HIF1 pathway Flashcards
What in vitro conditions are used to mimic hypoxic conditions
1% oxygen
What is anoxia
Severe/complete deprivation of oxygen
In what normal physiological conditions would hypoxia occur
Embryonic development, vigorous exercise, high altitude.
In general, what percentage range are cancer tissue oxygen levels found compared to in normal tissues?
1-2% (pancreatic is ~0.3%)
What effect does hypoxia have on radiotherapy?
Cells experiencing hypoxia are less likely to respond to radiotherapy
Why does hypoxia occur in solid tumours?
Rapid proliferation of cells leads to obstruction and compression of blood vessels. Also the vasculature is abnormal so is unable to supply the high demand for oxygen.
What is the distance from vasculature a cell needs to be to experience hypoxic conditions?
50 - 80 um
If a cell is 80um from the vasculature, does it experience chronic or acute hypoxia?
Chronic
What are HIFs?
Hypoxia-induced factors - Transcription factors that regulate transcription of hundreds of genes.
In what structural form do HIFs exist?
As heterodimers with an alpha and B subunit
What is the role of the HIF alpha subunit>
Sense oxygen levels
How is HIF-1a regulated under normoxic conditions?
1) Degredation: PHD proteins hydroxylate proline residues on HIF-1a. Hydroxylation causes pVHL to bind which ubiquitinates the TF, sending it for degredation at the proteosome. (Iron and 2-oxoglutarate are required for PHD activity)
2. Transcriptional deactivation: HIF-1a activity also regulated by factor inhibiting HIF (FIH) FIH hydroxylates HIF-1a at asparagine residues on its C terminus, blocking recruitment of coactivators rendering HIF-1a transcriptionally inactive.
How does HIF-1a regulation change following hypoxia
PHD and FIH are actively repressed, permitting HIF-1a protein to translocate to the nucleus and dimerise with HIF-1b. HIF-1a-ARNT heterodimer binds to hypoxia response elements concensus sequences on target genes.
How does HIF-1a expression change in cancer?
Upregulated due to hypoxic conditions in the tumour.
Loss of function mutations in VHL so no ubiquitination of HIF.
Inhibition of hydroxylases uninhibits HIF-1 transcription
Inhibition of tumour suppressor gene function
Constitutive activation of Ras-MAPK or PI3K-AKT pathways increase HIF-1a expression
What is commonly mutated in renal cell carcinoma and what is the result of this?
90% of tumours have somatic mutations in VHL gene. Germline inactivation is also associated with an increased risk of RCC. As a result HIF-1a cannot be tagged for ubiquitination and is constituitively expressed. Leads to the oversecretion of VEGF and greater vascular density.
How can HIF-1a be targeted in cancer therapy?
1) inhibition of HIF-1 dimerisation or coactivator recruitment
2) Inhibition of hypoxia response element transcriptional activity by small molecules
3) Activation of HI1-a protein degradation (PHD activators)
to a lesser extent:
4) Inhibition of signalling pathways upstream of HIF-1 (MAPK, PI3K)
5) Inhibition of chaperone proteins
6) Activation of p53
How is hypoxia detected in patients?
1) Oxygen tension electrodes
2) Immunohistochemistry markers
3) Gene signatures
4) Imaging
What are examples of immunohistochemistry markers of hypoxia>
Pimonidazole, EF5, Hypoxia-inducible proteins
What three hypoxia-induced proteins could be used for immunohistochemistry?
VEGF, CA9, GLUT1
How can PET imaging be used to detect hypoxic conditions?
Tag nitroimadazole with F18 to use as a PET radiotracer. This can be used in conjunction with FDG to show which nodules revealed by FDG are hypoxic
Why does chemotherapy not work as well on hypoxic tissues?
Because hypoxic cells tend to be less actively cycling, maybe quiescent, so won’t be targeted by chemo which targets proliferating cells.
