molecular basis of cancer Flashcards
name the 6 classic hallmarks of cancer
1) Sustaining proliferative signalling – don’t need as much nutrients and growth factors
2) Evading growth suppressors
3) Activating invasion and metastasis
4) Enabling replicative immortality
5) Inducing angiogenesis – development of new blood vessels (recruit other cells)
6) Resisting cell death
name the 2 emerging hallmarks of cancer and the 2 enabling characteristics of cancer
HALLMARKS
1) Avoiding immune destruction
2) Tumour-promoting inflammation – macrophages infiltrate, make them produce factors to promote growth instead of hindering it
ENABLING
1) Genome instability and mutation – thing that causes cancer
2) Deregulating cellular energetics – cancer cells can change the way they metabolise compared to normal cells – use glycolysis even when oxygen is present undergo ‘metabolic switch’
how do cells sustain proliferative signalling?
3 main ways:
modulation of growth factor (GF) provision
modulation of GF receptor activity
modulation of intracellular signalling pathways
how do cancer cells modulate growth factor (GF) provision?
they can:
1) stimulate normal cells in the microenvironment (stroma) to provide cells with GFs – eg macrophages releasing lots of GFs in tumour environment
2) produce their own mitogenic GFs and respond to them through autocrine mechanisms (self-sufficient process)
3) make other cells produce mitogenic GFs through paracrine mechanisms
how does GF signalling work?
GF binds receptor tyrosine kinase –> dimerisation –> autophosphorylation. Some proteins dock with active receptor and become activated by phosphorylation.
Some proteins act as substrates for receptor kinase and become tyrosine phosphorylated and phosphorylate other proteins (mitogen activated protein kinase pathway)
end result = final protein that phosphorylates and activates a TF = signal transduction in cell
how do cancer cells modulate GF receptor activity?
many GF receptors are proteins TKs. Overexpression of these allows tumours to respond to low levels of GF that wouldnt usually produce a response
name the effects of overexpression of GF receptors (protein TKs) and give examples
overexpression may lead to:
ligand independent signalling
structurally altered receptors (eg truncated EGF receptor is constitutively active)
give 2 examples of ligand-independent signalling
EGF-R is up-regulated in stomach, breast and brain tumours and HER2 is over expressed in breast cancer
give 2 examples of truncated proteins that have resulted in the modulation of GF receptor activity and explain how this has happened
EGF receptor mutated (with deletion of the EGF receptor part) forms ErbB oncoprotein - no receptor = ligand independent = dimerisation = constitutively active TK
Her2 gene point mutation leads to change in membrane embedded region (Val to Gln) forms Neu oncoprotein => dimerizaton and autophosphorylation (ligand independent) – antibody (herceptin) that stops EGF from binding Her2 no longer works in patient
which pathway is frequently modulated to increase the amount of GF that cancer cells can bind?
SOS-Ras-Raf-MAPK pathway
how is Ras normally activated?
Ras binds GTP (changing Ras’s conformation) allowing it to bind and activate Raf. Raf then dissociates (by GTP hydrolysis to GDP), is phosphorylated, activated and continues transducing.
Ras is still bound to GDP. SOS comes along and removes GDP. GTP now binds to Ras, reactivating it.
name the mutation in Ras that causes it to become hyperactive. explain why this happens and what the result of this is.
point mutation in protein resulting in changing Gly12 to any other a/a.
Locks Ras in hyperactive, permanently on state. GTP can’t hydrolyse to GDP, Ras keeps binding to Raf repetitively. This means pathway is constantly on, leading to uncontrolled growth.
what are the effects of mutations in PI3 Kinase and Ras?
PI3K can frequently mutate in many cancers, causing it to become more active. This means cells develop survival signals that protect against apoptosis.
name 2 effects of growth supressors
driving cells out of the cell cycle
inducing entry into a post-mitotic differentiated state
what are inhibitory signals and how do they work?
similar to GFs theyre generated by signals binding to surface receptors
purpose is to drive cells out of the cell cycle (eg G0) or differentiated state.
give an example of an inhibitory signal and its mode of action.
