Cholesterol transport and uptake - PowerPoint Presentation

1. Cholesterol transport and uptakeDr. Carolyn K. Suzuki1

2. To compare and contrast the properties of apolipoprotein particles (e.g. chylomicrons, LDL, HDL), with respect to their composition, metabolism and transport. To distinguish the different biochemical pathways that can be potentially targeted pharmacologically to control plasma apolipoprotein levels and manage cardiovascular disease.To predict the effect of LDL receptor mutations on the levels of intracellular cholesterol and the regulation of cholesterol synthesis within the cell. OBJECTIVES2

3. Anatomy of lipoprotein particles3

4. Major classes of lipoprotein particleschylomicronsVLDLs- very low density lipoproteinsLDLs- low density lipoproteinsHDLs- high density lipoproteinsPrincipal lipid components of lipoproteinstriacylglycerolscholesterol estersphospholipidsPrincipal protein components of lipoprotein particlesapolipoproteins- five classes A-Eimportant in release of lipoprotein particles from cellactivate lipid-processing enzymes in bloodmediate uptake of lipoprotein particles into cells Lipoprotein particlesLipoprotein particles- general characteristics and functionsspherical particles with varying amounts of lipid and protein maintain solubility of constituent lipidstransport of lipids in plasma4

5. Chylomicrons5

6. Major classes of lipoprotein particlesLippincott Fig. 18.196

7. Relative size and densities of lipoproteins7

8. Cholesterol is absorbed in the small intestine and assembled into chylomicronsplasma before acholesterol-rich mealplasma after acholesterol-rich meal8

9. Knuth N D , Horowitz J F J. Nutr. 2006;136:1498-1503Clearance of chylomicrons from plasmarepresents tissue uptake and chylomicron breakdown9

10. Some clinical manifestations of hyperlipidemia A. Cutaneous xanthomas linked to elevated plasma chylomicrons and/or LDL. B. Lipemic plasma (left), normal plasma (right).C. Lipemia retinalis, elevated plasma triglyceride.D. Tuberous xanthomas, usually on extensor surfaces.E. Palmar crease xanthomas.10

11. non-hepatic tissuesLDLHDLchylomicronLDL receptorcholesterolLPLFFAINTESTINEChylomicron metabolism starts in the intestine- LIVER(1) Chylomicrons are assembled in the intestine and contain apo B48(2) Chylomicrons are released into lymphC EC EC E(3) Chylomicrons acquire apo C-II and apo E from HDL in plasma11

12. non-hepatic tissuesLDLHDLchylomicronLDL receptorcholesterolLPLFFAINTESTINEChylomicron metabolism starts in the intestine- LIVER(1) Chylomicrons are assembled in the intestine and contain apo B48(2) Chylomicrons are released into lymphC EC EC E(3) Chylomicrons acquire apo C-II and apo E from HDL in plasma(4) Lipoprotein lipase on the surface of non-hepatic tissues, hydrolyzes triglycerides(see next slide)C EC EC EC EC ECE12

13. Lipoprotein lipase metabolizes chylomicrons on the cell surface of non-hepatic tissuestriacylglycerol (TG)apo CIIapo B48endothelial surface of non-hepatic cellmuscle & adipose tissueLIPOPROTEIN LIPASEon the surface of non-hepatic tissues, hydrolyzes TGLiverGlycolysisGluconeogenesisLipid synthesis glycerol + free fatty acids (FFA)TGfree fatty acidsFFA are taken up by non-hepatic cellsapo CII on chylomicrons (or VLDLs) binds to 13

14. EEEECCC5) Chylomicron remnants depleted of glycerol and FFA transfer apo C-II to HDLLIVERnon-hepatic tissuesC EC EC EC EC EC EChylomicron metabolism (cont’d)- formation of chylomicron remants, cholesterol delivery to liverINTESTINEC EC EC E1234chylomicron remnantsnascent chylomicronHDLcholesterolEEE6) Remnants w/ apoE and apoB48, bind to the apo E receptor on liver cells, resulting in the uptake of remnants14

15. ECIIA11b. HDLassembled in liver and intestinetransfers apo CII/E to nascent chylomicronstriacylglycerolcholesterol esterphospholipid1a. nascent chylomicronsassembled in intestinereleased into plasmaw/ apoB-48, which is unique to nascent formB483. lipoprotein lipase capillary walls, hydrolyzes TGdelivers FFA into adipose & muscleadipose &muscleFFA4. chylomicron remnantslack apoC-II,which is transferred to HDLapo CIIEB48Summary- chylomicron interactions with HDL ECIIB48E/CII from HDL2. mature chylomicronsapo E and C-II added from HDL apoC-II activates lipoprotein lipase5. mature HDLs re-acquire apo C-II, alsoacquires cholesterol from membranes, accumulates apoCII/ and E, transferring them to VLDL & LDL, functions in reverse transport of cholesterol to liver A1CIICIIE15

