Biochemoistry of Diabetic Complications

Question 1

Why don’t we get excessive ketosis in T2DM?

Answer 1

Low levels of circulating insulin inhibit HSL, inhibiting lipolysis so there are less FFA left to make ketones

Question 2

What are the small vessel complications of diabetes?

Answer 2

Retinopathy
Peripheral neuropathy
Nephropathy

Question 3

What are the large vessel complications of diabetes?

Answer 3

Peripheral vascular disease (Atherosclerosis)
Ischaemic heart disease
Stroke

Question 4

How does glucolipotoxicity lead to the development of micro/macrovascular complications?

Answer 4

Mitochondrial dysfunction
Oxidative stress
Inflammation
Leads to deregulation of cellular pathways
- Altered gene expression can promote thrombogenesis
- blood flow abnormalities can increase vasoconstriction
- increased vascular permeability

Question 5

What induces mitochondrial superoxide overproduction?

Answer 5

Excess glucose and FFA increase oxidative phosphorylation in mitochondria, which produces more ROS which exceed the body’s antioxidant capacity

Question 6

What are the effects of increased ROS?

Answer 6

• Oxidative stress = direct damage
• Activation of pro-inflammatory signalling pathways
• Activation of signalling pathways that impair insulin signalling in target tissues
  These all cause mitochondrial damage and impaired nutrient metabolism
• Beta-oxidation of FFA reduced and metabolites are diverted to other pathways, increasing ROS formation

Question 7

Quick review of how hyperglycaemia promotes ROS overproduction

Answer 7

Excess glucose causes more substrates that overload the mitochondria, making ROS -> upstream metabolites are diverted from glycolysis and go to overflow pathways (to be discussed)

Question 8

What are the 4 main overflow pathways for hyperglycaemia?

Answer 8

1. Polyol
2. Hexosamine
3. Protein kinase C
4. AGE

Question 9

How does the polyol pathway work?

Answer 9

Aldose reductase has a small affinity or glucose only and usually doesn’t convert much to sorbitol. With hyperglycaemia, increased sorbitol is made (polyol) which leads to oxidative stress (aldose reductase activity uses up antioxidants) and osmotic imbalance.

Question 10

How is AGE produced in hyperglycaemia?

Answer 10

Non-enzymatic glycation as a spontaneous process to form a Schiff base
This can be rearranged to an Amadori produce which is a ketoamine. (this is what happens in HbA1C!)
However with hyperglycaemia an IRREVERSIBLE formation of advanced-glycation end products (AGE)occurs over days.

Question 11

What is the effect of AGE?

Answer 11

• Structural and functional abnormalities in proteins
• Generate ROS
• Can generate an immune response
• Decrease renal clearance of excess AGE

Question 12

What proteins are modified by AGE?

Answer 12

• Intracellular proteins in cells that are insulin-independent
• Cell surface proteins
• Circulating proteins (albumin, apolipoproteins): circulating proteins that have been modified can activate AGE receptors on Macrophages, endothelial cells and vascular SM and activate signalling cascades

Question 13

What is the effect of this modification?

Answer 13

Increased stiffness of vessel walls

Atherosclerosis due to endothelial cell dysfunction

Question 14

How does the protein kinase C pathway work?

Answer 14

Hyperglycaemia increases PKC production, causing an increase in VEGF and TGF-beta and a decrease in eNOS (causes tissue iscaemia and endothelial injury)

Question 15

How does the hexosamine path work?

Answer 15

N-acetyl glucosamine is added to serine and threonine residues and causes pathologic changes in gene expression
Has also been linked to development of insulin resistance

Question 16

How does hyperglycaemia cause diabetic retinopathy?

Answer 16

Polyol pathway, PKC pathway and oxidative stress.

1. Vascular damage due to increased permeability and microaneurysm
2. Angiogenesis in response to VEGF etc. and macular oedema

Question 17

How does hyperglycaemia cause diabetic neuropathy?

Answer 17

Multifocal axon degeneration in peripheral nerves = sensory loss.

• Systemic oxidative stress
• AGE myelin accumulation causes demyelination and Schwann cell death

Question 18

How does hyperglycaemia cause diabetic nephropathy?

Answer 18

Thickening of basement membrane by AGE

Glomerular hypertrophy and damage later causes leakage to urine (ROS also contribute)

Question 19

Why does diabetes increase CVD risk?

Answer 19

1. Metabolic abnormalities (dyslipidaemia, chronic low-grade inflammation and hypertension)
2. Lipoprotein remodelling (dyslipidaemic triad)
3. Inflammatory processes (vessel wall inflammation causes endothelial dysfunction)

Question 20

What are the characteristics of endothelial cell dysfunction in diabetes?

Answer 20

Impaired relaxaion
Pro-thrombosis
Pro-inflammation
Insulin resistance

Question 21

How does endothelial cell dysfunction cause macrovascular complications?

Answer 21

Decreased vasodilation and insulin resistance causes dysfunction.
This simultaneously contributes to CHD/atherosclerosis and diabetes/dyslipidaemia

Question 22

What is the dyslipidaemic triad?

Answer 22

1. High TG levels (overproduction of VLDL)
2. Increased LDL (esp. small and dense)
3. Low levels of HDL cholesterol

Question 23

How does CETP contribute to cholesterol homeostasis?

Answer 23

Facilitates bi-directional cholesterol and TG transfer between lipoproteins
- Between ApoA1-containind HDL and Apo-B containing particles

Question 24

How do adiposity and insulin resistance cause dyslipidaemia?

Answer 24

1. Increased TG: FFA released by adipose tissue
2. VLDL increased production by liver
3. Decreased endothelial LPL = decreased TG clearance

Question 25

How does the production of specifically small and dense lipoproteins affect complications?

Answer 25

Small, dense LDL can cross the artery wall and is more susceptible to glycation, causing increased atherosclerosis
Small, dense HDL has reduced efficacy of reverse cholesterol transport and because the ApoA1 is unstable is renally cleared, reducing HDL-C