2 What are lipoproteins? Lipoproteins are protein-lipid complexes. Inner droplet of neutral (water-insoluble core lipids); primarily triglycerides and cholesteryl estersA solubilizing surface layer of phospholipids and unesterified cholesterolSpecific proteins (apolipoproteins) attached to the outer lipid layer through their specific lipophilic domains
3 The Players – Lipids Triacylglycerol Phospholipids Cholesterol Cholesteryl esters
4 The Players - Apolipoproteins Apo AI (liver, small intestine)Structural; activator of lecithin:cholesterol acyltransferase (LCAT)Apo AII (liver)Structural; inhibitor of hepatic lipase; component of ligand for HDL bindingApo A-IV (small intestine)Activator of LCAT; modulator of lipoprotein lipase (LPL)Apo A-V (liver)Direct functional role is unknown; regulates TG levels.
5 Apolipoproteins Apo B-100 (liver) Apo B-48 (small intestine) Structural; synthesis of VLDL; ligand for LDL-receptorApo B-48 (small intestine)Structural; synthesis of chylomicrons; derived from apo B-100 mRNA following specific mRNA editingApo E (liver, macrophages, brain)Ligand for apoE receptor; mobilization of cellular cholesterol
6 Apolipoproteins Apo C-I (liver) Apo C-II (liver) Apo C-III (liver) Activator of LCAT, inhibitor of hepatic TGRL uptakeApo C-II (liver)Activator of LPL, inhibitor of hepatic TGRL uptakeApo C-III (liver)Inhibitor of LPL, inhibitor of hepatic TGRL uptake
8 Lipoprotein Classes LDL HDL > 30 nm 20–22 nm 9–15 nm Chylomicrons, VLDL, and their catabolic remnantsLDLHDLLipoproteins are classified into four groups which differ primarily in the amounts of cholesterol, trigyleride, phospholipids, and types of apolipoproteins they containOriginal classification was a function of hydrated density.> 30 nm20–22 nm9–15 nmD<1.006 g/mlD= g/mlD= g/mlDoi H et al. Circulation 2000;102: ; Colome C et al. Atherosclerosis 2000; 149: ; Cockerill GW et al. Arterioscler Thromb Vasc Biol 1995;15:Lipids Online
9 Lipoprotein Metabolism Exogenous/chylomicron pathway (dietary fat)Endogenous pathway (lipids synthesized by the liver)HDL metabolism (apolipoprotein transfer, cholesteryl ester transfer, reverse cholesterol transport
10 Lipoprotein Metabolism Exogenous/chylomicron pathway (dietary fat)Endogenous pathway (lipids synthesized by the liver)HDL metabolism (apolipoprotein transfer, cholesteryl ester transfer, reverse cholesterol transport
11 TG Rich: VLDL Surface Monolayer Phospholipids (12%) Free Cholesterol (14%)Protein (4%)VLDL contain 60-70% triglyceridesProduced by the liverTransport endogenously synthesized triglycerides to peripheral tissuesHydrophobic Core Triglyceride (65%) Cholesteryl Esters (8%)Cholesterol and Atherosclerosis, Grundy)
12 VLDL MetabolismApo C’s and apoE and cholesteryl ester are acquired from HDL in circulationCholesterol and Atherosclerosis, Grundy)
13 Fatty Acid TransportApoC-II activates lipoprotein lipase which catalyses the hydrolysis of TGCholesterol and Atherosclerosis, Grundy)
14 VLDL Conversion to LDLFurther action on IDL by hepatic lipase loses additional apolipoproteins (apoE) becomes and is converted to LDLCholesterol and Atherosclerosis, Grundy)
15 CE Rich: LDL Surface Monolayer Phospholipids (25%) Free Cholesterol (15%)Protein (22%)Hydrophobic Core Triglyceride (5%) Cholesteryl Esters (35%)Major cholesterol carrying lipoprotein2/3 - 3/4 of serum cholesterol is carried by LDL50% of mass is cholesterolProduced as a product of VLDL metabolismDelivers cholesterol to peripheral tissues for biosynthesis and steroid hormone productionCholesterol and Atherosclerosis, Grundy)
16 LDL MetabolismLDL is removed by apoB100 receptors which are mainly expressed in the liverHepatic LipaseCholesteryl ester transfer proteinCholesterol and Atherosclerosis, Grundy)
17 X X LDL Uptake by Tissues Defects in the LDL receptor leads to familial hypercholesterolemiaCholesterol and Atherosclerosis, Grundy)
18 CE Rich: HDL Surface Monolayer Phospholipids (25%) Free Cholesterol (7%)Protein (45%)Hydrophobic Core Triglyceride (5%) Cholesteryl Esters (18%)Smallest of the lipoproteinsSynthesized by intestine and liver as nascent cholesterol-poor lipoproteinAccumulates cholesterol and cholesteryl esters through interactions with peripheral cells and other lipoproteinsParticipates in reverse cholesterol transport, removal of excess cholesterol from peripheral cells and delivery to the liver for metabolismCholesterol and Atherosclerosis, Grundy)
