Regulation of Cell Growth Flashcards
What are the 3 general considerations for cell growth?
Growth of population of cells: Hyperplasia and hypertrophy which depends on intra and extracellular signals
Growth at the cellular level (cell cycle)
G1, S, G2, M: controlled at 3 key check points
Apoptosis
Describe the phases of the cell cycle from mitosis to the S phase
Mitosis: nuclear membrane goes, chromatin condense.
Make 2 daughter cells and reenter cell cycle
Interphase: cell grow, macromolecules synthesised continuously
S: DNA replication so daughter cells= diploid.
Thymidine only incorporated into DNA at S phase
Describe the rest of the cell cycle between the S and M phase
G1: increase cell size but not organelles. Ps needed for DNA synthesis
G2: increase cell size and nucleus and organelles. PS needed for spindles
After M, you can come out of cell cycle into G0: removal of growth factor=cells undergo arrest
How can you use chemical analysis to figure out info about cells in the cell cycle and how much division is happening?
FACS uses laser to measure how much fluorescence a cell has (based on DNA amount)
Can work out the proportion of cells in which phase of cell cycle
Low division: increased proportion in G1 and few in S/G2/M
High rate division: most in S/G/M
Outline which drugs act on the cell cycle, specifically the S phase
S-Phase active: reduces the S phase
5-Fluorouracil (an analogue of thymidine blocks thymidylate synthesis). Treats cancer
Bromodeoxyuridine (another analogue. Detected by Abs to identify cells that have passed through the S-phase).
Outline which drugs act on the cell cycle, specifically the M phase
M-Phase active: inhibits M phase
Colchicine and Vinca alkaloids (stabilizes free tubulin, preventing microtubule polymerization and arresting cells in mitosis – used in karyotype analysis)
Paclitaxel (Taxol, stabilizes microtubules, preventing de-polymerization)
Paclitaxel, vinca alkaloids, tamoxifen treat cancer. Tamoxifen=estrogen antagonist, so reduces cell division
Give the 3 cell cycle checkpoints and explain what is checked for
1) Late G1 restriction pt: checks DNA not damaged, cell size, metabolite + nutrient stores
2) Just before mitosis: checks DNA completely replicated and not damaged
3) M: are all chromosomes aligned at spindle?
Main site for control of cell growth = M-G1 as cells responsive to growth factors
Cyclin dependent kinase activity controls cell cycle progression. How does this work?
Cellular control checkpoints involve cyclin dependent kinases
Active kinases have 2 components:
CDK: catalytic subunit (10 genes)
Cyclin: regulatory subunit (>20 genes)
Cyclin binds CDK to form active cyclin-CDK complex which can phosphorylate specific substrates
How is Cyclin-CDK activity regulated?
-Cyclical synthesis (gene expression) and destruction (by proteasome).
-Post translational modification by phosphorylation. Depending on modification site this causes activation, inhibition or destruction
-Dephosphorylation
-Binding of cyclin-dependent kinase inhibitors
What is the importance of the retinoblastoma protein?
RB is a key substrate for G1 and G1/S CDKs
RB is typically unphosphorylated, bound to E2F which stops S phase
Cyclin-CDK phosphorylates RB to release E2F.
E2F travels to nucleus and stimulates cyclin E and S phase protein expression (DNA polymerase, thymidine kinase etc.). This starts DNA replication
Loss of CDKIs = cancer. 2 families.
Describe the 2 families of Cyclin dependent kinase inhibitors- what is expression stimulated by etc
(CIP/KIP) family (aka CDKN1). Expression is stimulated weakly by TGFb and strongly by DNA damage (involving TP53)
Inhibits all other CDK-cyclin complexes
Gradually sequestered by G1 CDKs which activate later CDKs
Inhibitor of kinase 4 family (INK4) (aka CDKN2). Expression stimulated by TGFbeta. Specifically inhibit G1 CDKs
How do growth factors induce cyclin expression?
GF binds cell surface receptor=signal transduction which causes:
-Cascade of kinases which lead to waves of transcription in nucleus which make protein
-Some of these proteins stimulate transcription of different genes
Describe the sequence of events triggered by growth factors in summary
GF signalling activates early gene expression
Early gene products stimulate delayed gene expression (inc Cyclin D, CDK2/4, E2F transcription factors)
G1 cyclin-CDK hypophosphorylate RB, releasing E2F
E2F stimulates more Cyclin E and S-phase protein expression (DNA polymerase, thymidine kinase, PCNA etc.)
S and G2/M cyclin-CDK complexes build up in inactive forms. These switches are activated by post-translational modification or removal of inhibitors, allowing Sphase and mitosis
What are the options for a cell if there is DNA damage?
Stop cycle: cyclin dependent kinase inhibitors, CHEK2 etc.
Attempt DNA repair: nucleotide or based excision enzymes, mismatch repair etc.
Apoptosis if repair impossible: BCL2 family, caspases
What is TP53?
Tumour protein 53= transcription factor which is quickly degraded by proteasomes
If DNA damage detected= TP53 phosphorylation by kinases causes TP53 to act as transcription factor
-Expression of CDKIs arrest the cell cycle
-Expression of DNA repair proteins= excision repair
-Repair not possible = apoptosis