TGF-beta binds receptors and stimulates SMADs (proteins) which activate p15, p27 and p21. These proteins inhibit the cell cycle.
give 2 ways in which TGF-beta action is blocked
down regulation of TGF-beta receptor
mutation of TGF-beta receptor to inactive or less active form
describe the 2-hit hypothesis and give an example of it
hypothesis: cancers usually require more than 2 mutations to form
eg Rb mutation in one gene and develop mutation in other gene = more likely to develop retinoblastoma cancer
what is Rb? what are the effects of the loss of function in Rb?
Rb is the gatekeeper of the cell cycle, it decides which cells should(n’t) proceed through the cell cycle.
Loss of function = persistent cell proliferation.
with respect to E2F, how does Rb act as a tumour suppressor ?
Rb binds E2F (a TF) and stops it from transcribing DNA. The inactivation of E2F halts cell cycle.
how does EGF stop Rb from acting as a tumour suppressor?
EGF binds either EGF-R or HER2, stimulating the Ras-MAPK pathway. This results in CDK binding cyclin which then phosphorylates Rb. Rb can now not bind to E2F and there is transcription.
describe in detail the stages of the cell cycle after stimulation by a GF (include checkpoints).
GF stimulates CDK1 & CDK2 and cyclins D1, D2, D3 which push the cell into G1. Gets past checkpoint by phosphorylating Rb which dissociates from E2F = gene transcription.
Because there is gene transcription there is more CDK2/cyclin E which allows the cell to enter S phase. This produces CDK2 and cyclin A.
Second checkpoint where p53 checks for errors in DNA replication, if there is none then cell enters G2 and M phase where CDK1 and cyclin B are present => cell cycle can continue.
Antisignals TGFβ via SMADs upregulate p21 and p27 which bind CDK2/4-cyclin D (inhibits them), therefore can’t phosphorylate Rb.
Cell must avoid use of antisignals which it does by automatically altering/removing Rb (remember Rb plus E2F1 = no transc)
how do DNA viruses induce tumours?
Rb is inactivated by being complexed with a viral protein. In human cervical tumours this is the E7 protein produced by HPV or the E6 protein which binds p53 and promotes its destruction.
do you tumours under or over produce c-myc?
overproduce
describe the steps of apoptosis
trigger
chromatin condenses and cytoplasm shrinks
blebbing
nucleus fragments by caspases, DNA ladderings, cell fragmentation
phagocytosis by macrophages
name 4 possible external triggers of apoptosis
not enough GFs
hypoxia
activation of death receptors (eg by p53)
loss of adhesion (to other cells)
name 3 modulators of apoptosis
Bcl-2 family
p53
Mdm2
what are the effectors of apoptosis?
caspases which act on substrates and activate a proteolytic cascade
what the substrates for apoptosis? (ie which parts of the cell are broken down?)
many cellular proteins
DNA
what is the difference in the extrinsic and intrinsic pathways of apoptosis initiation?
extrinsic pathway uses fas ligand and receptor and tumour necrosis factor (TNF) and TNF receptor intrinsic pathway uses intracellular signals (p53, Bcl2, Bax)
describe the extrinsic caspase pathway
death ligand (fas/TNF) –> death receptors –> caspases (8 for extrinsic pathway which triggers 3,6,7 which cleave cell and DNA and result in apoptosis) –> cell death
describe the intrinsic caspase pathway
Apoptotic signal ER stress and DNA damage and cell cycle stress –> activate BAK & BAX –> downregulates Bcl2 –> BAK and BAX makes holes I mitochondria –> cytochrome C leaks out –> Apaf-1 with cytochrome C activate caspase 9 by apoptosome formation –>activates caspases 3,6,7 –> cell death
why are p53, MDM2 and Bax unregulated in cancer and Bcl2 upregulated?
p53, MDM2, Bax normally stimulate apoptosis – downregulated/mutated
Bcl2 normally inhibits apoptosis – upregulated
how does Bcl2 (the protein) inhibit apoptosis?
Bcl2 inhibits apoptosis by binding and inactivating pro-apoptotic proteins BAX and BAK, in the mitochondrial membrane.
BAX and BAK (pro-apoptotic) cause release of cytochrome c which activates caspases leading to apoptosis
