ChemPath: Lipoprotein metabolism, CVD and obesity

Question 1

What are the features of an atherosclerotic lesion?

Answer 1

• Fibrous cap
• Foam cells (macrophages full of cholesteryl ester)
• Necrotic core (full of cholesterol crystals)

Question 2

What is the biggest plasma lipoprotein?

Answer 2

Chylomicrons

Question 3

During what time will chylomicrons be most abundant?

Answer 3

After eating (they are present in very small amounts in the fasted state)

Question 4

Describe the uptake of cholesterol by the intestinal epithelium.

Answer 4

• Cholesterol entering the intestines will come from the diet and bile
• Cholesterol will be solubilised in mixed micelles
• It is then transported cross the intestinal epithelium by NPC1L1 (this is the main determinant of cholesterol transport)
• balance between NPC1L1 and ABC G5,G8 determines amount of cholesterol absorbed

Question 5

Name two transports that transport cholesterol back into the intestinal lumen.

Answer 5

ABC G5

ABC G8

Question 6

Where are bile acids absorbed?

Answer 6

Terminal ileum

Question 7

What happens when cholesterol arrives at the liver?

Answer 7

Downregulates the activity of HMG CoA reductase

NOTE: HMG CoA reductase is responsible for the production of cholesterol from acetate and mevalonic acid

If more cholesterol absorbed in SI, less will be made by liver

Question 8

What are the two fates of cholesterol that is either produced by or transported to the liver?

Answer 8

• Hydroxylation by 7a-hydroxylase to produce bile acids
• Esterification by ACAT to produce cholesterol ester which is incorporated into VLDLs along with triglycerids and ApoB

VLDL is precursor for LDL

Question 9

Which transfer protein is important in the packaging of VLDLs?

Answer 9

MTP

Question 10

Which transfer protein is important in the packaging of HDLs?

Answer 10

ABCA1

Question 11

What are the effects of CETP on the movement of substances between lipoproteins?

Answer 11

• Moves cholesterol from HDL → VLDL
• Moves triglycerides from VLDL → HDL

Question 12

Which receptor is responsible for the uptake of some HDLs by the liver?

Answer 12

SR-B1

Question 13

Describe the transport and metabolism of triglycerides.

Answer 13

• Triglycerides from fatty foods are hydrolysed to fatty acids, absorbed, and resynthesized into triglycerides which are transported by chylomicrons into the plasma
• Chylomicrons are hydrolysed by lipoprotein lipase into free fatty acids
• Some free fatty acids are taken up by the liver, and some by adipose tissue
• The liver resynthesizes fatty acids into triglycerides and packages them into VLDLs
• VLDLs are acted upon by lipoprotein lipase to liberate free fatty acids

Question 14

List the three causes of familial hypercholesterolaemia (type II).

Answer 14

• Caused by autosomal dominant gene mutations in:
  
  • LDL receptor
  • ApoB
  • PCSK9

Question 15

List some mutations that are implicated in polygenic hypercholesterolaemia.

Answer 15

• NPC1L1
• HMGCR
• CYP7A1

Question 16

What is familial hyperalphalipoproteinaemia?

Answer 16

• Increase in HDL caused by deficiency of CETP
• This is associated with longevity

Question 17

What is phytosterolaemia?

Answer 17

• Increased plasma concentrations of plant sterols due to mutations in ABC G5 and ABC G8

NOTE: this condition is associated with premature atherosclerosis

Question 18

Dsecribe the function of the LDL receptor.

Answer 18

• LDLs bind to LDLR in coated pits which then undergo endocytosis (thereby uptaking the LDL into the liver)

forms liposomes

Question 19

List some clinical features of familial hypercholesterolaemia.

Answer 19

• Xanthelasma
• Corneal arcus
• Tendon xanthomata

Heterozygous is much more common for FH than homozygous

Question 20

What is PCSK9?

