Cholesterol and Lipoproteins

Theme: Chest Pain

Objectives 1- Lipoproteins characteristics (Structure and Composition) 2- Metabolism of Lipoproteins Chylomicrons, HDL, LDL, IDL, vLDL and HDL) including regulatory mechanisms 3- Lipoprotein receptors and receptor mediated endocytosis 4- Transport of Cholesterol by Lipoproteins 5- Biochemical aspects of Atherosclerosis

LIPOPROTEINS spherical particles with a hydrophobic core (TG and esterified cholesterol) apolipoproteins on the surface large: apoB (b-48 and B-100) smaller: apoA-I, apoC-II, apoE classified on the basis of density and electrophoretic mobility (VLDL; LDL; IDL;HDL;

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

Lipoprotein structure

The apolipoproteins major components of lipoproteins often referred to as aproteins classified by alphabetical designation (A-E) the use of roman numeral suffix describes the order in which the apolipoprotein emerge from a chromatographic column responsible for recognition of particle by receptors

Apoproteins of human lipoproteins A-I (28,300)- principal protein in HDL 90 –120 mg% in plasma; activates LCAT A-II (8,700) – occurs as dimer mainly in HDL 30 – 50 mg %; enhances hepatic lipase activity B-48 (240,000) – found only in chylomicron <5 mg %; derived from apo-B-100 gene by RNA editing; lacks the LDL receptor-binding domain of apo-B-100 B-100 (500,000) – principal protein in LDL 80 –100 mg %; binds to LDL receptor

Apoproteins of human lipoproteins C-I (7,000) – found in chylomicron, VLDL, HDL 4 – 7 mg %; may also activate LCAT C-II (8,800) - found in chylomicron, VLDL, HDL 3 – 8 mg %; activates lipoprotein lipase C-III (8,800) - found in chylomicron, VLDL, IDL, HDL 8 15 mg %; inhibits lipoprotein lipase D (32,500) - found in HDL 8 – 10 mg %; also called cholesterol ester transfer protein (CETP) E (34,100) - found in chylomicron, VLDL, IDL HDL 3 – 6 mg %; binds to LDL receptor H (50,000) – found in chylomicron; also known as b-2-glycoprotein I (involved in TG metabolism)

Major lipoprotein classes Chylomicrons (derived from diet) density <<1.006 diameter 80 - 500 nm dietary triglycerides apoB-48, apoA-I, apoA-II, apoA-IV, apoC-II/C-III, apoE

Chylomicron Synthesis , RER-Golgi (Apo-B-48, cytosine to uracil) Assembly, enzymes in SER, (microsomal triacylglycerol transfer protein, MTP) Modification of nascent chylomicron , addition of apo-E and apo-CII source is HDL, (necessary for activation of Lipo-protein lipase) LPL regulation, Km large in adipose, small in heart (energy requirement of heart muscle…) Chylomicron remnants, less TAG, -apoC, apoE for receptor binding in liver, receptor mediated endocytosis.

Cholesterol and lipid transport Chylomicrons

Major lipoprotein classes VLDL density >1.006 diameter 30 - 80nm endogenous triglycerides apoB-100, apoE, apoC-II/C-III formed in the liver as nascent VLDL (contains only triglycerides, apoE and apoB)

VLDL nascent VLDLs interact with HDL to generate mature VLDLs (with added cholesterol, apoC-II and apoC-III) mature VLDLs are acted upon by LpL to generate VLDL remnants (IDL) IDL are further degraded by hepatic triglyceride lipase (HTGL) to generate LDLs

Major lipoprotein classes IDL (intermediate density lipoproteins) density: 1.006 - 1.019 diameter: 25 - 35nm cholesteryl esters and triglycerides apoB-100, apoE, apoC-II/C-III

Major lipoprotein classes LDL (low density lipoproteins) density: 1.019 - 1.063 diameter: 18-25nm cholesteryl esters apoB-100 < 130 LDL cholesterol is desirable, 130-159 is borderline high and >160 is high

Metabolism of LDL Receptor mediated endocytosis High cholesterol due to LDL function has following consequences Inhibition of HMG COA reductase LDL receptor synthesis inhibition (transcription…) ACAT Chemically modified LDL (macrophage scavenger receptors SR-A, accumulation in macrophages----foam cells-atherosclerosis

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

Representation of the LDL receptor (839 aa) extracellular domain is responsible for apo-B-100/apo-E binding intracellular domain is responsible for clustering of LDL receptors into coated pit region of plasma membrane

Major lipoprotein classes HDL (high density lipoproteins) density: 1.063-1.210 diameter: 5-12nm cholesteryl esters and phospholipids apoA-I, apoA-II, apoC-II/C-III and apoE

HDLs Several subfamilies exist Discoidal HDL : contains cholesterol, phospholipid, apoA-I, apoA-II, apoE and is disc shaped; it is formed in liver and intestine It interacts with chylomicron remnants and lecithin-cholesterol acyl transferase (LCAT) to form HDL3

HDLs HDL3 composed of cholesterol, cholesterol ester, phospholipid and apoA and apoE interacts with the cell plasma membranes to remove free cholesterol reaction with LCAT converts HDL3 to HDL2a (an HDL with a high apoE and cholesterol ester content) cholesterol ester-rich HDL2a is then converted to triglyceride-rich HDL2b by concomitant transfer of HDL cholesterol esters to VLDL and VLDL triglycerides to HDL with the help of CETP

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)

Reverse Cholesterol Transport Inverse relation ship between plasma HDL and atherosclerosis Efflux of cholesterol from periphery to HDL (ABCA1), Esterification by LCAT, binding of HDL2 (rich in cholesterol esters) to liver and steroidogenic cells, transfer of cholesterol to these cells release of lipid depleted HDL3

Lipoproteins (a)- Lp(a) another atherogenic family of lipoproteins(at least 19 different alleles) they consist of LDL and a protein designated as (a) the apoA is covalently linked to apoB-100 by a disulfide linkage unusual in that it contains a kringle protein motif/domain (tri-looped structure with 3 intramolecular disulfide bonds Large amounts in plasma are associated with high risk association with premature coronary artery disease and stroke Elevated level slows breakdown of blood clot. Similar to plasminogen, competes with it for binding with fibrin

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

Frederickson -WHO classification Type I: incr. chylomicrons, reduced HDL, absence of lipoprotein lipase; deficiency of apo CII (hyperchylomironemia) Type II-A: raised LDL; decreased catabolism of LDL (receptor deficiency or polygenic) Type II-B: raised VLDL + LDL; often reduced HDL; increased production of VLDL + impaired LDL catabolism Type III: raised IDL (dysbetalipoproteinemia); abnormal apolipoprotein E; impaired catabolism of IDL; elevated cholesterol and triglycerides (formerly known as broad beta disease)

Frederickson -WHO classification Type IV: raised VLDL; often reduced HDL; impaired VLDL catabolism; dietary indiscretion ( formerly known as hyperprebetalipoproteinemia) Type V: raised chylomicrons + VLDL; reduced HDL; reduced lipoprotein lipase + VLDL hypersecretion (formerly known as mixed lipemia)

Disorders of Cholesterol and Lipoproteins in 3 Categories Specific Familial/Genetic Disorders These comprise a small minority of patients Secondary to other Diseases Diabetes, Hypothyroid, Nephrotic syndrome, Renal Failure, Lupus Dietary/Polygenic --(most common) This is the great majority of patients with elevated cholesterol/LDL

Thank You