Cholesterol_Transport_FA23 (1).pptx

CHOLESTEROL TRANSPORT Vanessa De La Rosa, PhD CV I •Differentiate the function and metabolism of different blood lipoproteins, including chylomicrons, VLDL, IDL, LDL, and HDL. SESSION OBJECTIV ES •Compare and contrast forward and reverse cholesterol transport, including lipoprotein composition...

CHOLESTEROL TRANSPORT Vanessa De La Rosa, PhD CV I •Differentiate the function and metabolism of different blood lipoproteins, including chylomicrons, VLDL, IDL, LDL, and HDL. SESSION OBJECTIV ES •Compare and contrast forward and reverse cholesterol transport, including lipoprotein composition and metabolism. •Describe the function of apolipoproteins and differentiate their roles in cholesterol transport. •Describe functions of essential enzymes in lipoprotein metabolism, including lipoprotein lipase, hepatic lipase, lecithin-cholesterol acyl transferase (LCAT), phospholipid transfer protein (PLTP), and cholesterol ester transfer protein (CETP). •Describe and compare the effects of disrupted lipid transport on lipid profiles (dyslipidemia). CHOLESTEROL PROPERTIES AND TRANSPORT • Cholesterol is water insoluble • Transported from tissues of origin (intestine & liver) to peripheral tissues- forward transport • Excess transported back to liver for excretion- reverse transport • Transported within plasma by lipoprotein particles • Dyslipidemias are a class of disorders defined by alterations in lipid levels CLINICAL RELEVANCE 1 Lipoprotein and lipid levels are used to predict coronary heart disease risk 2 Dyslipidemias are a class of genetic disorders defined by alterations in lipid profiles BLOOD LIPOPROTEINS THAT TRANSPORT LIPIDS 8 major classes of lipoproteins ELECTROPHORE LIPOPROTEIN TIC MOBILITY Least dense; larger Most dense; smaller • • • • Core of hydrophobic lipids (cholesterol esters and TGs) Outer shell of polar lipids and apolipoproteins Major carriers of lipids are chylomicrons, VLDL, and HDL Each lipoprotein associated with specific TG LIPID (%) a CHOL PL 80–95 2–7 3–9 Chylomicrons Origin Chylomicron remnants Slow pre-β VLDL Pre-β 55–80 5–15 10–20 IDL Slow pre-β 20–50 20–40 15–25 LDL β 5–15 40–50 20–25 HDL2 α 5–10 15–25 20–30 HDL3 α Lip(a) Pre-β FUNCTION Deliver dietary lipids Return dietary lipids to the liver Deliver endogenou s lipids Return endogenou s lipids to the liver; precursor of LDL Deliver cholesterol to cells Reverse cholesterol transport Reverse cholesterol transport CLINICAL CHEMISTRY: LIPOPROTEINS (A) Ultracentrifugation separates lipoproteins by density (B) Agarose gel electrophoresis separates based on charge Not easily adapted for clinical setting! APOLIPOPROTEINS • Provide structure • Enzyme co-factors for normal lipoprotein metabolism • Ligands on lipoprotein surface that target receptors in peripheral tissues Apolipoprotei n Function Apo A-1 Required for HDL formation; activates LCAT Apo B-48 Mediates synthesis and secretion of chylomicrons Apo B-100 Mediates VLDL synthesis and secretion; binds LDL receptor IDL, LDL, VLDL and mediates LDL endocytosis Builds (VLDL) and binds LDL receptor (larger Apo-B has more work) Apo C-2 Binds and activates LPL Chylomicrons, HDL, IDL, VLDL Co-factor (activates LPL) Mediates endocytosis of Chylomicrons, Endocytosis of Apo E Found in Mnemonic HDL Acquires cholesterol from periphery Chylomicrons Builds (chylomicron) microsom al triglycerid e transfer protein (MTP) FORWARD TRANSPORT OF LIPIDS Two pathways for forward transport Endogeno us Exogenous As lipoproteins progress through transport, become denser, with less TGs, and more cholesterol and protein Lipids transported to peripheral tissues for storage or energy 8 Acquire s ApoE and C-II from HDL 1. EXOGENO US TRANSPO RT PATHWAY •CM are considerably less dense than water and float without centrifugation •High CM results in “milky” plasma and accumulate as a floating creamy layer Receptor mediated endocytosi s 2. ENDOGENOUS PATHWAY • TG-rich (smaller than chylomicron; relative greater content of cholesterol & CE) • Contains apoB-100 (apoB-48 of chylomicrons is a truncated version) • VLDL metabolism results in CE rich IDL Nascent VLDL ApoB-100 Loss of ApoE • Hepatic delivery