19 HDL MetabolismNascent HDL (lipid-poor apoA-I) is produced by the liver and intestinePathways involved in the generation and conversion of HDL. Mature HDL3 and HDL2 are generated from lipid-free apoA-I or lipid-poor pre-ß1-HDL as the precursors. These precursors are produced as nascent HDL by the liver or intestine or are released from lipolysed VLDL and chylomicrons or by interconversion of HDL3 and HDL2. ABC1-mediated lipid efflux from cells is important for initial lipidation; LCAT-mediated esterification of cholesterol generates spherical particles that continue to grow on ongoing cholesterol esterification and PLTP-mediated particle fusion and surface remnant transfer. Larger HDL2 particles are converted into smaller HDL3 particles on CETP-mediated export of cholesteryl esters from HDL onto apoB-containing lipoproteins, on SR-BI–mediated selective uptake of cholesteryl esters into liver and steroidogenic organs, and on HL- and EL-mediated hydrolysis of phospholipids. HDL lipids are catabolized either separately from HDL proteins (ie, by selective uptake or via CETP transfer) or together with HDL proteins (ie, via uptake through as-yet-unknown HDL receptors or apoE receptors). The conversion of HDL2 into HDL3 and the PLTP-mediated conversion of HDL3 into HDL2 liberated lipid-free or poorly lipidated apoA-I. A part of lipid-free apoA-I undergoes glomerular filtration in the kidney and tubular readsorption through cubilin. For further details, see text and Table 1 . Blue arrows represent lipid transfer processes, and red arrows represent protein transfer processes. TGRL indicates triglyceride-rich lipoproteins
21 Hepatic Cholesterol Synthesis Only pathway for cholesterol degradationRate LimitingEnergetically expensive; prefer to conserve what is already made/acquired – LDL receptor pathwayCholesterol and Atherosaclerosis, Grundy)
22 LDL Cellular Metabolism LDL are taken up by the LDL Receptor into clathrin-coated pitsCholesterol and Atherosaclerosis, Grundy)
23 Endothelial Dysfunction Increased endothelial permeability to lipoproteins and plasma constituents mediated by NO, PDGF, AG-II, endothelin.Up-regulation of leukocyte adhesion molecules (L-selectin, integrins, etc).Up-regulation of endothelial adhesion molecules (E-selectin, P-selectin, ICAM-1, VCAM-1).Migration of leukocytes into artery wall mediated by oxLDL, MCP-1, IL-8, PDGF, M-CSF.Ross, NEJM; 1999
24 Formation of Fatty Streak SMC migration stimulated by PDGF, FGF-2, TGF-BT-Cell activation mediated by TNF-a, IL-2, GM-CSF.Foam-cell formation mediated by oxLDL, TNF-a, IL-1,and M-CSF.Platelet adherence and aggregation stimulated by integrins, P-selectin, fibrin, TXA2, and TF.Ross, NEJM; 1999
25 Formation of Advanced, Complicated Lesion Fibrous cap forms in response to injury to wall off lesion from lumen.Fibrous cap covers a mixture of leukocytes, lipid and debris which may form a necrotic core.Lesions expand at shoulders by means of continued leukocyte adhesion and entry.Necrotic core results from apoptosis and necrosis, increased proteolytic activity and lipid accumulation.Ross, NEJM; 1999
26 Development of Unstable Fibrous Plaque Rupture or ulceration of fibrous cap rapidly leads to thrombosis.Occurs primarily at sites of thinning of the fibrous cap.Thinning is a result of continuing influx of and activation of macrophages which release metalloproteinases and other proteolytic enzymes.These enzymes degrade the matrix which can lead to hemorrhage and thrombus formationRoss, NEJM; 1999
27 Role of LDL in Atherosclerosis Steinberg D et al. N Engl J Med 1989;320:EndotheliumVessel LumenLDLLDL Readily Enter the Artery Wall Where They May be ModifiedIntimaModified LDLModified LDL are ProinflammatoryHydrolysis of Phosphatidylcholine to LysophosphatidylcholineOther Chemical ModificationsOxidation of Lipids and ApoBAggregationRole of LDL in inflammationLDL readily enters the artery wall by crossing the endothelial membrane. Once in the arterial wall, if LDL accumulates, it is subject to a variety of modifications. The best known of these is oxidation, both of the lipids and of the apo B. LDL is also subject to aggregation, and its phospholipids are subject to hydrolysis by phospholipases to form lysophosphatidylcholine. Several other chemical modifications have also been reported. The net effect of these changes is the production of a variety of modified LDL particles, and the evidence is now very strong that these modified LDL particles are proinflammatory.Reference:Steinberg D, Parthasarathy S, Carew TE, Khoo JC, Witztum JL. Beyond cholesterol: modifications of low-density lipoprotein that increase its atherogenicity. N Engl J Med 1989;320:Lipids Online
