HIF mechanism and regulation

Question 1

Oxygen is an essential component for life on earth

Answer 1

proving a fundamental element in the survival of aerobic organisms

Question 2

In healthy humans

Answer 2

physiological oxygen gradients exist throughout the tissues and cells of the body

Question 3

The partial pressure of oxygen (PO2) in the atmosphere

Answer 3

sits at approximately 156mmHg

Question 4

Dropping to 100mmHg in the alveoli and

Answer 4

as low as 25mmHg in the retina

Question 5

These respective oxygen levels are normal for these regions

Answer 5

facilitating cellular respiration by the mitochondria of each cell to generate energy in the form of ATP through oxidative phosphorylation

Question 6

In normal oxygen environments

Answer 6

mitochondria consume 90% of the O2 available achieving a net production of 34 ATP per glucose molecule through oxidative phosphorylation

Question 7

However in circumstances

Answer 7

such as travelling to high altitude or in certain lung diseases tissues and cells of the body can become hypoxic

Question 8

Hypoxia is defined as the physiological condition whereby

Answer 8

oxygen demand exceeds available supply

Question 9

insufficient ATP production by mitochondria during hypoxia can

Answer 9

alter cellular homeostasis ultimately leading to a state of bioenergetic crisis

Question 10

during chronic hypoxic conditions

Answer 10

cells can experience oxidative stress due to impaired energy production

Question 11

knock on effects such as

Answer 11

cell death, tissue damage and organ dysfunction

Question 12

to prevent these drastic consequences therefore

Answer 12

animals have evolved the ability to adapt to chronic hypoxia on a cellular level in order to ensure homeostasis and survival

Question 13

hypoxia inducible factor (HIF)

Answer 13

highly conserved
heterodimeric TF
exists in 3 isoforms
HIF-1, HIF-2, HIF-3

Question 14

HIF-1

Answer 14

central player in cellular response to hypoxia
constitutive B subunit
oxygen-sensitive a subunit

Question 15

discovery of HIF-1 pathway

Answer 15

combination effort by three clinician scientists
Gregg Semenza
Peter Ratcliffe
William Kaelin Jr

Question 16

shared 2019 nobel prize in phys or medicine for

Answer 16

seminal research on how cells sense and adapt to oxygen availability

Question 17

In physiological normoxia…. 90%

Answer 17

90% oxygen consumed by mitochondria to produce ATP

Question 18

Remaining 10%

Answer 18

used in degradation of constitutively expressed HIF1a mRNA

Question 19

In normoxic conditions… prolyl

Answer 19

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)

Question 20

PHD also exists in three isoforms

Answer 20

PHD1, 2, 3
Can hydroxylate 2-oxyglutarare
Ascorbate
Iron

Question 21

Hydroxylated HIF-1a recognised and targeted for…..

Answer 21

proteosomal degradation by the von hippel lindau (VHL) tumour suppressor protein

component of the E3 ubiquitin lipase complex (maxwell and ratcliffe, 1999)

Question 22

in particular, hydroxylation of the proline 564 on HIF-1a

Answer 22

is critical for VHL interaction (Jaakola, 2001)

Question 23

factor inhibiting hif (FIH)

Answer 23

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

Question 24

Thus together, 2-oxyglutarate-dependent-dioxygenases…..

Answer 24

PHD and FIH work effectively to prevent accumulation and transactivation of the HIF1 pathway in normoxic conditions

Question 25

In hypoxic conditions....mito

Answer 25

mitochondria consume 100% of the oxygen available to the cell in order to maintain cellular respiration

Question 26

HIF degradation by oxygen-dependent hydroxylases is prevented

Answer 26

in hypoxic conditions

Question 27

the oxygen-dependent hydroxylation of HIF1a subunit by......

Answer 27

PHD and FIH is reduced resulting in rapid HIF1a stabilisation and accumulation

Question 28

HIF1a migration into the nucleus and dimerisation with.....

Answer 28

HIF1B to induce an adaptive transcriptional response through CBP/p300 interaction (Semenza, 1993)

Question 29

HIF complex binds to hypoxia response elements (HRE).....in

Answer 29

promoter regions of target genes to promote transcription of genes involved in adaptive responses to hypoxia (Semenza, 1992)

Question 30

Target genes

Answer 30

VEGF (angiogenesis) iNOS (vasodilation) EPO (erythropoiesis) glycolytic enzymes, GLUT1&3, LDH (glycolysis)

Question 31

HIF master regulator

Answer 31

associated with regulation of over 200 genes in adaptive cellular response to hypoxia

Question 32

Promotion of erythropoiesis

Answer 32

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

Question 33

Formation of new blood vessels (angiogenesis)

Answer 33

vascular endothelial growth factor (VEGF) promotes development of extensive vascular network enhance oxygen supply to tissues cell survival and function

Question 34

vasodilation (iNOS)

Answer 34

increase blood flow optimise O2 transport to tissues support tissue oxygenation cellular function under hypoxic conditions

Question 35

in combo, EPO, VEGF and iNOS....

Answer 35

results in net oxygen delivery to tissue in order to maintain survival

Question 36

HIF mediates glycolysis.....the metabolic

Answer 36

the metabolic pathway involved in generation of energy without relying on oxygen-dependent processes

Question 37

under hypoxic conditions HIF promotes expression of glycolytic enzymes...

Answer 37

facilitating the conversion of glucose to ATP in the absence of oxygen

Question 38

metabolic adaptation ensures cellular energy production...

Answer 38

in oxygen deprived environments

Question 39

HIF upregulates glucose transporters

Answer 39

GLUT1&3

Question 40

Increased LDH expression...

Answer 40

to ensure efficient conversion of pyruvate to lactate preventing buildup of pyruvate in absence of TCA cycle and allowing glycolysis to proceed

Question 41

In this way HIF promotes....

Answer 41

ATP production in order to maintain cellular homeostasis

Question 42

While the HIF pathway is highly adapted and regulated mechanism in cellular response to hypoxia

Answer 42

dysregulation involves VHL disease

Question 43

VHL disease is a rare genetic disorder...

Answer 43

caused by defective oxygen dependent gene expression

Question 44

In VHL disease, the cell believes...

Answer 44

it is hypoxic when it is not leading to aberant HIF pathway activation triggering expression of genes involved in various cellular responses to hypoxia

Question 45

Processes triggered in VHL disease

Answer 45

angiogenesis erythropoeisis glycolysis adaptive mechanisms

Question 46

patients with VHL disease suffer an increased risk of....

Answer 46

tumour and cyst development especially in highly vascularised tissues

Question 47

hemangioblastomas

Answer 47

hallmark feature of VHL high predisposition to developing these vascular tumours in CNS brain and spinal cord