tutorial 1 - liver as an examplar Flashcards
aim of G1 *
grow and have the products for replication
are the cells dormant in G0 *
no, they produce proteins and metabolites needed in the cell or elsewhere
example of when cells would need to leave G0 and enter the cell cycle
proliferation of liver hepatocytes after damage by alcohol, hepatitis or partial hepatectomy
How do tyrosine kinase-mediated signals trigger cell cycle entry from Go in mammalian cells, e.g. regenerating liver?
ligand is a dimer that binds to receptor = receptor dimerization and cross-phosphorylation
via the Ras/RAf/ERK MAPK pathway
this causes production of c myc
which triggers production of cyclin d, activation of cdk4/6 and entry into G1
How is the cyclical activation of the Cdks via pRb proteins achieved?
cyclical activation of cdks occurs by cyclin production and destruction
each cdk/cyclin complex stimulates the production of the next cyclin In the cycle = progression
different cyclin/cdk complexes act at different stages of the cell cycle
positive phos by CAK and dephosphorylation of the negative regulatory site by cdc25 are needed for cdk activation
pRb forms inactive complexes with TF of E2F family
cdks progressively hyperphosphorylate pRb = release of active DP/E2F = transcription of genes for next cyclin
each time there is progressively more E2F released, until the last time when it is a different TF
what are typical targets of cdks *
proteins involved in DNA replication
nuclear breakdown
protein synthesis
How does activation of the PI-3-kinase pathway occur and how does it affect the cell cycle? *
PI3’K phosphorylates PIP2 to PIP3 = activation of pdk1 and akt/PKB
this is opposed by the PIP3 phosphtase (PTEN)
overexpression of Akt has an anti-apoptotic effect - why *
Akt phosphorylates FOXO TF
blocking transcription of the negative cell cycle regulators p27kip and apoptotic protein Fas-L
akt phosphorylates bad so bcl-2/xl are active = anti-apoptotic
akt phosphorylates and inhibits caspase 9
How do cell cycle inhibitors such as Cip/Kip and INK4 control Cdk activity?
they inhibit regulators of the cell cycle
the INK4 family inhibit cdk4/6 in G1
p27kip1 is transcribed by FOXO
when p27kip1 complexes with cyclinD/ckd4/6 it is active and promote cell progression; however when p27 is phosphorylated by GSK3b (which is inhibited by akt) it is inactivated; therefore akt inhibits gsk3b = no phosphorylation of p27 = active complex of p27 and cyclin D = progression of cell cell - therefore akt is anti-apoptotic
How do cells check for DNA damage and what do they do if they detect it?
p53 is a tumour suppressor gene that regulates the cell cycle and conserves genomic stability by preventing mutation
it can activate DNA repair proteins, it can cause growth arrest by holding cell in G1/S upon dna damage recognition(iff it holds the cell here for long enough, the DNA repair proteins will have time to fix the damage and the cell will be able to continue the cycle), can initiate apoptosis
p53 becomes activated in response to stress eg DNA damage - it is stabalised and accumulates is phosphorylated and activated as a transcription regulator
MAPK, ATR and ATM are kinases that can activate p53 and are recruited and activated by dna damage eg double strand breaks
activation of p53 leads to activation of a number of genes whose products trigger cell cycle arrest, apoptosis or DNA repair
how is p53 affected in liver cancer *
it is functionally attenuated by hep B virus X protein and is a mutational target of aflatoxin B1
targets of mutations in cancer *
Genes encoding proteins that promote cell growth or survival are common targets of activating mutation in cancers – oncogenes. In contrast, genes encoding proteins that restrain cell growth are frequently mutated to reduce their activity, thus favouring growth – tumour suppressors.
oncogenes *
EGFR/HER2, mutationally activated or overexpressed in many breast cancers
Ras, mutationally activated in many cancers (>15%)
CyclinD1, overexpressed in 50% of breast cancers
B-Raf, mutationally activated in melanomas;; v-Raf (deletion of regulatory domain)
c-Myc, overexpressed in many tumour
tumour suppressors *
p53 - mutated in 50% of cancers
PTEN - dephosphorylates phosphoinositides at 3’-position - loss of PTEN activates the PKB/Akt pathway, promotes cell survival.
pRb, inactivated in many cancers
p27KIP1, underexpression correlates with poor prognosis in many malignancies
which cancers is PTEN affected in *
- mutated in multiple advanced cancers - frequency similar to p53.
Deletion in chromosomal region 10q22-25 observed in many cancers: prostate, renal, endometrial, melanoma, glioma, meningioma
