Cholesterol and Steroid Metabolism UNIT III: Lipid Metabolism.
Cholesterol Absorption, Synthesis, & Metabolism I Chapter 34 Nov. 4 th 2011.
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Transcript of Cholesterol Absorption, Synthesis, & Metabolism I Chapter 34 Nov. 4 th 2011.
Cholesterol Background
• Atherosclerotic vascular disease
• Stabilizes cell membrane
• Precursor to bile salts and steroid hormones
• Cholesterol precursors converted to ubiquinone, dolichol, & vitamine D
Cholesterol BackgroundSynthesis
• Obtained through diet or synthesis
• Synthesized in many cells, but mostly in the liver and intestine
• Acetyl coenzyme A (acetyl CoA) is the precursor to cholesterol synthesis
Cholesterol Background(Transport)
• Chylomicrons & VLDL transport cholesterol to other cells through the bloodstream
• Chylomicrons package cholesterol in intestine, while VLDL package in liver
• Triacylglycerols are also transported by Chylomicrons and VLDL• HDL – reverse cholesterol transport
Student Learning Outcomes
• Describe the rate-limiting step in cholesterol synthesis and how the HMG-CoA reductase is regulated
• Briefly describe the fates of cholesterol• Describe the VLDL to LDL pathway• The role of HDL– RCT, apoprotein & lipid exchange
• Explain what occurs during receptor mediated endocytosis
• Describe the aspects of Atherosclerosis
Cholesterol Synthesis• Perhydrocyclopentanophenanthrene structure
consists of four fused rings• Cholesterol contains a hydroxyl group at C3, double
bond between C5 & C6, eight-membered hydrocarbon chain at C17, & methyl groups at C10 & C13
CholesterolPerhydrocyclopentanophenanthrene
Fig. 1Fig.2
Cholesterol Synthesis Stage I: Transcription Control
• Feedback regulatory system• Rate of HMG-CoA reductase mRNA synthesis controlled
by sterol regulatory element binding protein (SREBP)• Once in the Golgi, SERBP is cleaved twice by S1p & S2P to
release the transcription factor
Fig. 4A
Cholesterol Synthesis Stage I: Proteolytic Degradation of HMG-
CoA Reductase
• When sterol present, enzyme undergoes sterol accelerated ERAD (ER associated degradation)
• HMG-CoA is ubiquitinated and extracted from membrane where it is then degraded by proteosomes
Fig. 4B
Cholesterol Synthesis Stage I: Regulation by Covalent
Modification
• Short-term regulation by phosphorylation & dephosphorylation
• Adenosine monophosphate (AMP) activated kinase phosphorylates HMG-CoA
• Glucagon, sterols, glucocorticoids & low ATP levels inactivate HMG-CoA
• Insulin, thyroid hormone, high ATP levels activate enzyme
Fig. 4C
Cholesterol Synthesis Stage 2: Mevalonate to 2 Activated
Isoprenes
• Transfer 3 ATP to Mevalonate in order to activate C5 & OH-group of C3
• Phosphate group at C3 & Carboxyl group of C1 leave, which produces a double bound
• This allows for two active isoprenes
Fig.5
Cholesterol Synthesis Stage 3: Condensation of Isoprenes to for
Squalene• 1) Head to tail attachment of
isoprenes to form Geranyl pyrophosphate
• 2) Head to tail condensation of Geranyl pyrophosphate and isopentenylpyrophosphate to form Farnesyl pyrophosphate
• 3) Head to head fusion of two Farnesyl pyrophosphate to form squalene
Fig.6
Cholesterol Synthesis Stage 4: Squalene to Four-Ring Steroid
Nucleus
• Squalene monooxygenase adds oxygen to form an epoxide• Unsaturated carbons (double bonds) are aligned to allow
cyclization and formation of lanosterol• After many reaction get cholesterol
Fig. 7
Cholesterol Esters
• Acyl-CoA:cholesterol acyl transferase (ACAT) is an ER membrane protein
• ACAT transfers fatty acid of CoA to C3 hydroxyl group of cholesterol
• Excess cholesterol is stored as cholesterol esters in cytosolic lipid droplets
Fig. 8
Bile Salts
• Bile acids & salts are effective detergents• Synthesized in the liver• Stored & concentrated in the gallbladder• Discharged into gut and aides in absorption of
intraluminal lipids, cholesteral, & fat soluble vitamines• Bile acid refers to the protonated form while bile salts
refers to the ionized form– The pH of the intestine is 7 and the pKa of bile salts is 6,
which means that 50% are protonated• These terms are sometimes used interchangeably
