HIF mechanism and regulation Flashcards
Oxygen is an essential component for life on earth
proving a fundamental element in the survival of aerobic organisms
In healthy humans
physiological oxygen gradients exist throughout the tissues and cells of the body
The partial pressure of oxygen (PO2) in the atmosphere
sits at approximately 156mmHg
Dropping to 100mmHg in the alveoli and
as low as 25mmHg in the retina
These respective oxygen levels are normal for these regions
facilitating cellular respiration by the mitochondria of each cell to generate energy in the form of ATP through oxidative phosphorylation
In normal oxygen environments
mitochondria consume 90% of the O2 available achieving a net production of 34 ATP per glucose molecule through oxidative phosphorylation
However in circumstances
such as travelling to high altitude or in certain lung diseases tissues and cells of the body can become hypoxic
Hypoxia is defined as the physiological condition whereby
oxygen demand exceeds available supply
insufficient ATP production by mitochondria during hypoxia can
alter cellular homeostasis ultimately leading to a state of bioenergetic crisis
during chronic hypoxic conditions
cells can experience oxidative stress due to impaired energy production
knock on effects such as
cell death, tissue damage and organ dysfunction
to prevent these drastic consequences therefore
animals have evolved the ability to adapt to chronic hypoxia on a cellular level in order to ensure homeostasis and survival
hypoxia inducible factor (HIF)
highly conserved
heterodimeric TF
exists in 3 isoforms
HIF-1, HIF-2, HIF-3
HIF-1
central player in cellular response to hypoxia
constitutive B subunit
oxygen-sensitive a subunit
discovery of HIF-1 pathway
combination effort by three clinician scientists
Gregg Semenza
Peter Ratcliffe
William Kaelin Jr
shared 2019 nobel prize in phys or medicine for
seminal research on how cells sense and adapt to oxygen availability
In physiological normoxia…. 90%
90% oxygen consumed by mitochondria to produce ATP
Remaining 10%
used in degradation of constitutively expressed HIF1a mRNA
In normoxic conditions… prolyl
prolyl hydroxylase domain (PHD) enzymes use oxygen as substrate in order to add specific proline residues to the O2 dependent degradation domain (ODD) of the HIF-1a subunit (ratcliffe and kaelin, 2001, independently)
PHD also exists in three isoforms
PHD1, 2, 3
Can hydroxylate 2-oxyglutarare
Ascorbate
Iron
Hydroxylated HIF-1a recognised and targeted for…..
proteosomal degradation by the von hippel lindau (VHL) tumour suppressor protein
component of the E3 ubiquitin lipase complex (maxwell and ratcliffe, 1999)
in particular, hydroxylation of the proline 564 on HIF-1a
is critical for VHL interaction (Jaakola, 2001)
factor inhibiting hif (FIH)
is a second enzyme involved in HIF regulation where hydroxylation of asparagine residues on HIF1a blocks association with transcriptional activators CBP/p300 to inhibit transcriptional HIF activation
Thus together, 2-oxyglutarate-dependent-dioxygenases…..
PHD and FIH work effectively to prevent accumulation and transactivation of the HIF1 pathway in normoxic conditions
In hypoxic conditions….mito
mitochondria consume 100% of the oxygen available to the cell in order to maintain cellular respiration
HIF degradation by oxygen-dependent hydroxylases is prevented
in hypoxic conditions
the oxygen-dependent hydroxylation of HIF1a subunit by……
PHD and FIH is reduced resulting in rapid HIF1a stabilisation and accumulation
HIF1a migration into the nucleus and dimerisation with…..
HIF1B to induce an adaptive transcriptional response through CBP/p300 interaction (Semenza, 1993)
HIF complex binds to hypoxia response elements (HRE)…..in
promoter regions of target genes to promote transcription of genes involved in adaptive responses to hypoxia (Semenza, 1992)
Target genes
VEGF (angiogenesis)
iNOS (vasodilation)
EPO (erythropoiesis)
glycolytic enzymes, GLUT1&3, LDH (glycolysis)
HIF master regulator
associated with regulation of over 200 genes in adaptive cellular response to hypoxia
Promotion of erythropoiesis
EPO stimulates production of RBC in response to low O2 enhancing oxygen carrying capacity of blood and ensuring efficient oxygen delivery to tissues and supporting cellular adaptation to hypoxia
Formation of new blood vessels (angiogenesis)
vascular endothelial growth factor (VEGF)
promotes development of extensive vascular network
enhance oxygen supply to tissues
cell survival and function
vasodilation (iNOS)
increase blood flow
optimise O2 transport to tissues
support tissue oxygenation
cellular function under hypoxic conditions
in combo, EPO, VEGF and iNOS….
results in net oxygen delivery to tissue in order to maintain survival
HIF mediates glycolysis…..the metabolic
the metabolic pathway involved in generation of energy without relying on oxygen-dependent processes
under hypoxic conditions HIF promotes expression of glycolytic enzymes…
facilitating the conversion of glucose to ATP in the absence of oxygen
metabolic adaptation ensures cellular energy production…
in oxygen deprived environments
HIF upregulates glucose transporters
GLUT1&3
Increased LDH expression…
to ensure efficient conversion of pyruvate to lactate preventing buildup of pyruvate in absence of TCA cycle and allowing glycolysis to proceed
In this way HIF promotes….
ATP production in order to maintain cellular homeostasis
While the HIF pathway is highly adapted and regulated mechanism in cellular response to hypoxia
dysregulation involves VHL disease
VHL disease is a rare genetic disorder…
caused by defective oxygen dependent gene expression
In VHL disease, the cell believes…
it is hypoxic when it is not leading to aberant HIF pathway activation triggering expression of genes involved in various cellular responses to hypoxia
Processes triggered in VHL disease
angiogenesis
erythropoeisis
glycolysis
adaptive mechanisms
patients with VHL disease suffer an increased risk of….
tumour and cyst development
especially in highly vascularised tissues
hemangioblastomas
hallmark feature of VHL
high predisposition to developing these vascular tumours in CNS
brain and spinal cord
