CMB2001/L06 Transcriptional Responses to Stress & Infection II Flashcards
What is the role of E3 Ubiquitin ligase?
Facilitates the attachment of ubiquitin chains to target proteins
Define hypoxia.
Lowering of the oxygen concentrations compared to sea level +-20.9% oxygen
Give 3 physiological processes that require a hypoxia response.
Placenta development
Heart
Bone
Vasculature development
What does hypoxia response allow adaptation to? (2)
High altitude living
Intense muscle exercise
Give 3 cellular responses to hypoxia.
Restoration of oxygen homeostasis
Cell survival
Cell death
What is the HIF system?
Hypoxia Inducible Factor
Heterodimeric transcription factor HIF-a and HIF-1B
Where is HIF-1a expressed?
Ubiquitously in all tissues
Where is HIF-2a expressed?
Restricted to certain tissues
Where is HIF-3a expressed?
Restricted to certain tissues
Lacks C-terminus transactivation domain
How does HIF-3a function?
As a dominant negative inhibitor for HIF-1a and HIF-2a
Has been shown to activate a different set of genes in hypoxia
What do Proline Hydroxylases (PHDs) require to function?
Oxygen
How is HIF-1a regulated?
Transcription
Translation - evades block
Post-translational regulation - lack of oxygen inactivates PHD proteins
Describe canonical regulation of HIF (in normoxia).
Oxygen sensing
Ubiquitination and degradation
Stabilisation in hypoxia
Dimerisation with HIF-1B
Transcriptional activation
Target genes
In what form can asparagine interact with target gene mRNA?
Unhydroxylated
Name 3 pathways controlled by HIF.
Oxygen supply
Transcription
Cellular metabolism
Cell growth
HIF control
Cell death
How does hypoxia aid cancer? (3)
Activation of HIF stimulates angiogenesis to bring oxygen and nutrients to tumour
Regulates other tumour characteristics: evasion and metastasis
Enhanced growth and survival and increased glycolysis
Describe the structure of p53.
Typical for transcription factors
Distinct DNA binding and multimerisation domains
Transactivation domain (TAD)
P - proline-rich domain
NLS- nuclear localisation sequence
Tet - tetramerisation domain
Give 3 positive roles of p53.
Tumour suppression
Stem cell modulation
Development
Fertility
Give 3 unfavourable roles of p53.
Ischemia
Treacher Collins syndrome
Neurodegeneration
Aging
What is the role of ARF?
p14^ARF - tumour suppressor
Disrupts interaction between p53 and Mdm2
Increases levels of transcriptionally active p53
When is p14^ARF expression induced?
As a result of increased cellular proliferation
What is the role of Mdm2? (3)
E3 ubiquitin ligase
Promote ubiquitination of p53 for degradation by proteasome
Keeps p53 levels low in undamaged cells
Describe how Mdm2 inhibits p53.
DNA damage causes phosphorylation of p53 at Ser15 by ARM or ATR kinases
Mdm2 phosphorylated
Disrupts interaction between p53 and Mdm2
Describe the Mdm2 feedback loop.
p53 stimulates Mdm2 expression
Negative feedback loop limiting extent of p53 activation
Give an example of negative feedback among transcription factor pathways.
p53 induces its inhibitor Mdm2 causing its proteolytic degradation
HIF1a induces PHD proteins that cause its proteolytic degradation
NF-kB induces its inhibitor IkBa that removes it from nucleus and retains it in cytoplasm
Explain how cancer cells inhibit p53.
Mutation causes loss of ARF/ATM function
Mdm2 amplification
Viral infection of E6
Mutation or defects with p53
Arrests apoptosis
What is Li-Fraumeni Syndrome?
Hereditary cancer disposition syndrome
Most commonly caused by mutation in gene TP53 for p53
How do the NF-kB and HIF pathways differ?
NF-kB activation involves degradation of an inhibitor (IkBa)
HIF and p53 are degraded continuously in unstimulated cells and become stabilised upon exposure to activating stimulus