28 Role of LDL in Atherosclerosis EndotheliumVessel LumenMonocyteMacrophageMCP-1Adhesion MoleculesFoam CellIntimaModified RemnantsCytokinesCell Proliferation Matrix DegradationDoi H et al. Circulation 2000;102:Growth Factors MetalloproteinasesRemnant LipoproteinsRemnantsThe remnants of VLDL and chylomicrons are also pro-inflammatoryVLDL remnants and chylomicron remnants behave in much the same way as LDL. They enter the subendothelial space, where they become modified, and the modified remnants stimulate MCP-1, promote the differentiation of monocytes into macrophages, and are taken up by the macrophages to form foam cells. Like LDL, the remnant lipoproteins are proinflammatory and proatherogenic.References:Doi H, Kugiyama K, Oka H, Sugiyama S, Ogata N, Koide SI, Nakamura SI, Yasue H. Remnant lipoproteins induce proatherothrombogenic molecules in endothelial cells through a redox-sensitive mechanism. Circulation 2000;102:Lipids Online
29 HDL Prevent Foam Cell Formation LDLMiyazaki A et al. Biochim Biophys Acta 1992;1126:73-80.EndotheliumVessel LumenMonocyteModified LDLMacrophageMCP-1Adhesion MoleculesCytokinesIntimaHDL Promote Cholesterol EffluxFoam CellHDL prevent formation of foam cellsPerhaps the best-known function of HDL is the promotion of cholesterol efflux from cells. Efflux of cholesterol from foam cells leads to a reduction in foam cell formation; although the macrophages may accumulate, they are not converted into foam cells. As a result, the inflammatory process is arrested to a certain extent. Therefore, HDL is anti-inflammatory and also protects against the development of atherosclerosis.Lipids Online
30 Atherosclerosis and lipoprotein metabolism Atheromatous disease is ubiquitous and underlies the commonest causesof death (e.g. myocardial infarction) and disability (e.g. stroke) in industrialcountriesHypertension and dyslipidemia are ones of the most important risk factors, amenable to drug therapyATHEROMA is a focal disease of the intima of large and medium-sized arteriesA t h e r o g e n e s i s involves several stages:endothelial dysfunction with altered PGI2 and NO synthesismonocyte attachmentendothelial cells bind LDLoxidatively modified LDL is taken up by macrophageshaving taken up oxidised LDL, these macrophages (now foam cells) migratesubendotheliallyatheromatous plaque formationrupture of the plaque
31 Atherosclerosis and lipoprotein metabolism LIPIDS, including CHOLESTEROL (CHO) and TRIGLYCERIDES (TG), are transported in the plasma as lipoproteins, of which there are four classes:- chylomicrons transport TG and CHO from the GIT to the tissues, wherethey are split by lipase, releasing free fatty acids.There are taken up in muscle and adipose tissue. Chylomicron remnants are taken up in the liver- very low density lipoproteins (VLDL), which transport CHO and newly synthetised TG to the tissues, where TGs are removed as before, leaving:- low density lipoproteins (LDL) with a large component of CHO, some of which is taken up by the tissues and some by the liver, by endocytosis via specificLDL receptors- high density lipoproteins (HDL).which absorb CHO derived from cell breakdown in tissues and transfer it to VLDL and LDL
32 Atherosclerosis and lipoprotein metabolism There are two different pathways for exogenous and endogenous lipids:THE EXOGENOUS PATHWAY: CHO + TG absorbed from the GIT are transported in the lymph and than in the plasma as CHYLOMICRONS to capillaries in muscle and adipose tissues. Here the core TRIGL are hydrolysed by lipoprotein lipase, and the tissues take up the resulting FREE FATTY ACIDSCHO is liberated within the liver cells and may be stored, oxidised to bile aids or secreted in the bile unalteredAlternatively it may enter the endogenous pathway of lipid transpor in VLDL
