Plasma lipoproteins & atherosclerosis by. Prof.Dr. Maha M. Sallam

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1 Biochemistry Department Plasma lipoproteins & atherosclerosis by Prof.Dr. Maha M. Sallam 1

2 1. Recognize structures,types and role of lipoproteins in blood (Chylomicrons, VLDL, LDL and HDL). 2. Explain the modes of transport of lipids between tissues and their metabolism. 3. Describe the various types of hyperlipoprotenemias 4. Determine the pathological effects of alterations in structure of lipoproteins or their receptors and their relation to atherosclerosis.

3 Lipoproteins are complexes of proteins and lipids which transport lipids in blood from tissue of origin to tissues where they are consumed.

4 The protein and lipid in each lipoprotein particle are held by non covalent forces

5 Lipoprotein Structure Spherical molecules of lipids and proteins (apoproteins) Outer coat: -Amphipathic lipids -Phospholipids -Free Cholesterol + -Apoproteins Inner core: -Hydrophobic lipid -TAG -Cholesterol ester

6 Hydrophobic lipid TAG & Cholesterol ester Amphipathic lipids

7 Classification of Lipoprotein 1-Chylomicrons CM: derived from small intestine and carries exogenous dietary TAG 2-Very low density lipoproteins (VLDL) derived from liver and carries endogenous TAG

8 3-Low density lipoproteins (LDL): carries cholesterol from liver to tissues (Bad Lipoprotein) 4-High density lipoproteins (HDL) : carries cholesterol from tissues to liver to be excreted in bile (Good Lipoprotein)

10 By 2 methods : 1-Ultacentrifugation 2- Electrophoresis

11 Ultracentrifugation Chylomicrons VLDL LDL HDL This method separates lipoproteins according to their density. The higher the fat content the lower the density of the lipoprotein particle.

13 Electrophoresis Movement of lipoproteins in an electric field is depending on the charges located on their proteins. Higher protein content (negatively charged) moves faster towards the anode

14 Functions of lipoproteins 1- Transport of triacylglycerol (TAG rich particles) Chylomicrons: transport TAG of dietary origin VLDL:transport TAG of endogenous hepatic origin 2- Transport of cholesterol (cholesterol rich particles ) LDL: Carries cholesterol from liver to extrahepatic tissues. HDL: Carries cholesterol from peripheral tissues to liver. 3-Transport of fat-soluble vitamins and fat-soluble drugs 4-Delivery of cholesterol for steroid hormone producing tissues.

1- Structural some are essential structural component of the lipoprotein particles and can not be produced without them, others are transferred freely between

15 1- Structural some are essential structural component of the lipoprotein particles and can not be produced without them, others are transferred freely between lipoprotein 2-Regulatory activators or coenzymes for enzymes involved in lipoprotein metabolism 3- Ligands Functions of Apoproteins recognition sites for cell- surface receptors

16 Apoproteins Five major classes A.to...E with subclasses Integral proteins po B Peripheral Apo Proteins Apo B-48 Apo B-100 Apo A I,II, IV Apo C I, II,III Apo D Apo E

17 Lipoprotein Metabolism

20 Chylomicrons (CM) CM are assembled in intestinal mucosal cells, enter the lymphatic then the blood. CM have :Lowest density, largest size ( 1 micrometer), highest % of lipids and lowest % of proteins.

21 Carry TAG of (dietary origin) to peripheral tissues. Nascent (newly formed) CM: contains Apo B-48 (48% of the protein coded by Apo B gene)

22 Mature CM : contains Apo B-48 plus Apo C-II and Apo E ( taken from HDL during circulating in blood ). CM are responsible for physiological milky appearance of plasma (up to 2 hours after meal).

23 Lipoprotein lipase is required to degrade TAG into glycerol and fatty acids. As CM circulate in blood 90% of their TAG in the core are degraded by lipoprotein lipase, and decrease in size.

24 Apo C ( but not Apo E) returns back to HDL. The remaining particles are called CM remnants, that are rapidly removed from blood by the liver by endocytosis by specific receptors that recognize Apo E protein.

