 CDKs activated in response to?
• G1 CDKs are activated in response to environmental signals, late CDKs by preceding kinase activities.
What is RB hyperphosphorylated dephosphorylated by?
Protein phosphatase 1
G1 CDKs ___ and late CDKs ___.
G1 CDKs hypophosphorylate, and late CDKs hyperphosphorylate.
Unphosphorlyated RB binds to __ transcription factor and what does this prevent?
Its stimulation of S-phase protein expression.
What does released E2F stimulate?
Expression of more cyclin E and S-phase proteins e.g. DNA polymerase, thymidine kinase, PCNA etc. DNA replication starts.
Overview:
What does Rb control and when?
What can cyclin-CDK phosphorylate and what effect does this have?
- Rb controls cell cycle in all cells, when hypophophorylated/unphosphorylated it binds to E2F transcription factor
- Cyclin-CDK can phosphorylate RB, releasing E2F leading to expression of Cyclin E and other S phase proteins
Growth factor signalling activates?
Early gene expression (transcription factors – FOS, JUN, MYC)
What do early gene products stimulate?
Early gene products stimulate delayed gene expression (includes Cyclin D, CDK2/4 and E2F transcription factors)
What is E2F sequestered by?
• E2F sequestered by binding to unphosphorylated retinoblastoma protein (RB)
What do G1. cyclin-CDK complexes do to RB?
What does G1/S cyclin-CDK complexes do to RB?
G1 cyclin-CDK complexes hypophosphorylate RB and then G1/S cyclin-CDK complexes hyperphosphorylate RB releasing E2F
What does E2F stimulate the expression of?
E2F stimulates expression of more Cyclin E and S-phase proteins (e.g. DNA polymerase, thymidine kinase, Proliferating Cell Nuclear Antigen etc.)
What do S phase cyclin-CDK and G2/M cyclin-CDK complexes do?
What are these switches activated by?
• S-phase cyclin-CDK and G2/M cyclin-CDK complexes build up in inactive forms.
These switches are activated by post-translational modification or removal of inhibitors, driving the cell through S-phase and mitosis.
What happens when there is DNA damage?
- Stop the cycle- by (cyclin dependent kinase inhibitors, CHEK2 etc.)
- Attempt DNA repair- (nucleotide or base excision enzymes, mismatch repair etc.)
- if repair impossible Programmed Cell Death (BCL2 family, caspases)
What is DNA damage is detected at checkpoint during DNA repair?
• If DNA damage is detected at a checkpoint, the cell arrests and there is also an attempt to repair the damaged DNA.
If DNA is fully repaired what happens to the cell?
• If the DNA is fully repaired, the cell re-enters the cell cycle whilst if complete repair is not achieved, the cell undergoes apoptosis so as not to transmit mutated DNA to daughter cells.
When DNA is intact what happens to the TP53 gene?
When DNA is intact the TP53 gene is continuously destroyed by a proteosome.
When DNA is damaged (e.g. by __) what is activated?
• When DNA is damaged by e.g. mutagens this activates kinase activation
What will kinase phosphorlyate and what is the effect of this?
Kinase will phosphorylate the TP53 gene so it can no longer be destroyed by the proteosome and become activated and conduct the following actions
• Actions of TP53:
o Expression of CKI: cause Cell cycle arrest
o Repair not possible: Apoptosis
o DNA Repair e.g.Excision repair- so can then return to cell cycle
What mutations are the most frequent in cancer?
TP53 in Cancer
• TP53 loss-of-function mutations are amongst the most frequent in cancer
Describe the roles of TP53.
Prevent cell cycle arrest- faster growth
• Prevent apoptosis- so cells don’t die
Prevent DNA repair- more mutations.
Why does preventing DNA repair cause more mutations?
Prevent DNA repair- more mutations
o more heterogeneity
o more adaptation
o cancer progression
What is chemotherapy?
Chemotherapy
• Traditional chemotherapeutic drugs act on the cell cycle
What is the objective of chemotherapy?
• Objective: stop proliferation, induce apoptosis
What is the role of S-phase drugs?
S-phase drugs cause DNA damage.
Give examples of S-phase drugs and describe what they do?
5-fluorouracil (prevents synthesis of thymidine) o Cisplatin (binds to DNA causing damage and blocking repair)
What do M-phase drugs target?
• M-Phase drugs target the mitotic spindle
Give examples of M-phase drugs and their role.
Vinca alkaloids- stabilize free tubulin, prevent microtubule polymerization, arrest cells in mitosis
o Paclitaxel (Taxol)- stabilizes microtubules, preventing de-polymerization, arrests cell in mitosis
• Not just cancer: colchicine (similar mode of action to vinca alkaloids) is used for immune-suppression
Summary:
What does growth factors binding to receptors induce?
What does G1 and G1/S cyclin-CDK complexes do?
What does release of E2F stimulate?
What happens next?
If all DNA is replicated what do G2/M cyclin-CDK complexes cause?
What happens if chromosomes are aligned on spindle?
If process fails what does TP53 initiate?
- Growth factors binding to receptors induce gene expression
- G1 and G1/S Cyclin-CDK complexes phosphorylate RB in the absence of inhibition by CKIs (expression of these is regulated by TP53 or TGF)
- E2F released, stimulating expression of genes required for S-phase
- Cell replicates DNA (expression of S-phase Cyclin-CDK complexes)
- If all DNA replicated, G2/M Cyclin-CDK complexes cause cell to enter mitosis. If chromosomes aligned on spindle, exit from mitosis is triggered
- If process fails, TP53 initiates apoptosis
