Lecture 14 - cancer Flashcards
Anoikis
Cells which lose contact with the ECM undergo it (apoptosis) - a means to help prevent metastasis
Nascent neoplastic cells
Cells which have their cancerous activity initiated - typically self-destruct by apoptosis
Bcl-2: what is it, how was it first identified, what is it generated by and what are its most common mutations
B cell lymphoma proteins that control the mitochondrial (intrinsic) pathway of apoptosis
Bcl-2 was first identified as an oncogene in human follicular B-cell lymphoma
Defective VDJ arrangement (the gene rearrangement to produce a diverse repertoire of b cells) that occurs during b-cell development
- Reciprocal translocation between t(14;18)
- 14q32 IgH heavy chain JH region
- 18q21 Bcl2 gene
- Generated by defective VDJ (variable, diversity, joining) rearrangements during B-cell development
BH-3 proteins: what are they, what do they promote and what do they inhibit?
Proteins that interact with pro/anti-apoptotic multi-domain Bcl-2 proteins via their BH3-domain
Pro-apoptotic proteins (ie Bax and Bak)
Inhibit the anti-apoptotic activity of Bcl-XL and Bcl-2
Bax and Bak: what are they and what do they do?
Pro-apoptotic proteins
Directly permeabilise the outer mitochondrial membrane (causing cytochrome c release - apoptosis)
What methods of Bcl-2 protein control are there?
Multiple mechanisms exist to regulate the expression and activity of BH3-only proteins downstream of cell stress signals
- Transcription
- Degradation control
- BH3 proteins can be sequestered in inactive forms or require proteolytic cleavage for activation
What models are there for how BH3-only proteins activate apoptosis and what are they based on?
- Direct binding and activation of BAX/BAK
- Neutralization of anti-apoptotic BCL-2 family proteins and displacement of BAX/BAK
Based on the assumption that the key role of BH3-only proteins in apoptosis is the interaction through their BH3-domains with multidomain BCL-2 family proteins
In the first model, the priority and significance in mediating the irreversible decision to die is dependent on binding to multidomain pro-apoptotic proteins BAX and BAK
The second model is based on the near-universal property of BH3-only proteins to interact with the anti-apoptotic BCL-2 family members, and BAK is postulated to be released from complexes with Bcl-XL and Mcl-1
Both models agree that BAX and BAK are activated as the result of binding - according to the direct activation model, BAX and BAK are postulated to be conformationally activated as a result of binding with activator BH3-only proteins (such as tBID and BIM).
PI3K-Akt: what is it, what is its role in tumour formation, where can PI3K be found, what is the mechanism behind its role in tumour formation, and so what are PI3K, PTEN, and PIP3?
Phosphatidylinositol-3 kinase, protein kinase B which regulates cell survival and proliferation
Dysregulated almost in all human cancers
PI3K has an SH2 domain so it can be found at the membrane after being recruited to it by Ras where it can complete its function
Activation of RTKs like the insulin-like growth factor receptor (IGFR) causes PI3K to be potently stimulated, resulting in its p110 catalytic domain phosphorylating phosphatidylinositol (4,5) diphosphate (PIP2), generating a second messenger phosphatidylinositol (3,4,5) triphosphate aka PIP3 - PIP3 is recognized by proteins like the ser/thr protein kinase Akt/PKB and Akt is phosphorylated and activated by two PIP3-dependent kinases at the membrane
- PI3K is an oncoprotein
- PTEN is a phosphatase which returns PIP3 to PIP2 - it is a tumour suppressor
Bad: what is it regulated by and what does it do in the absence of a survival signal?
Phosphorylation by Akt
Associates with mitochondrial-associated Bcl-2 proteins and activate mitochondrial outer membrane permeability (MOMP)
p53: what is it, where are the majority of its mutations in cancer, what results from the mutation, and what does it do?
Tumour suppressor protein
Majority of mutations - missense in the DNA binding domain
Somewhat of a gain of function in its mutation
p53 protein levels are elevated by a number of noxious or stressful stimuli or danger signals, both external (including the majority of chemotherapeutic agents and radiotherapy) and internal in origin (these include oncogene activation for instance), a number of cellular responses dependent on p53 are then possible influenced by the intensity and duration of the stimulus and cell-type. Cells may undergo cell cycle arrest, this buys time to repair DNA, and return to proliferation or become irreversibly growth-arrested (senescence).
Proteasomal degradation is the principle mechanism regulating p53 protein levels. Mdm2 is an E3 ligase responsible for conjugating poly-ubiquitin to p53. Poly-ubiquitylated p53 is degraded by the proteasome. In ‘happy’ cells p53 levels are kept very low by this mechanism. Interference though with this pathway allows p53 mechanisms to rise rapidly. Q: How then do the various inputs create a pause in p53 degradation?
Critical transducer of abnormality signals - While a useful adaptation by vertebrates that has allowed them to live longer than invertebrates despite the insults thrown by nature, p53 as the single watchman in the cell (or guardian of the genome) is a weak link in the line of defence against neoplasia. In a single stroke, loss of p53 greatly increases a cells chance to become neoplastically transformed by rendering it resistant to a number of insults such as loss of survival signalling or hypoxia that ordinarily would hamper tumour progression were p53 present to induce apoptosis. Likewise p53 mutant cells are blinded to the activation of oncogenes. Perhaps worse, loss of p53 allows cells to forego DNA damage repair and this enables the accumulation of the multiple mutations which drive tumour progression. p53 loss also confers resistance to chemotherapy and radiotherapy.
Control of p53 levels by various kinases
A: Ser/thr phosphorylation of the N-terminus of p53 by damage sensors e.g. Chk2, ATM, ATR blocks association with Mdm2 and thereby spares p53 from proteasomal degradation.
why Q: Why then does p53 dominate the regulation of apoptosis?
A: Because it transduces a plethora of stress/danger signals and promotes extrinsic and intrinsic apoptosis signalling and suppresses survival signalling.
How many pathways for apoptosis are there?
Numerous
ABT-263 is a small molecule BH3 mimetic which competes with BH3 proteins for binding to certain anti-apoptotic Bcl2 proteins
ABT-263 is a small molecule BH3 mimetic which competes with BH3 proteins for binding to certain anti-apoptotic Bcl2 proteins
Targeting resistance to apoptosis for cancer therapy
Bcl-2, Bcl-XL, and Bcl-w
Mcl-1, Bcl-A1
Nutlin-2 binds the p53-binding pocket of Mdm2 inhibiting its interaction with p53 thereby preventing p53 degradation and inducing apoptosis in cancer cells.
Nutlin-2 binds the p53-binding pocket of Mdm2 inhibiting its interaction with p53 thereby preventing p53 degradation and inducing apoptosis in cancer cells.
