lecture 6 - HIF and p53 Flashcards
what protein facilitates the attachment of ubiquitin chains to target protein
E3 ubiquitin ligase
what is hypoxia
lowering of the oxygen concentrations compared to the normal levels cells are exposed to
anoxia is the
absence of oxygen completely
as a tumor grows the core becomes hypoxic the tumor then initiates
the hypoxia response
3 cellular responses to hypoxia
restoration of oxygen homeostasis
cell survival
cell death
HIF system
hypoxia inducible factor
HIF is a heterodimeric transcription factor
HIF-alpha and HIF-1beta
3 alpha subunits (1alpha, 2alpha, 3 alpha)
HIF-1alpha: expressed in all tissues
HIF structure
helix-loop-helix: mediates DNA binding
C-terminal transactivation domain (CTAD): allows them to activate transcription
key thing they have which allows them to repond to oxygen levels is
ODD (oxygen dependent degradation) domain
HIG-1beta doesnt have an
ODD domain
HIF-1alpha regulation at normal levels of oxygen
PHD (proline hydroxylase) mediate hydroxylation (add OH) of HIF-1a at at proline residues in the ODD domain
hydroxylated prolines allow recruitment of
VHL (Von Hippel Lindau protein) which is an E3 ubiquitin ligase which stimulates ubuiqitination of HIF-1a and degrades it
HIF-1a is being made continuously by the cell but its also being
continuously degraded
if O2 levels drop the PHD and FIH cant function as
they require the oxygen in order to induce the hydroxylated so HIF-1a stabilises and can carry out its functions
FIH (factor inhibiting HIF) hydroxylases an asparagine domain in the CTAD this
inhibits transcriptional activity
VHL binds to
hydroxylates prolines
when HIF-1a is stabilised is dimerizes with HIF-1b
dimer is the active transcription factor
CTAD isnt hydroxylated by FIH so it can bind to
p300/CBP do activator proteins to regulate transcription
HIF controls many targets
oxygen supply
transcription
cellular metabolism
cell growth/death
HIF control
p53 (tumour suppressor and transcription factor) functions as a
tetramer
p53 responds to DNA damage
normally its inactive and bound to inhibitor mdm2
if the cell recieves a DNA damage stimulus mdm2 becomes
inactivated and p53 becomes active
primary response of p53 is to induce
cell cycle arrest:
cell stops proliferating and allows a chance for the DNA repair and the the cell cycle restarts
p53 also induces
apoptosis if DNA damage is too severe
eliminate the damages cell
p53 prevents the development of
cancer
role of mdm2
p53 is being made continuously by the cell but being continuously degraded as mdm2 is also a
E3 ubiquitin ligase, promotes the ubiquitination of p53 leading to its degradation by proteasome
keeps p53 levels low in undamaged cells
p53 and mdm2 get phosphorylated by ATM kinases. what does this phosphorylation disrupt
the interactions between p53 and mdm2
p53 also regulates the activity of mdm2 this forms
a negative feedback loop that limits the extent of p53 activation
p14ARF is induced by increased cellular proliferation. p14ARF binds to
mdm2 and blocks it form binding to p53 and increases levels of p53
inactivation of p53 causes most cancer types
can occur through mutation of ARF or ATM kinase
causes amplification of mdm2 so p53 is inactive
mutations occur in hotspots which are mainly in the
DNA-binding domain of p53
Li-fraumeni sydrome (LFS)
born with mutations in one copy of their p53 gene in DNA binding domain
they have a high percentage of cancer development and early onset
illustrates how efficient these pathways normally are