Answer 20

• A protein that binds to LDL receptors (especially on liver) and degrades them

NOTE:
gain of function mutations result in increased breakdown of LDLR and hence increased plasma LDL levels –> Rare cause of FH

loss of function mutation cause decreased plasma LDL levels

Question 21

List the key features of the following forms of familial hypertriglyceridaemia:

• Familial Type I
• Familial Type IV
• Familial Type V

Answer 21

Familial Type I:

• decreased clearance of chylomicrons
• Caused by deficiency of lipoprotein lipase and ApoC II
• NOTE: lipoprotein lipase degrades chylomicrons and ApoC II is an activator of lipoprotein lipase

high chylomicrons

Familial Type IV:

• Characterised by increased synthesis of triglycerides

high VLDL

Familial Type V:

• Characterised by deficiency of ApoA V

high chylomicrons + VLDL

• NOTE: these hypertriglyceridaemias show different patterns when the plasma is left overnight to separate

Question 22

What is familial combined hyperlipidaemia?

Answer 22

Some people in the family have high cholesterol and others have high triglycerides

Question 23

What is familial dysbetalipoproteinaemia (type III)?

Answer 23

• Due to aberrant form of ApoE (E2/2)
• NOTE: normal form is ApoE (E3/3)
• A diagnostic clinical feature of yellowing of the palmar crease (palmar striae) + eruptive xanthoma on elbow

Question 24

List some causes of secondary hyperlipidaemia.

Answer 24

• Pregnancy
• Hypothyroidism
• Obesity
• Alcohol
• Diabetes
• Nephrotic syndrome

Question 25

List four causes of hypolipiademia and their underlying genetic defect.

Answer 25

Aβ-lipoproteinaemia:

• Autosomal recessive
• Extremely low levels of cholesterol
• Due to deficiency of MTP

Hypoβ-lipoproteinaemia:

• Autosomal dominant
• Low LDL
• Caused by mutations in ApoB

Tangier disease:

• Low HDL
• Caused by mutation of ABCA1

Hypoα-lipoproteinaemia:

• Sometimes caused by mutation of ApoA1

Question 26

Describe the role of LDL in atherosclerosis.

Answer 26

• LDL becomes oxidised once it has got through the vascular endothelium
• Once oxidised it is taken up by macrophages
• Within the macrophages, the LDLs become esterified and you develop foam cells

Question 27

List some lipid-lowering drugs and their effect on lipid levels.

Answer 27

• Statins - reduce LDLs, increased HDLs, slight increased in triglycerides
• Fibrates - lower triglycerides, little effect on HDL + LDL
• Ezetimibe - reduces cholesterol absorption (blocks NPC1L1) –> reduce LDL
• Colestyramine - resin that binds to bile acids and reduces their absorption (so liver makes more bile acid from cholesterol) –> reduce LDL

Question 28

List three types of bariatric surgery.

Answer 28

• Gastric banding
• Roux-en-Y gastric bypass
• Biliopancreatic diversion

Question 29

What is the definition of success in bariatric surgery?

Answer 29

More than 50% reduction in excess weight

Question 30

Obesity treatment

Answer 30

low calorie diet
+ exercise

Orlistat –> inhibits pancreatic lipase

Bariatric surgery –> if BMI >=40

Question 31

List some beneficial effects of bariatric surgery.

Answer 31

• Reduced diabetes risk
• Reduced serum triglycerides
• Increased HDLs
• Reduced fatty liver
• Reduced blood pressure

Question 32

What are the types of lipoprotein

Answer 32

Chylomicron - largest

Very-low density Lipoprotein

Low-density lipoprotein

High density lipoprotein - smallest

Question 33

What do the different lipoproteins mainly carry

Answer 33

Chlomicrons - <5% triglycerides

VLDL - 55% triglycerides

LDL - ‘bad’ cholesterol + 29% triglycerides

HDL - ‘good’ cholesterol + 11% of triglycerides

Note: chylomicrons have short half life in plasma

Question 34

Role of HDL

Answer 34

carry cholesterol from periphery to liver

Question 35

Role of LDL

Answer 35

Carry cholesterol from liver to periphery

delivers to cells with LDL receptors

Question 36

Causes of primary hypercholesterolaemia

Answer 36

Familial hypercholesterolaemia

Polygenic hypercholesterolaemia

Familial hyperalphalipoproteinaemia

Phytosterolaemia