Lecture - Biochemical Consequences Of O2 Deficiency And Excess

Question 1

When O2 is toxic: retinopathy of prematurity

Answer 1

• loss of placental/maternal growth factors
• high O2 induces loss of retinal vessel
• High pO2 suppresses local growth factor production

Question 2

When O2 is toxic Bronchopulmonary dysplasia

Answer 2

• disrupted VEGF-nitric oxide signaling in endothelial colony-forming cells
• failure of normal alveolar development
• pulmonary hypertension

Question 3

Retinopathy in hyperoxia is dependent on growth factors

Answer 3

• in an IGFBP-3 null mice, the hyperoxic effect is enhanced, whereas in wt mouse the hyperoxic effect is resisted

Question 4

Reactive oxygen species

Answer 4

• in mitochondria
• plasma membrane: NADPH oxidase
• species include: hydrogen peroxyde, superoxide and hydroxyl radical

Question 5

Oxygen sensors

Answer 5

• prolyl hydroxylase: modulate stability of HIF, O2-dependent transctiption
• Heme oxygenase: putative O2 sensor of carotid body
• cytochrome oxidase: key O2 binding protein of mitochondria

Question 6

Role of mitochondria in ROS formation

Answer 6

• O2 leaks away because produced at too high level
• converted to ROS in mitochondria
• ROS also form at plasma membrane in hyperoxia - NADPH ocidase

Question 7

Reactive oxygen species

Answer 7

• superoxide
• hydrogen peroxide
• Hydroxyl radical
• Peroxynitrite
• pathophysiological mechanism via uncontrolled reactions with proteins
• anti-ROS defence: glutathione, redox proteins
• physiological mechanism via controlled reaction with proteins
• ROS-regulated signaling pathways mediated by redox proteins

Question 8

Oxidation of glutathione

Answer 8

• protects from pro-oxidants
• 2 GSH + ROOH -> GSSG + H2O + ROH
• regeneration of reduced gluthathione is by using NADPH and the enzyme glutathione reductase

Question 9

Hyperoxia-induced response : antioxidant response

Answer 9

• High O2 -> Metabolism by NADPH oxidase -> ROS -> EGFR-dependent signalling
• activation of transcription factor Nrf2
• Nrf2-mediated activation of AREs

Question 10

Hypoxemia: HIF-dependent response

Answer 10

• expression of growth factors
• cell survival and expansion
• erythrocyte generation: EPO
• angiogenesis
• oxygen delivery
• metabolism

Question 11

HIF dependent control of angiogenesis

Answer 11

• low oxygen stimulates production of growth factors
• growth factors promote angiogenesis
• high oxygen suppresses production of growth factors
• withdrawal of growth factors impairs angiogenesis

Question 12

do HIFs modulate ocygen induced retinal damage

Answer 12

• DMOG can rescue the hyperoxia phenotype but this depends on HIFa
• if no HIFa, DMOG cannot rescue the hyperoxia retinal damage

Question 13

What is DMOG?

Answer 13

• DMOG inhibits two components of cellular O2 sensing: prolyl Hydroxylase and Asn-yl Hydroxylase aka factor inhibiting HIF
• ## in presence of DMOG, hepatic HIF promotes production of angiogenic growth factor

Question 14

Hypoxia uncouples glycolysis from CAC

Answer 14

• low O2 -> HIF stabilized -> enhanced PDK expression
• PDK phosphorylates and de-activates PDH
• pyruvate retained in cytoplas, converted to lactate
• glycolysis uncouples from CAC
• glycolytic enzymes are upregulated

Question 15

What reactions does CSE catalyze

Answer 15

• Cystathione breakdown: yields cysteine and a-ketobutyrate
• L-cysteine breakdown: yields pyruvate, NH2, H2S
• CSE catalyzes the synthesis of hydrogen sulfide

Question 16

How is H2S linked to hypoxia

Answer 16

• Heme-oxygenase 2 - oxygen dependent enzyme
• CO is released
• Carbon monoxide inhibits CSE and thus inhibits H2S synthesis from cysteine

= H2S is poisonous

• nitric oxide is another signal of hypoxia in the carotid body

Question 17

Summary: oxygen excess

Answer 17

• high oxygen levels can be toxic
• oxygen treatment can disrupt generation of key growth factor signals
• high tissue oxygen levels can promote generation of ROS
• ROS stabilizes transcription factor Nrf2
• Nrf2 controls gene expression

Question 18

Oxygen deficiency summary

Answer 18

• oxygen sensors for control of gene expression: prolyl hydroxylase, asparaginyl hydroxykase
• normal high oxygen levels: PH and AH promote degradation of HIF
• low oxygen levels stabilize HIF
• HIF controls expression of genes
• Acute response pathways: oxygen sensing mediated by heme oxygenase
• intracellular CO negatively modulates H2S production in carotid body
• When O2 is low CO prodiction falls and H2S level rise
• ## H2S activates Ca2+ induced nerve signal to respiratory centre to increase resp rate