16. BBB(5) LDLbinds receptor on cellse.g. fat,muscleBBLIVER(8) cholesterol is excreted as bileVLDL and LDL metabolism starts in the liver(6) LDL is taken up by cells, increasing intracellular cholesterolLDLHDLVLDLLDL receptorcholesterolLPLFFAC EC E(4) apo C-II and apo-Eare transferred from VLDL to HDL resulting in LDLBB(1) assembly and export of nascent VLDL containing apoB100 B(2) nascent VLDL acquires apoC-II and apoE from HDLC EC EC EC EBBnon-hepatic tissuee.g. fat and muscle(3) Lipoprotein lipase hydrolyzes TGs, FFA are taken up, LDL circulatesC EBCEBCEB(7) LDL and HDL bind specific receptors and mediate uptake in the liver non-hepatic tissue16

17. B100(1b) nascent VLDLsassembled in liver mediated by apoB100(1a) HDLsassembled in liver transfer apoCII/E to VLDLsECIIA1Summary- VLDL and LDL interactions with HDL(5) LDLs are derived from from VLDLs thatNo longer contain apoCII and EB100FFA(3) lipoprotein lipasehydrolyzes TGFFA are delivered to adipose tissue & muscleadipose &muscleCII + EA1ECII(4) mature HDLsre-acquireapoCII/E from VLDLs ECIIB100(2) mature VLDLsapoE and CII are acquired from HDLapoCII activates lipoprotein lipase17

18. apo E receptorapo E receptorLIVERLDL receptorchylomicronremnantsB48EEB100LDLlipoprotein uptakecholesterol ester metabolismbile storageSummary- lipoprotein particle receptors in liverPCSK9mature HDLEEAIAIICII18

19. General characteristics of HDLs synthesized in the liver and intestine secreted directly into the blood from liver and intestine protein rich express apo-AI and AII, apo-CII and apo-E nearly devoid of cholesterol and cholesterol estersattempts to increase HDL by increasing AI synthesisdevelopment of CETP inhibitorsdata show that people with CETP deficiency have increased HDL, lower riskof heart diseaseR & Dnew cholesterol lowering drugsHDL-apolipoprotein exchange HDL transfers apo-CII and apo-E to chylomicrons Chylomicrons return apo-CII to mature HDLs HDL transfers apo-CII and apo-E to VLDLs VLDL returns apo-CII and apo-E to HDLsHDL and cholesterol/cholesterol ester exchange HDL can acquire cholesterol from chylomicrons, VLDLs or membrane and convert them to cholesterol esters Cholesterol esters in HDL can be transferred to VLDLs and LDLs by cholesterol ester transfer protein (CETP)HDL and reverse cholesterol transport- HDLs that are rich in cholesterol esters are returned to liver 19

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21. Cholesterol paradigm of atherosclerosisBrown and GoldsteinNobel Prize in Physiology and Medicine 1985Familial Hypercholesterolemia (FH) Elevated total cholesterol >300 mg/dL in adults >250 mg/dL in children Dominant inheritance Heterozygotes (1 in 500) heart attacks at 30-40 yrs Homozygotes (1 in million) heart attacks in childhoodTheir hypothesis: FH is caused by defects in the regulation of cholesterol synthesis21

22. LDL receptor mediates cellular uptake of cholesterol by"receptor-mediated endocytosis"When the LDL receptor functions normally-increased blood cholesterol leads toincreased LDL uptake into cells, resulting in increased cholesterol in cells and inhibition of cholesterol synthesisRemember from our last lecture- when intracellular cholesterol is highexpression of cholesterol synthesis genes is blockedHMG CoA reductase is degradedFamilial Hypercholesterolemia (FH) Caused by mutations in the gene encoding the LDL receptor(also known as the apoB-100/apoE receptor)1722

23. LDL receptorLDL binding domainbinds apolipoprotein N-linked oligosaccharide domainrequired for LDL binding Transmembrane domainO-linked oligosaccharide domainCytosolic domainhighly conserved requires for endocytosis

24. Class I- No receptors synthesized. Mutations in LDLR promoter, frameshift or splicing mutations.Class 2- Receptors are synthesized but retained intracellularly in the endoplasmic reticulum or Golgi complexClass 3- Receptors reach the cell surface but lack normal LDL bindingClass 4- Receptors reach the cell surface and bind LDL but are not clustered in coated pits and endocytosed.LDL receptorAll above mutations lead to high blood cholesterol levels

25. Receptor-mediated endocytosis of LDLLippincott Fig. 18-2025

26. 26Receptor-mediated endocytosis of LDLLDL particleclathrincoated vesicleendosomeendocytosis ofLDL bound receptorinto cellLDL receptorsrecycle to plasma membraneLDL dissociates from receptorin endosome

27. endosome fuses w/lysosomeACATacyl CoA cholesterolacetyltransferasecholesterolcholesterolester

28. down-regulation ofcholesterol synthesis genes

29. Regulation of cholesterol synthesis and uptake

30. PCKS9- another drug target for reducing LDL levels30PCKS9 is a normal human protein that leads to LDL receptor degradation in lysosomesPCKS9LDLreceptorLDL receptors bound to PCKS9 are degraded in lysosomeslysosomesLDLreceptorrecyclingendosomes reduced plasma LDLSearch for inhibitors of PCKS9

31. Where are chylomicrons synthesized?Review- you tell me !!!!Where are VLDL particles synthesized?Non-functional LDL receptors result in:Lower or higher plasma levels of cholesterol?Lower of higher intracellular levels of cholesterolWhich lipoprotein particle is the largest?Which is the smallest?

32.