33 Atherosclerosis and lipoprotein metabolism ENDOGENOUSPATHWAYEXOGENOUSPATHWAY
34 Atherosclerosis and lipoprotein metabolism THE ENDOGENOUS PATHWAYCHO and newly synthetised TG are transported from the liver as VLDL to muscle and adipose tissue, there TG are hydrolysed and the resultingFATTY ACIDS enter the tissuesThe lipoprotein particles become smaller and ultimetaly become LDL ,which provides the source of CHO for incorporation into cell membranes, for synthesis of steroids, and bile acidsCells take up LDL by endocytosis via LDL receptors that recognise LDL apolipoproteinsCHO can return to plasma from the tissues in HDL particles and the resulting cholesteryl esters are subsequently transferred to VLDL or LDLOne species of LDL – lipoprotein - is associated with atherosclerosis (localised in atherosclerotic lesions). LDL can also activate platelets, constituting a further thrombogenic effect
35 Dyslipidemia Dyslipidemia can be primary or secondary. The normal range of plasma total CHO concentration < 6.5 mmol/L.There are smooth gradations of increased risk withelevated LDL CHO conc, and with reduced HDL CHO conc.Dyslipidemia can be primary or secondary.The primary forms are genetically determinedSecondary forms are a consequence of other conditionssuch as diabetes mellitus, alcoholism, nephrotic sy,chronic renal failure, administration of drug…
36 Lipid-lowering drugs Several drugs are used to decrease plasma LDL-CHO Drug therapy to lower plasma lipids is only one approach to treatmentand is used in addition to dietary managementand correction of other modifiable cardiovascular risk factors
37 increase in synthesis of CHO receptors + increased clearance of LDL LIPID-LOWERING DRUGS: StatinsHMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase inhibitors. The reductase catalyses the conversion of HMG-CoA to mevalonic acid; blocks the synthesis of CHO in the liver:Simvastatin + pravastatin + atorvastatindecrease hepatic CHO synthesis: lowers total and LDLincrease in synthesis of CHO receptors+ increased clearance of LDLStimulates the exprssion of more enzyme restores CHO synthesis to normal.Several studies demonstrated positive effects on morbidity and mortality.Reltatively few side-effects...However, adverse effects: myopathy (incr in pts given combined therapy with nicotinic acid or fibrates. Should not be given during pregnancy.
38 Promising pharmacodynamic actions: LIPID-LOWERING DRUGSStatinsPromising pharmacodynamic actions:improved endothelial functionreduced vascular inflammation and platelet aggregabilityantithrombotic actionstabilisation of atherosclerotic plaquesincreased neovascularisation of ischaemic tissueenhanced fibrinolysisimmune suppressionosteoclast apoptosis and increased synthetic activity inosteoblasts
39 LIPID-LOWERING DRUG Statins Pharmacokineticswell absorbed when given orallyextracted by the liver (target tissue), undergo extensive presystemic biotransformationSimvastatin is an inactive pro-drug
40 LIPID-LOWERING DRUG Statins C l i n i c a l u s e sSecondary prevention of myocardial infarction and stroke in patients who have symptomatic atherosclerotic disease (angina, transient ischemic attacks) following acute myocardial infarction or strokePrimary prevention of arterial disease in patients who are at high risk because of elevated serum CHO concentration, especially it there are other risk factors for atherosclerosisAtorvastatin lowers serum CHO in patients with homozygous familiar hypercholesterolemia
41 LIPID-LOWERING DRUG Statins A d v e r s e e f f e c t s:mild gastrointestinal disturbancesincreased plasma activities in liver enzymessevere myositis (rhabdomyolysis)and angio-oedema (rare)
42 LIPID-LOWERING DRUGS: Fibrates - stimulate the β-oxidative degradation of fatty acids- liberate free fatty acids for storage in fat or for metabolism instriated muscle- Are ligands for nuclear txn receptor, peroxisome proliferator-activated recptor-α (PARP-α)- increase the activity of lipoprotein lipase,hence increasing hydrolysis of triglyceride in chylomicronsand VLDL particles.reduce hepatic VLDL production and increase hepatic LDLuptake.Produce a modest decrease in LDL (~ 10%) and increase in HDL (~ 10%).But, a marked decrease in TGs (~ 30%).
