Lipoprotein Structure, Function, and Metabolism Lipid Transport Lipoprotein Structure, Function, and Metabolism

Introduction Fats are triacylglycerols containing saturated fatty acids - solid at room temp - usually from animal source (however, coconut & palm oil are saturated). Oils are triacylglycerols containing mono- or polyunsaturated fatty acids - liquid at room temp - usually from plant sources (however, fish oils are polyunsaturated). Phospholipids are triacylglycerols that have had a FA replaced with a phosphate linked FA group. The major dietary sterol is cholesterol.

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.  

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

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.

Enzymic digestion of lipids in small intestine 2-monoacylglycerol

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.

Lipid transport in the circulation Lipids are insoluble in plasma. In order to be transported they are combined with specific proteins to form lipoproteins: Proteins (apoproteins) Non polar lipids in core (TAG and cholesterol esters) Cholesterol Apoproteins are only weakly associated with a particular lipoprotein and are easily transferred to another lipoprotein of a different class. Apoproteins have various functions including:  Structural role  Binding sites for receptors  Activators or co-enzymes for enzymes involved with lipid metabolism 

The five classes of lipoprotein (all contain characteristic amounts TAG, cholesterol, cholesterol esters, phospholipids and apoproteins) Class Diameter (nm) Source and function Major apoliproteins Chylomicrons (CM) 500 Intestine. Transport of dietary TAG A, B48, C(I,II,III) E Very low density lipoproteins (VLDL) 43 Liver. Transport of endogenously synthesised TAG B100, C(I,II,III) , E Low density lipoproteins (LDL) 22 Formed in circulation by partial breakdown of IDL. Delivers cholesterol to peripheral tissues B100 High density lipoproteins (HDL) 8 Liver. Removes “used” cholesterol from tissues and takes it to liver. Donates apolipoproteins to CM and VLDL A, C(I,II,III), D, E Increasing density

Apolipoproteins Provide structural stability to Lp Serve as ligands for interaction w/Lp receptors that help determine disposition of individual particles Act as cofactors for enzymes involved in plasma lipid and Lp metabolism

There are many types of apolipoproteinsa Apoprotein Lipoproteins Function(s) Apo B-100 VLDL, IDL, LDL Secretion of VLDL from liver Structural protein of VLDL, IDL, and HDL Ligand for LDL receptor (LDLR) Apo B-48 Chylomicrons, remnants Secretion of chylomicrons from intestine; lacks LDLR binding domain of Apo B-100 Apo E Chylomicrons, VLDL, IDL, HDL Ligand for binding of IDL & remnants to LDLR and LRP Apo A-I HDL, chylomicrons Major structural protein of HDL Activator of LCAT Apo A-II Unknown Apo C-I Modulator of hepatic uptake of VLDL and IDL (also involved in activation of LCAT) Apo C-II Activator of LPL Apo C-III Inhibitor of LPL activity

Plasma Lipoproteins Structure LP core Triglycerides Cholesterol esters LP surface Phospholipids Proteins cholesterol

Composition and properties of human lipoproteins Lipoprotein class Density (g/mL) Diameter (nm) Protein % of dry wt Phospholipid % Triacylglycerol % of dry wt HDL 1.063-1.21 5 – 15 33 29 8 LDL 1.019 – 1.063 18 – 28 25 21 4 IDL 1.006-1.019 25 - 50 18 22 31 VLDL 0.95 – 1.006 30 - 80 10 50 chylomicrons < 0.95 100 - 500 1 - 2 7 84 Composition and properties of human lipoproteins most proteins have densities of about 1.3 – 1.4 g/mL and lipid aggregates usually have densities of about 0.8 g/mL

Plasma Lipoproteins Classes & 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

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

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

Plasma Lipoproteins Classes & 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

Plasma Lipoproteins Classes & 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

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

LDL molecule

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

Plasma Lipoproteins Classes & 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

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)

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

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

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.

LDL Peak Particle Diameter (nm) LDL Particle Size and Apolipoprotein B Predict Ischemic Heart Disease: Quebec Cardiovascular Study 6.2 (p<0.001) Apo B 2.0 >120 mg/dl 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 Münster (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. 1.0 1.0 <120 mg/dl >25.64 <25.64 LDL Peak Particle Diameter (nm)

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

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

Cholesterol and lipid transport by lipoproteins

Cholesterol and lipid transport by lipoproteins

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

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

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

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

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

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

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

Overview of Lipoproteins

LDL = Low Density Lipoprotein Made in: the Liver as VLDL Arise from: VLDL once it has lost a lot of its TG’s Secreted into: the bloodstream Rich in: Cholesterol Function: Deliver cholesterol to all body cells

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

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

A Healthy Endothelium produces: á PGI2 á NO Maintaining an anti-coagulant, anti-thrombotic surface

â PGI2 â NO TNFa VCAM-1 A Dysfunctional Endothelium has decreased: Increased: pro-inflammatory molecules: MCP-1 TNFa VCAM-1 Shifting to a pro-coagulant, pro-thrombotic surface

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.

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

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

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 = á VLDL (áTG) á LDL â HDL

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

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

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)

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

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

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.

Diet Methionine (a.a.) Homocysteine SAM CH3 SAH cysteine sulfate Enzymes B12, Folate Homocysteine SAM 1. Norepinephrine 2. Guanidinoacetate 3. Serotonin 4. Serine Enzyme B6 cysteine CH3 1. Epinephrine 2. Creatine 3. Melatonin 4. Choline SAH sulfate

Dietary/Lifestyle Prevention/Intervention of Heart Disease Maintain Endothelial Function Platelet Activity Decrease LDL Increase HDL Increase Antioxidants â High Blood Pressure w-3 PUFAs â w -6 PUFA â Saturated Fat á MUFA/ â PUFA á MUFA/ â w -6 PUFA â Homocysteine B6, B12, Folic Acid á Phytochemicals â Cholesterol á w-3 PUFAs (fish) áVegetables áPhytochemicals Aspirin á w-3 oils (fish) á Exercise áFruits Stop smoking á Fiber â Trans Fats âBody weight if overweight

Know Your Lipid Profile Fasting Blood Level Ideal, Healthy Level Total Cholesterol < 200 mg/dl LDL-Cholesterol < 100 mg/dl HDL-Cholesterol ≥ 60 mg/dl Triglycerides < 150 mg/dl

Know Your Diabetes, Metabolic Risk Fasting Healthy Pre-Diabetes Diabetes (Metabolic Syndrome) Blood Glucose < 110 mg/dl 110-125 mg/dl ≥ 126 mg/dl 2 hr GTT < 140 mg/dl 140-200 mg/dl > 200 mg/dl Triglyceride < 150 mg/dl > 150 mg/dl Typically elevated HDL ≥ 60 mg/dl M < 40 mg/dl F < 50 mg/dl Typically low

The Metabolic Syndrome Abdominal Obesity Men Women > 40 inch waist > 35 inch waist Triglycerides ≥ 150 mg/dL HDL cholesterol < 40 mg/dL < 50 mg/dL Blood Pressure ≥ 130/ 85 mm Hg Fasting Blood Glucose 110-125 mg/dL

Know Your Blood Pressure Category Systolic (mm/Hg) Diastolic (mm/Hg) Normal 120 or less 80 or less High Normal 130-139 85-89 High Blood Pressure 140 or more 90 or more Strive for blood pressure of 120/80 or less