L41 - Pathophysiology of diabetes

Question 1

Summarize all the major pathways for glucose to induce damage? (4)

Answer 1

1) Glycation pathway:
i) Reactive carbonyl species (Amadori products)
ii) Advanced Glycation Endproducts (AGE)
2) Polyol pathway: DAG synthesis and PKC activation:
i) increase oxidative stress
ii) osmotic pressure change
iii) Vascular occlusion
3) Hexoasmine pathway: transcription:
i) Increase plasminogen activator inhibitor (PAI)
ii) Increase TGF-B for inflammation
4) PARP activation from DNA damage >> activate all above pathways via GAPDH

Question 2

Describe non-enzymatic glycation?

Answer 2

Glucose react and added to macromoelcules e.g. protein, lipid, nucleic acid without enzyme

Occurs intra and intracellularly

Question 3

Describe the formation of Reactive carbonyl compounds from glycation reactions?

Answer 3

NH2-group on protiens + glucose >> Schiff base adducts >> Amadori products >> Reactive carbonyl compounds

Question 4

List the 3 types of reactive carbonyl products formed by glycation?

Answer 4

• Auto-oxidation reaction: make GLYOXAL
• Glycolysis: intermediate Glyceraldehyde 3-phosphate not broken down fast enough > METHYLGLYOXAL
• Reaction with protein via Amadori pathway > 3-deoxyglucosone

Question 5

Describe the formation of AGE from Amadori products?

Answer 5

Amadori products undergo lots of rxns (e.g. Re-arrangements; Elimination; Condensation; oxidation)

Form extremely reactive carbonyl compounds >> crosslinking and aggregate into AGE

Question 6

Describe how AGE is normally eliminated in body?

Answer 6

Normal AGE production kept at minimum: reactive carbonyl species inactivated by reductases

>> AGE degraded by cells, excreted by kidneys

Question 7

Outline cellular damages caused by hyperglycaemia-caused AGE production?

Answer 7

1a) Impair binding of LDL to LDL-R: abolish LDL-receptor recognition
1b) Impair degradation of LDL by fibroblast
2) Inhibit lateral association of Type 4 collagen: disrupt Heparin sulphate, proteoglycan binding to Basement membrane
3) Alter packing of Type 1 collagen and laminin = abolish cell-matrix interaction
4) Dampen Nitric Oxide action

Question 8

Outline the cellular effects of AGE on Macrophage/ monocyte/ endothelial cells?

Answer 8

Macrophage/ monocyte:

• Proliferation + secrete inflammatory cytokines

Endothelial cells:

• Decrease thrombomodulin (anti-coagulant)
• Increase Tissue Factor production

>>> increase coagulation

Question 9

Describe how the Polyol pathway affect glycolysis and PPP pathway under hyperglycaemia?

Answer 9

Hyperglycaemia:

Increase G 6-P made >> feedback inhibition on Glycolysis and PPP

Glucose forced to accumulate into Polyol pathway >> cause depletion of NADPH (anti-oxidation) and NAD (less for glycolysis)

Question 10

Describe how the Polyol pathway causes PKC activation?

Answer 10

Increase conversion of Glyceraldehyde 3-Phosphate to Dihydroxyacetone phosphate (instead of pyruvate)

Dihydroxyacetone phosphate >>> Glycerol-3 phosphate >>> diacylglycerol synthesis >> PKC activation

Question 11

Summarize the intracellular effects of the polyol pathway?

Answer 11

1) Competes with glycolytic pathway for NAD
2) Accumulation of glycerol 3-phosphate >> more PKC activation >> vascular damages
3) Use NAPDH = oxidative damage (GSH)
4) Accumulation of sorbitol >> change osmotic pressure in lens >> cataracts

Question 12

List the damages caused by PKC activation on vascular function.

Answer 12

• Vascular occlusion (due to procoagulant activity)
• capillary occlusion (due to extracellular matrix synthesis)
• Vascular permeability
• Blood flow abnormality
• Inhibits NO formation by ↓expression of eNOS = vasoconstrict

Question 13

Outline the pathways for polyol pathway to cause oxidative damage?

Answer 13

Depletion of NADPH

>> Less reduction of GSSH into GSH

>> Less intracellular oxidative compound inactivation

Question 14

Describe the hexosamine pathway to increase pro-inflammatory gene transcription?

Answer 14

1) Increase conversion of glucose to glucosamine 6-phosphate
2) More UDP- N-acetylglucosamine formed >> More O-linked glycosylation of nuclear transcription factors
3) Activation of glycosylated transcription factors e.g. Plasminogen activation inhibitor (> less fibrinolysis) and TGF-B

Question 15

Summarize the intracellular and vascular damages caused by hyperglycemia via different pathways?

Answer 15

Question 16

What is the link between increased normal glycose metabolism and all the pathways for hyperglycemia damage?

Answer 16

Hyperglycaemia: overloads normal mitochondiral respiration and electron transport chain

>> Increase superoxide production***

>> Nuclear DNA damage cause increase DNA repair signal Polyadenine ribose polymerase (PARP)***

>> Polyribosylation cause Inactivation of Glyceraldehyde 3phosphate dehydrogenase (GAPDH)***

>> lead to AGE, hexosamine, Polyol pathways…etc

*** = bridge

Question 17

Explain why not all cells are equally affected by damaging effects of hyperglycemia?

Answer 17

Only cells that cannot control influx and transport of glucose in hyperglycemia are most affected

e.g. endothelial cells in blood vessels

Question 18

Describe the polyribosylation reaction of GADPH?

Answer 18