25 The receptors are recycled again to the cell membrane CM remenants are hydrolyzed by lysosomes in liver. What are the products of hydrolysis?

26 Q: What is the clearing factor? Lipoprotein lipase (LPL) LPL is synthesized by adipocytes, cardiac and skeletal muscle cells

Lipoprotein lipase (LPL) It is then transferred to the luminal surface of the capillaries to be attached by heparan sulfate to the capillary wall of most tissues

27 Lipoprotein lipase (LPL) It is then transferred to the luminal surface of the capillaries to be attached by heparan sulfate to the capillary wall of most tissues (cardiac, skeletal muscles, adipose tissue and lactating mammary gland) liver lacks this enzyme. Apo CII is cofactor activating LPL. Insulin enhances LPL synthesis in adipocytes

28 It hydrolyzes TAG of CM & VLDL releasing fatty acids and glycerol so it clears plasma from lipoproteins. Fatty Acids are taken up by cells 1) Stored: adipose tissue 2) Oxidized for energy: in muscle, or 3) Carried by albumin (Long- chain FA) until their uptake. Glycerol is taken up by the liver for a) Lipid synthesis, b) Glycolysis, c) Gluconeogenesis.

31 Very Low Density Lipoproteins (VLDL) They are assembled and secreted by the liver into the blood. They carry TAG of hepatic origin to peripheral tissues Nascent VLDL: contain Apo B-100 (100% of the protein coded by Apo Dr. B Maha gene) Sallam

32 Mature VLDL: contain Apo B-100 plus Apo C-II and Apo E (from HDL) ApoC-II activates lipoprotein lipase that degrades TAG into glycerol and fatty acids. As TAG are degraded, VLDL become smaller in size and more dense.

33 Modification of circulating VLDL Apo CII and Apo E return back to HDL Transfer of TAG from VLDL to HDL in exchange with Cholesterol Esters (CE) by cholesterol ester transfer protein ( a plasma protein). VLDL IDL LDL

34 Modification of circulating VLDL Transfer of TAG from VLDL to HDL in exchange with CE by cholesterol ester transfer protein. With these modifications the VLDL are converted in the plasma to LDL.

36 Uptake of LDL LDL particles contain much less TAG and more cholesterol and cholesterol ester than their VLDL precursors.

37 They provide cholesterol to the tissues by binding to cell membrane LDL receptors (in both liver and tissues ( that recognize Apo B -100 ) by receptor mediated endocytosis. ( which is highly specific and regulated). Another method of uptake of chemically modified LDL is by Machrophage scavenger receptors ( not regulated)

38 Uptake of LDL Receptor-Mediated Endocytosis ( regulated) Macrophage Scavenger Receptors (not regulated)

39 Receptor-Mediated Endocytosis (regulated)

40 LDL

41 LDL receptors are ve charged glycoproteins that are clustered in pits on cell membrane coated with the protein clathrin that stabilizes the shape of the pit. LDL-receptor complex is endocytosed inside the cells The vesicle containing LDL rapidly loses its clathrin coat and fuses with other similar vesicles forming endosomes. The receptor migrate to one side of the endosome whereas LDLs stay free in the lumen of the vesicle.

42 LDL receptors are recycled to cell membrane, whereas LDL in the vesicle are transferred to lysosomes where they are degraded by lysosomal enzymes releasing :Free Cholesterol, Phospholipids, Fatty Acids, Amino acids ( All are utilized by the cell. ) Excess cholesterol will : 1-inhibit synthesis of new LDL receptors protein thus decrease entry of LDL ( down regulation). 2- inhibit HMG CoA reductase 3- be esterified to Cholesterol Ester by ACAT to be stored

43 Macrophage Scavenger Receptors (not regulated)

45 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 apob 100 protein --> oxldl.

46 Ox LDL is engulfed by scavenger receptors of macrophages in subendothelial space which are not down regulated in response to increased intracellular cholesterol. CE accumulate in these cells and cause their transformation into foam cells. Foam cells accumulate in subendothelial layer releasing growth factors and cytokines that stimulate migration of smooth muscle cells from the media to the intima with the formation of atherosclerotic plaque

Chapter VIII: Dr. Sameh Sarray Hlaoui

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