PI3K inhibitors Flashcards
What is the general substrate of PI3K
phosphatidyl-inositol
Where can the inositol head group be phosphorylated
3,4,5 free OH groups
Which OH groups cannot be phosphorylated on the inositol head group of PI
Positions 2 and 6
Nor 1 because it’s already bound to the phosphate group linking it to the DAG lipid anchor
What determines where a phosphorylated PtdIns will localise intracellularly?
The positions in which it is phosphorylated
How does PI3P concentration change in response to growth factor stimulation
Doesn’t change, considered to be constitutive lipid
How do PIP3 and PI(3,4)P2 concentrations change after growth factor stimulation
Levels of the lipids rise rapidly following growth factor stimulation
what is the result of a PtdIns being phosphorylated at the 3-OH position?
It’s no longer a substrate for phospholipase
What kind of proteins do PIP3 and PI(3,4)P2 bind to
Proteins with a plextrin homology domain which is found in protein kinases, adaptor proteins and regulators of small GTPases
What protein domains do PI(3)P bind to? and where are these found?
FYVE - Mostly in proteins involved in membrane trafficking
PX - in proteins that regulate NADPH oxidase complex, endocytic trafficking
What are the four members of the class I PI3K isoforms
p110alpha/PIK3CA, p110beta/delta/gamma
What are the 3 members of the class II PI3Ks
C2alpha, C2beta, C2gamma
What is the class III PI3K
vps34
What is the substrate of the class I PI3Ks and what do they produce from it
PI(4,5)P2 is the substrate and it forms PIP3
How does PIP3 bind to downstream protein substrates
Via the proteins PH domain
What is the substrate of class II/III PI3Ks
PI and converts it to PI(3)P which binds its target proteins by FYVE and PX domains.
Why is there a delay in the production of PI(3,4)P2 compared to PI(3,4,5)P3 following growth factor stimulation?
PIP3 is produced first, then through the action of 5-phosphatase PIP3 is used to make PI(3,4)P2
What are the class 1A PI3Ks
p110alpha, beta and delta
what is the class 1B isoform of PI3K
p110 gamma
How are class 1A PI3Ks activated?
By tyrosine kinase receptors, the SH2 domain of the regulatory subunit on each isoform binds to the phosphorylated kinase domain on the tyrosine kinase
Also activated by Ras/Rac/cdc42
How are class 1B PI3Ks activated?
By GPCRs, also by Ras/Rac/cdc42
What is the lipid phosphatase that regulates the production of PIP3
PTEN
Which position on PIP3 is dephosphorylated by PTEN
At the 3 position
Can PI3K signalling cause treatment resistance
Yes, following hormonal therapy, chemotherapy, radiotherapy and targeted agents
Which parts of the PI3K pathway often become mutated in cancer
Oncogenic tyrosine kinases and Ras
PIK3CA/p110 alpha frequently mutated in cancers
Akt/PKB is activated as a result of mutation or upstream PI3K activation
What are the hotspots in which PIK3CA becomes mutated in solid tumours?
The helical domain and the kinase domain.
What kind of mutations are found in PIK3CA in segmental overgrowth syndromes?
Mosaic mutations in PIK3CA during embryonic development
Why does the inhibition of mTORC1 using rapamycin actually increase the amount of PI3K signalling
Because: Downstream of mTORC1 is s6K1, a kinase which inhibits IRS-1 which is responsible for PI3K activation. By inhibiting mTORC1, this negative feedback loop no longer functions and PI3K signalling is switched on and other targets of AKT can be targeted (not just mTOR)
Can cells survive with a low amount of PI3K activity? and does this effect proliferation?
Cells can survive and proliferate normally with <10% of their class 1A PI3K activity
What kind of PI3K inhibitor is Piqray?
A p110 alpha inhibitor.
What combination therapy is used in ER+ breast cancer with Piqray and why is this the case?
p110 alpha inhibitors in ER+ breast cancer increases the expression of ER. Therefore, when combined with hormonal therapy there is a synergistic effect on tumour regression
Where is p110 delta mainly expressed?
Leukocytes
Does PI3K delta expression increase in leukemic cells compared to normal ones?
No, its present at high levels in both cell types
In leukemia, is PI3K delta mutated?
Very rarely
Which specific haem-malignancies are PI3Kdelta inhibitors effective in
B cell malignancies. Specifically B cell chronic lymphocytic leukaemia (CLL) and B cell follicular lymphoma
Why is PI3Kdelta inhibition only effective in B cell malignancies?
In CLL, B-cells have a particular life cycle, going between the lymph nodes and bone marrow and then into circulation. To access the lymph nodes/bone marrow they respond to chemokines, to stay they resond to adhesion molecules, for co stimulation they respond to B-cell antigen receptors; all of which rely on PI3K delta. If this is blocked, the cells can no longer respond to the signals and accumulate in the circulation. In the circulation they are vulnerable to combination therapy (anti CD20 immunotherapy)
Why are normal B cells dependent on PI3Kdelta for survival?
Signalling through adhesion molecules, chemokines, co-stimulatory molecules and B-cell antigen receptors all rely on PI3K delta for the modulation of these signals
Are PI3K delta inhibitors responsible for the cytotoxic effect on B-cells
No, only make them vulnerable to the therapy used in combination
What effect does PI3K delta inhibition have on different T cell populations and what effect does this have on solid tumours?
Greatly inhibits Tregs whilst only having a limited inhibition on effector T cells (Th and cytotoxic T cells) This tips the balance towards an active, anti-cancer response.
