Cell cycle regulation Flashcards
What is cell cycle machinery influenced by?
cancer associated proteins
oncogenes
tumour suppressors
Restriction point
between G1 and S
cell is fully comitted and continues cell cycle without any extrinsic factors
CDKs
cyclin dependent kinases
deployed by cell cycle machinery
depend on accessory proteins called cyclins
CDK/cyclin complexes
responsible for sending out signals to the proteins that carry out work to move cells through the cell cycle
Why are cyclins needed for CDKs to perform their function?
cyclins activate catalytic activity of CDK partners
cyclins help with substrate recognition of the complexes in the cell
What does cyclin B pair with?
CDK1
What does cyclin D pair with?
CDK4/6
What does cyclin E pair with?
CDK2
What does cyclin A pair with?
CDK1/2
What happens to cyclin levels as the cell cycle progresses?
falls due to ubiquitin dependent degradation
What are D cyclins regulated by?
growth factors
integrin mediated ECM attachments
What does the removal of growth factors lead to?
rapid collapse of cyclin D1 levels
What cancers is cyclin D1 shown to be everexpressed in?
breast
lung
melanoma
oral squamous cell carcinomas
What does the over expression of cyclin D1 in pancreatic cancer cells cause?
increased cell proliferation
increased anchorage independent growth
reduced chemosensitivity and elevated survival in the presence of cisplatin
shRNA effect on cyclin D1
partially reduces protein expression in gastric carcinoma cells + reduced cell proliferation in vitro
decreased tumour formation
induction of apoptosis due to G1 arrest
What is the role of cyclin E?
phosphorylation of substrates required for entry into S phase
What happens when cyclin E1 is overexpressed?
found in many different tumour types, oncogenic role in ovarian cancer
associated with increased mRNA expression
occurs in early lesions
What does cleavage of cyclin E result in?
expression of low molecular weight cyclin E
- stable protein with a high affinity for CDK2
observed in cancer + resistance to therapy
What factors in normal cells control cell cycle progression?
mitogenic signals promote
TGF-b inhibits it
How is TGF-b involved in carcinoma pathogenesis?
strongly increase levels of P15
inhibits cyclin B CDK4/6 complexes so cells can’t reach the R point
What does weak induction of P21 lead to?
stronger induction upon DNA damage
cell cycle is halted until genome is repaired
How do mitogenic factors progress the cell cycle?
Akt phosphorylates p21 in the nucleus and it is translocated to the cytoplasm
Akt phosphorylates p27 in the cytosol which prevents nuclear translocation
What are mitogenic factors?
small bioactive protein or peptide that induces a cell to begin cell division, or enhances the rate of division
Akt
What does p27 mis-localisation lead to?
clinical progression when p27 was localised in the nucleus only
What makes a patient more likely to have a more invasive cancer?
cytoplasmic p27 as it is used as a prognostic marker
Checkpoints
surveillance mechanisms to monitor each step of the cell cycle progression
G1/S checkpoint
cell will not be permitted to enter S phase if the genome is in need of repair
S checkpoint
DNA replication will be paused in response to DNA damage (this can cause the doubling of the time required to complete DNA synthesis)
G2/M checkpoint
a cell will not process through G2 to M until the DNA replication of S phase has been completed and entrance in M phase is blocked if the DNA is damaged
Spindle assembly checkpoint
a cell is not permitted to enter anaphase until all of its chromosomes are properly assembled on the mitotic spindle during metaphase
How do cancer cells have proliferative advantage?
- incompatible with normal cell cycle progression
- checkpoint controls block advance through the cell cycle if DNA has been damaged
pRb
nuclear phosphoprotein absent/present in a defective form in many tumours
molecular governer of the R point
How does pRb govern the R point?
cells can only go through if pRb becomes inactivated through hyperphosphorylation
when un/hypophosphorylated it binds to E2Fs, when hyperphosphorylated it dissociates
How is pRb phosphorylation tightly controlled?
- early and mid G1 → D type cyclin and CDK4/6 initiate pRb phosphorylation
- cyclin E levels increase at the R point → cyclin E/CDK2 mediate pRb hyperphosphorylation
How does pRb in early-mid G1 prevent cell cycle progress?
pRb binds to E2Fs preventing the transcription of E2F-dependent genes
decrease in trasncription of genes needed for S phase
pRb at the R point
pRb hyperphosphorylation → E2Fs are released → transcription of genes mediating G1/S transition