Synthesis of Bile Salts
• Rate-limiting step performed by the 7α-hydroxylase (CYP7A1) and is regulated by bile salt concentration
• End product: Cholic acid series & Chenocholic acid series• Bile salts can be conjugated & become better detergents
Fig. 9 Fig. 10
Cholesterol Transport by Blood Lipoproteins
• Cholesterol, cholesterol esters, triacylglycerols, & phospholipids are insoluble and must travel via lipoproteins
VLDL to LDL
• The TG, free & esterified cholesterol, FA, & apoB-100 are packaged into nascent VLDL• Nascent VLDL are secreted to bloodstream and acquire apoCII & apoE from HDL to form a
mature VLDL• Hepatic triglyceride lipase (HTGL) hydrolyzes additional triglycerides to produce LDL• 40% of LDL transported to extrahepatic tissues• Excess LDL is taken up by macrophages
Fig. 14
Reverse Cholesterol Transport (RCT)
• HDL removes cholesterol from cells and returns it to the liver• ABC1 transport protein uses ATP hydrolysis to move cholesterol from
inner leaflet to outer leaflet of membrane• HDL receives cholesterol and uses the LCAT enzyme to modify & trap the
cholesterol
Oram, JF & Vaughan, AM. (2000) ABCA1-mediated transport of cellular cholesterol & phospholipids to HDL apolipoproteins. Curr Opin Lipidol. June;11(3):253-60
Fate of HDL
• HDL can bind to specific hepatic receptors, but primary HDL clearance occurs through uptake by scavenger receptor SR-B1
• Present on many cells• SR-B1 can be upregulated in cells that require more cholesterol• SR-B1 is not downregulated when cholesterol levels are high
HDL binds SR-B1 receptor
Transfers cholesterol & cholesterol ester to cell
Depleted HDL dissociates & re-enters circulation
HDL Interactions with Other Particles
• HDL transfers apoE & apoCII to Chylomicrons & VLDL• HDL either transfers cholesterol & cholesterol esters directly to liver or by
means of CETP to VLDL (or other TG-rich lipoproteins)• In exchange, HDL receives triacylglyceroles• Prior to CETP mature HDL particles are HDL3, post CETP they become larger
and are called HDL2
Fig. 16Fig. 17
Receptor-Mediated Endocytosis of Lipoproteins
• LDL receptor are located at coated pits, which also contain clathrin
• Vesicles fuse with lysosome where cholesterol esters are hydrolyzed into cholesterol & re-esterified by ACAT
• This avoids damaging effects of high concentrations of free cholesterol on membrane
• Unlike cholesterol esters of LDL, these cholesterol esters are monosaturated
Fig. 18
Feedback Regulation of Receptors
• Regulation by SREBP or its cofactor• Low levels of cholesterol leads to up regulation
of receptor genes– Increase amount of cholesterol in cells
• High levels suppress expression of receptor genes– Reduces amount of cholesterol that enters cells
Lipoprotein Receptors• LDL receptor most well
characterized & contains 6 different regions
• LDL receptor-related proteins are structurally related but recognize more ligands
• Macrophage scavenger receptor : SR-AI & SR-A2– Take up oxidatively
modified LDL– When engorged with lipids
macrophages become foam cells
Anatomical & Biochemical Aspects of Atherosclerosis
• Initial step is formation of fatty streak (foam cells) in subintimal space
• Foam cells separate endothelial cells exposing them to blood, which leads to plaques & thrombin at these sites
• When plaque content exposed to procoagulant elements in circulation, acute thrombus formation occurs
• Further thrombus formation leads to complete occlusion of lumen & eventually AMI or CVA
Fig 21. Layers of arterial wall
Key Concepts• HMG-CoA conversion to mevalonate is the rate limiting step
of cholesterol synthesis– HMG-CoA reductase regulated by feedback, degradation,
modification• Cholesterol fate: membranes, esters, biliary cholesterol, bile
salts– Bile salts aide in absorption of lipids
• Hydrolysis of VLDL leads to LDL, which transport TG & CE to peripheral cells & macrophages
• HDL involved in RCT & apoprotein/lipid exchange• LDL enters cells via receptor-mediated endocytosis• Excess LDL taken up by macrophage leads to the formation
of foam cells, which is the beginning of atherosclerosis