Lipid Transport

1. Lipid Transport Lipoprotein Structure, Function, and Metabolism

3. Health issues Excessive dietary fat intake is associated with obesity, diabetes, cancer, hypertension and atherosclerosis. Not more than 35% of energy intake should come from fat. Saturated fat should not make up more than 15% of the total fat intake. Omega-3 fatty acids (20 carbons) from fish may protect against atherosclerosis. American Heart association recommends 2-3 fish meals per weak. Fish oil supplements should be avoided because they may be contain concentrated toxins accumulated by the fish. �

4. FUNCTIONS OF LIPIDS: Major components of cell membranes.  Required to solubilise fat soluble vitamins  Biosynthetic precursors (e.g. steroid hormones from cholesterol)  Protection (e.g. kidneys are shielded with fat in fed state)  Insulation

5. LIPID DIGESTION Stomach - lingual lipase and gastric lipase attack triacylglycerols and hydrolyse a limited number of FA.  Small Intestine - acid chyme (stomach contents) stimulates mucosa cells to release hormone (choleocystokinin) which stimulates gall bladder and pancreas to release bile and digestive enzymes respectively (bile acids help emulsify fat droplets thus increasing their surface area). Other mucosa cells release secretin which causes pancreas to release bicarbonate rich fluid to neutralise chyme.

6. Enzymic digestion of lipids in small intestine

7. Enzymic digestion generates more polar products that form mixed micelles of free fatty acids, 2-monoacylglycerol, cholesterol & bile salts that are adsorbed (except bile salts which pass through to ileum � see later). Once adsorbed fatty acids and 2-monoacylglycerol are recombined to form triacylglycerol. Triacylglycerol + cholesterol + phospholipid + proteins form a lipoprotein complex called a chylomicron which transports the lipids in the circulation.

9. The five classes of lipoprotein(all contain characteristic amounts TAG, cholesterol, cholesterol esters, phospholipids and apoproteins)

12. Plasma LipoproteinsStructure LP core Triglycerides Cholesterol esters LP surface Phospholipids Proteins cholesterol

14. Plasma LipoproteinsClasses & Functions Chylomicrons Synthesized in small intestine Transport dietary lipids 98% lipid, large sized, lowest density Apo B-48 Receptor binding Apo C-II Lipoprotein lipase activator Apo E Remnant receptor binding

15. Chylomicron formed through extrusion of resynthesized triglycerides from the mucosal cells into the intestinal lacteals flow through the thoracic ducts into the suclavian veins degraded to remnants by the action of lipoprotein lipase (LpL) which is located on capillary endothelial cell surface remnants are taken up by liver parenchymal cells due to apoE-III and apoE-IV isoform recognition sites

16. Chylomicron Metabolism Nascent chylomicron (B-48) Mature chylomicron (+apo C & apo E) Lipoprotein lipase Chylomicron remnant Apo C removed Removed in liver

17. Plasma LipoproteinsClasses & Functions Very Low Density Lipoprotein (VLDL) Synthesized in liver Transport endogenous triglycerides 90% lipid, 10% protein Apo B-100 Receptor binding Apo C-II LPL activator Apo E Remnant receptor binding

18. Plasma LipoproteinsClasses & Functions Intermediate Density Lipoprotein (IDL) Synthesized from VLDL during VLDL degradation Triglyceride transport and precurser to LDL Apo B-100 Receptor binding Apo C-II LPL activator Apo E Receptor binding

19. Plasma LipoproteinsClasses & Functions Low Density Lipoprotein (LDL) Synthesized from IDL Cholesterol transport 78% lipid, 58% cholesterol & CE Apo B-100 Receptor binding

20. LDL molecule

21. VLDL Metabolism Nascent VLDL (B-100) + HDL (apo C & E) = VLDL LPL hydrolyzes TG forming IDL IDL loses apo C-II (reduces affinity for LPL) 75% of IDL removed by liver Apo E and Apo B mediated receptors 25% of IDL converted to LDL by hepatic lipase Loses apo E to HDL

22. Plasma LipoproteinsClasses & Functions High Density Lipoprotein (HDL) Synthesized in liver and intestine Reservoir of apoproteins Reverse cholesterol transport 52% protein, 48% lipid, 35% C & CE Apo A Activates lecithin-cholesterol acyltransferase (LCAT) Apo C Activates LPL Apo E Remnant receptor binding

23. Functions of HDL transfers proteins to other lipoproteins picks up lipids from other lipoproteins picks up cholesterol from cell membranes converts cholesterol to cholesterol esters via the LCAT reaction transfers cholesterol esters to other lipoproteins, which transport them to the liver (referred to as �reverse cholesterol transport)

26. LDL Metabolism LDL receptor-mediated endocytosis LDL receptors on �coated pits� Clathrin: a protein polymer that stabilizes pit Endocytosis Loss of clathrin coating uncoupling of receptor, returns to surface Fusing of endosome with lysosome Frees cholesterol & amino acids