43 fenofibrate clofibrate gemfibrozil ciprofibrate LIPID-LOWERING DRUGSFibratesO t h e r e f f e c t s :improve glucose toleranceinhibit vascular smooth muscle inflammationfenofibrate clofibrategemfibrozil ciprofibrate
44 A d v e r s e e f f e c t s: LIPID-LOWERING DRUGS Fibrates in patients with renal impairment myositis (rhabdomyolysis)myoglobulinuria, acute renal failureFibrates should be avoided in such patients and also in alcoholics)mild GIT symptoms
45 LIPID-LOWERING DRUGS 1st-line defense for: Fibrates1st-line defense for:*mixed dyslipidemia (i.e. raised serum TG and CHO)* patients with low HDL and high risk of atheromatous disease (often type 2 diabetic patients)* patients with severe treatment- resistant dyslipidemia (combination with other lipid-lowering drugs).* Indicated in patients with VERY HIGH [TG]s who are at risk for pancreatitis
46 Bile acid binding resins (Anion-exchange resins) LIPID-LOWERING DRUGSBile acid binding resins (Anion-exchange resins)sequester bile acids in the GIT prevent their reabsorptionand enterohepatic recirculationThe r e s u l t is:decreased absorption of exogenous CHO and increased metabolism of endogenous CHO into bile acid acidsincreased expression of LDL receptors on liver cellsincreased removal of LDL from the bloodreduced concentration of LDL CHO in plasma(while an unwanted increase in TG)
47 Anion-exchange Resins Increase the excretion of bile acids, causing more CHO to be converted to BAs.The decr in hepatocyte [CHO] compenatory incr in HMG CoA reductase activity and the number of LDLRs.Because these resins don’t work in patients with homozygous familial hypercholesterolemia, increased expression of hepatic LDLRs is the main mechanism by which resins lower plasma CHO.
48 LIPID-LOWERING DRUGS Colestyramin colestipol C l i n i c a l u s e s: Bile acid binding resinsColestyramin colestipolanion exchange resinsC l i n i c a l u s e s:heterozygous familiar hypercholesterolemiaan addition to a statin if response has been inadequatehypercholesterolemia when a statin iscontraindicateduses unrelated to atherosclerosis, including:pruritus in patients with partial biliary obstructionbile acid diarrhea (diabetic neuropathy)
49 LIPID-LOWERING DRUGS A d v e r s e e f f e c t s: Bile acid binding resinsA d v e r s e e f f e c t s:GIT symptoms - nauzea, abdominal bloating,constipation or diarrhea, bec resins not absorbed.resins are unappetizing. This can be minimized bysuspending them in fruit juiceinterfere with the absorption of fat-soluble vitaminsand drugs (chlorothiazide, digoxin, warfarin)These drugs should be given at last 1 hour before or 4-6 hours after a resin
50 Others LIPID-LOWERING DRUGS Nicotinic acid inhibits hepatic TG production and VLDLSecretion (by ~ 30-50%)modest reduction in LDL and increase in HDL.Nicotinic acid was the 1st lipid-lowering drug to decr overall mortality in patients with CAD.But its use is limited by the desirableA d v e r s e e f f e c t s:flushing, palpitations , GIT disturbances.Currently, nicotinic acid is rarely used.
51 Others LIPID-LOWERING DRUGS Fish oil (rich in highly unsaturated fatty acids)the omega-3 marine TG- reduce plasma TG but increase CHO (CHO is more strongly associated wih coronary artery disease)the effects on cardiac morbidity or mortality is unproven( although there is epidemiological evidence that eating fish regularly does reduce ischemic heart disease)
52 Others LIPID-LOWERING DRUGS Inhibitors of Intestinal CHO Absorption: Ezetimibe:Reduces CHO and phytosterol absorption and decreases LDL CHP by ~18%, but with little change in HDL CHO.May be synergistic with statins: so good for combination therapy.
53 Drug CombinationsSevere hyperlipidemia often requires multiple LLDs to get the job done.As usual, combinations should involve drugs with different mechanisms of action (e.g., statins with fibrates).Even though some combinations (foregoing) may increase risk of, say, myopathy, the benefits of lowering LDL CHO outweigh the small incr in adverse effects.Recent trial with gemfibrozil (fibrate) decr myocardial infarction, stroke, and overall mortality in men with CAD assoc with low HDL (this drug inc HDL CHO w/o decr LDL CHO).