of IDL via LDL receptor or hepatic lipase • Results in CE rich LDL • LDL-C (LDL cholesterol) delivered to peripheral tissues via LDL receptor • LDL receptor has lower affinity for ApoB100 compared to apoE = longer residence time in circulation (days) compared to IDL (hours) • LDL DOES NOT alter the clarity of plasma even at greatly increased 50% 10 RECEPTOR MEDIATED ENDOCYTOSIS OF LDL LDL receptors modulate plasma LDL levels High dietary intake of saturated fats and cholesterol raises plasma LDL levels primarily by down-regulating hepatic LDL receptors Non-hepatic cells obtain cholesterol primarily from plasma FAMILIAL HYPERCHOLESTEROLEMIA • Autosomal dominant disorder • Caused by gene mutations in LDL receptor • Divided into 5 classes based on phenotype (>900 mutations identified) • Severely elevated serum cholesterol levels and normal triglyceride levels SUMMARY OF DISORDERS IN FORWARD TRANSPORT 1. Anderson’s disease presents in childhood; defect in secretion of apoB-48–containing lipoproteins  fat malabsorption and low levels of chol and TGs 2. Mutations in apoB gene or mutations in MTTP gene (inability to assemble apoB containing lipoproteins) Low TGs and cholesterol levels 3. LPL or ApoC-II deficiency; Inability to clear chylomicrons or VLDL High triglyceride with normal cholesterol 4. LDL receptor mutations  elevated cholesterol with normal TG levels (see slide 31) 5. Relatively rare; ApoE mutation (E2) elevated LDL and TGs 6. Autosomal dominant disorder of the apoB gene; missense mutation interferes with the recognition of apoB-100 by the LDLR TRANSPORT OF CHOLESTEROL AND TRIGLYCERIDES: REVERSE TRANSPORT Cholesterol in macrophage foam cells in the arterial wall and excess cholesterol in peripheral tissues is transported to the liver 14 HDL • HDL is involved in reverse cholesterol transport • Small particle, consisting mostly cholesterol, with only traces TGs • ApoA-I is the major apolipoprotein • Synthesized by several mechanisms • Contains apoA1, CII, E (ACE) • Cholesteryl Ester Transfer Protein (CETP) • transfers CE in exchange for TG • exchange activity primarily regulated by relative levels of circulating TG-rich lipoproteins additional cholesterol converted to CE by LCAT forming mature HDL HDL HDL exchanges apoproteins and lipids with other blood lipoproteins ABCA1 apoA-I interacts with ATP-binding cassette transporter (ABCA1) to extract phospholipids and cholesterol HDL CHOLESTEROL Lecithin-cholesterol acyltransferase (LCAT) • Circulates in blood and associated with HDL • Activated by ApoA-I • Produces majority of CE in circulating lipoproteins • Promotes removal of cholesterol from peripheral cells by HDL transfers fatty acid to hydroxyl group on Aring of cholesterol • Prevents passive diffusion of cholesterol from HDL back to peripheral cells 17 SUMMARY OF DISORDERS IN REVERSE TRANSPORT Disorders result in abnormal HDL 1) Tangier disease is an autosomal recessive deficiency of ABCA1 complete absence of HDL 2) autosomal recessive deficiency of LCAT most cholesterol remains unesterified; HDL low 3) autosomal recessive deficiency of CETP transfer of cholesterol esters is inhibited; high HDL-C 4) rare familial disorder  TG content of all lipoproteins increased 5) autosomal dominant disorder low HDL-C in the presence of normal VLDL cholesterol and LDL cholesterol levels SUMMARY GOOD VS. BAD •Elevated levels of plasma LDL-C are atherogenic •Cholesterol carried in LDL (LDL-C) has the most direct relationship to atherosclerosis. •Oxidized LDL contributes to the formation of fatty streaks •Microphages bind to and internalize oxidizedfatty acids within LDLs to become subintimal foam cells •Accumulation of lipid-laden macrophages or foam cells in the subintimal space •HDL removes cholesterol from cholesterol-laden cells to the liver and is considered vasoprotective CLINICAL CHEMISTRY: LIPID PROFILING Total cholesterol Indirect Total triglyceridesenzymatic assays HDL cholesterol (precipitation and non-specific assays) LDL cholesterol (calculated from Friedewald equation) Expanded lipid profiles that directly measure lipids, particle size and number, or lipid sub-fractions, improve lipid analysis and CHD risk profiling