27. Coordinate Control of Cholesterol Uptake and Synthesis Increased uptake of LDL-cholesterol results in: inhibition of HMG-CoA reductase reduced cholesterol synthesis stimulation of acyl CoA:cholesterol acyl transferase (ACAT) increased cholesterol storage TG + C -> DG + CE decreased synthesis of LDL-receptors �down-regulation� decreased LDL uptake

28. Heterogeneity of LDL-particles Not all LDL-particles the same Small dense LDL (diameter <256A) Large buoyant LDL (diameter >256 A) Lamarche B, St-Pierre AC, Ruel IL, et al.�A prospective, population-based study of low density lipoprotein particle size as a risk factor for �Can J Cardiol�2001;17:859-65. 2057 men with hi LDL, 5 year follow-up Those with elevated small dense LDL had RR of 2.2 for IHD compared to men with elevated large buoyant LDL Detection expensive Treatment for lowering small dense LDL similar to lowering all LDL (diet, exercise, drugs) Some drugs (niacin, fibrates) may be more effective at lowering small dense LDL.

29. LDL Particle Size and Apolipoprotein B Predict Ischemic Heart Disease: Quebec Cardiovascular Study LDL Particle Size and Apolipoprotein B Predict Ischemic Heart Disease: Quebec Cardiovascular Study In another analysis from the Quebec Cardiovascular Study, men were stratified by apo B level and LDL particle size. High apo B was associated with CHD, and the presence of both high apo B and small, dense LDL was associated with a marked increase in CHD risk. One interpretation of these findings is that concomitant interventions should be used both to lower apo B, such as with a statin, and to improve LDL particle size, such as with fibrates or high-dose statins. However, another interpretation is that if apo B is reduced to less than 120 mg/dL, LDL particle size no longer has an effect, perhaps because if there are few enough apo B-containing particles, it may not matter how atherogenic these particles are. This is a fairly controversial area, although a number of other epidemiological studies also suggest that this might be true. However, the effects of triglyceride level, for instance, which is strongly correlated to LDL particle size, appear to be considerably more important in people with high LDL/HDL ratio, apo B level, or total cholesterol level, as has been seen in the observational Paris Prospective Study and Prospective Cardiovascular M�nster (PROCAM) Study and the interventional Helsinki Heart Study. Reference: Lamarche B, Tchernof A, Moorjani S, Cantin B, Dagenais GR, Lupien PJ, Despres JP. Small, dense low-density lipoprotein particles as a predictor of the risk of ischemic heart disease in men: prospective results from the Quebec Cardiovascular Study. Circulation 1997;95:69-75.LDL Particle Size and Apolipoprotein B Predict Ischemic Heart Disease: Quebec Cardiovascular Study In another analysis from the Quebec Cardiovascular Study, men were stratified by apo B level and LDL particle size. High apo B was associated with CHD, and the presence of both high apo B and small, dense LDL was associated with a marked increase in CHD risk. One interpretation of these findings is that concomitant interventions should be used both to lower apo B, such as with a statin, and to improve LDL particle size, such as with fibrates or high-dose statins. However, another interpretation is that if apo B is reduced to less than 120 mg/dL, LDL particle size no longer has an effect, perhaps because if there are few enough apo B-containing particles, it may not matter how atherogenic these particles are. This is a fairly controversial area, although a number of other epidemiological studies also suggest that this might be true. However, the effects of triglyceride level, for instance, which is strongly correlated to LDL particle size, appear to be considerably more important in people with high LDL/HDL ratio, apo B level, or total cholesterol level, as has been seen in the observational Paris Prospective Study and Prospective Cardiovascular M�nster (PROCAM) Study and the interventional Helsinki Heart Study. Reference: Lamarche B, Tchernof A, Moorjani S, Cantin B, Dagenais GR, Lupien PJ, Despres JP. Small, dense low-density lipoprotein particles as a predictor of the risk of ischemic heart disease in men: prospective results from the Quebec Cardiovascular Study. Circulation 1997;95:69-75.

30. HDL Metabolism: Functions Apoprotein exchange provides apo C and apo E to/from VLDL and chylomicrons Reverse cholesterol transport

31. Reverse cholesterol transport Uptake of cholesterol from peripheral tissues (binding by apo-A-I) Esterification of HDL-C by LCAT LCAT activated by apoA1 Transfer of CE to lipoprotein remnants (IDL and CR) by CETP removal of CE-rich remnants by liver, converted to bile acids and excreted

32. Cholesterol and lipid transport by lipoproteins

33. Cholesterol and lipid transport by lipoproteins

36. The LDL receptor characterized by Michael Brown and Joseph Goldstein (Nobel prize winners in 1985) based on work on familial hypercholesterolemia receptor also called B/E receptor because of its ability to recognize particles containing both apos B and E activity occurs mainly in the liver receptor recognizes apo E more readily than apo B-100

37. Atherosclerosis hardening of the arteries due to the deposition of atheromas heart disease is the leading cause of death caused by the deposition of cholesteryl esters on the walls of arteries atherosclerosis is correlated with high LDL and low HDL

38. Factors promoting elevated blood lipids age men >45 years of age; women > 55 years of age family history of CAD smoking hypertension >140/90 mm Hg low HDL cholesterol obesity >30% overweight diabetes mellitus inactivity/ lack of exercise

39. HMG CoA reductase 3 different regulatory mechanisms are involved: covalent modification: phosphorylation by cAMP-dependent protein kinases inactivate the reductase. This inactivation can be reversed by 2 specific phosphatases degradation of the enzyme � half life of 3 hours and the half-life depends on cholesterol levels gene expression: cholesterol levels control the amount of mRNA

42. HMG CoA reductase inhibitors Precaution: mild elevation of serum aminotransferase (should be measured at 2 to 4 month intervals) minor increases in creatine kinase (myopathy, muscle pain and tenderness) do not give during pregnancy

43. Low-Density Lipoproteins (LDLs) �Bad� cholesterol Delivers cholesterol to cells Can increase build-up of plaque High levels of LDL associated with increased risk for cardiovascular disease

44. High-Density Lipoproteins (HDLs) �Good� cholesterol Made by liver Circulates in the blood to collect excess cholesterol from cells Returns cholesterol to liver for excretion in bile Highest protein content

47. HDL = High Density Lipoprotein Made in: the Liver and Small Intestine Secreted into: the bloodstream Function: Pick up cholesterol from body cells and take it back to the liver = �reverse cholesterol transport� Potential to help reverse heart disease

50. Cardiovascular Disease (CVD) Main type of CVD is Atherosclerosis (AS) Endothelial dysfunction is one of earliest changes in AS Mechanical, chemical, inflammatory mediators can trigger endothelial dysfunction: High blood pressure Smoking (free radicals that oxidatively damage endothelium) Elevated homocysteine Inflammatory stimuli Hyperlipidemia

53. Pro-Inflammatory Molecules Chemokines = monocyte chemoattractant protein 1 (MCP-1) Inflammatory cytokines = tumor necrosis factor ? (TNF??) Adhesion molecules = intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) Overexpression of all these inflammatory mediators is commonly seen in atherosclerotic lesions.

54. Endothelial Dysfunction( endothelial activation, impaired endothelial-dependent vasodilation) � endothelial synthesis of PGI2 (prostacylcin), & NO (nitric oxide) PGI2 = vasodilator, �platelet adhesion/aggregation NO = vasodilator, �platelet & WBC (monocyte) adhesion � Adhesion of monocytes onto endothelium --> transmigration into subendothelial space (artery wall) --> change to macrophages Endothelial dysfunction --> increased flux of LDL into artery wall

57. Oxidation of LDL (oxLDL) Oxidation = process by which free radicals (oxidants) attack and damage target molecules / tissues Targets of free radical attack: DNA - carbohydrates Proteins - PUFA�s>>> MUFA�s>>>>> SFA�s LDL can be oxidatively damaged: PUFA�s are oxidized and trigger oxidation of apoB100 protein --> oxLDL OxLDL is engulfed by macrophages in subendothelial space

60. Atherosclerotic Plaque Continued endothelial dysfunction (inflammatory response) Accumulation of oxLDL in macrophages (= foam cells) Migration and accumulation of: smooth muscle cells, additional WBC�s (macrophages, T-lymphocytes) Calcific deposits Change in extracellular proteins, fibrous tissue formation High risk = � VLDL (�TG) � LDL � HDL

61. Antioxidant Defense Systems 1. Prevent oxidation from being initiated 2. Halt oxidation once it has begun 3. Repair oxidative damage

62. Antioxidant Mechanisms Antioxidant vitamins (vitamins C, E, carotenoids) Flavanoids and other phytochemicals Antioxidant enzyme systems Minerals required: Mn, Cu, Zn, Se

65. Factors Associated with CVD Genetic Variables Being male Being post-menopausal female Family history of heart disease before the age of 55 (some are associated with genetic defects in LDL receptors)

66. Factors Associated with CVD � Dietary 1. Elevated levels of LDL --More LDL around to potentially oxidize and accumulate in artery wall 2. Low levels of HDL --HDL carries cholesterol from artery walls back to the liver 3. Low levels of antioxidant vitamins --Vit. E, Vit. C, Beta-carotene 4. Low levels of other dietary antioxidants --Phenolics, flavanoids, red wine, grape juice, vegetables, fruits

67. Factors Associated with CVD High blood pressure � Damages the artery wall allowing LDL to enter the wall more readily Cigarette Smoking Cigarette smoke products are oxidants and can oxidize LDL Cigarette smoking compromises the body�s antioxidant vitamin status, especially Vit. C Damages the artery wall Activity Level Exercise is the most effective means of raising HDL levels Obesity

68. Homocysteine Levels Normal byproduct of certain metabolic pathways Normally metabolized to other products Elevated levels cause damage to artery walls = increased the oxidation of LDL Elevated homocysteine levels are significantly correlated with increased risk to heart disease. Vitamins B6, B12, and Folic acid normalize homocysteine levels.

70. Dietary/Lifestyle Prevention/Intervention of Heart Disease

71. Know Your Lipid Profile

72. Know Your Diabetes, Metabolic Risk

73. The Metabolic Syndrome

74. Know Your Blood Pressure