Lipoprotein physiology Flashcards
What are the 2 main lipid species that are transported in lipoproteins?
- The insoluble ones
* Cholesterol esters and triglycerides
How does the body get rid of cholesterol?
- Remember that cholesterol is not able to be oxidized in fatty acid oxidation.
- Instead, it is secreted as bile salts and excreted in the stool (at least the bit that isn’t reabsorbed in the terminal ileum)
What are the 5 general classes of lipoprotein particles?
- Chylomicrons
- VLDL - very low density lipoproteins
- Remnant particles and Intermediate Density lipoproteins
- (LDL) - Low Density Lipoproteins
- (HDL) - High Density Lipoproteins
What are chylomicrons?
- Responsible for rise of Tg levels after a meal
- Made from GI tract from dietary fat
- Physically large and have far more triglyceride than cholesterol (10:1)
What’s up with the VLDL?
- Source of basal triglyceride production
- Made by the liver
- 5:1 Tg to cholesterol
- Deliver triglyceride to peripheral tissues between meals
- Made at lower levels in the post-meal period
What makes up an HDL?
- These are the trash trucks of lipid metabolism
- Collect cholesterol from peripheral tissues
- Transport cholesterol back to liver
- Provide a reservoir of phospholipids for other lipoprotein particles
- Exchange triglyceride and apo-proteins with other particles in the circulation
- Higher HDL means less atherosclerotic risk
What makes up an LDL?
- Produced from metabolism of VLDL
- More cholesterol than Tg
- VERY atherogenic
- Less Tg means smaller and more dense
- Small, dense LDL are espcecially aterogenic
- Cleared from circulation by liver
What makes up an IDL?
- Metabolic byproducts of the metablosim of chylomicrons and VLDL
- As Tg rich lipoproteins deliver Tg to peripheral tissues they become physically smaller
- They also become more cholesterol enriched as they lose Tg
- Mid-sized and about 50/50 Tg/cholesterol
- These are ATHEROGENIC
Describe in general the chylomicron pathway
• This pathway is how the body handles dietary fats
• Chylomicros are mostly triglyceride with some cholesterol, but most diets have way more triglyceride than cholesterol
• In the intestine, dietary triglyceride is hydrolyzed to monoacylglycerol and free fatyy acids through the action of pancreatic lipase
• The lipids and free fatty acids diffuse across the intestinal wall
• Once intracellular, they are rebuilt into triglycerides and packaged into chylomicrons
• APOPROTEIN B48 is the important protein here intracellularly
• The chylomicrons pick up C-2 and E (other apo-proteins) in the central circulation
○ Pick these up from HDL particles
• C-2 is a co-factor for LPL
○ LPL = lipoprotein lipase
• LPL is what breaks down the triglycerides in the chylomicron and if this can’t interact with C-2 there is hypertryglyceridemia
• Remnant particles of chylomicron breakdown are taken up by the liver
• Not normally present when taken in the fasting state
• This is why we ask patients to fast before we check lipid levels
What are the really important apo-proteins in the chylomicron pathway
- APOPROTEIN B48 is the important protein here intracellularly - it is the scaffold on which the reassembled triglycerides from the diet are built into the chylomicron
- C-2 - important item picked up by the chylomicron in the central circulation FROM HDL - an important cofactor for LPL (lipoprotein lipase) and degradation of chylymicron to get triglycerides into tissues
- E - the other protein picked up in circulation from HDL
Describe in general the VLDL pathway
- VLDL is a triglyceride rich lipoprotein synthesized by the liver
- 50% triglyceride, 10% cholesterol
- B100 is the all-important apoprotein here
- Full length gene produce, B48 in chylomicron pathway is a PTM shortened version
- Like the chylomicron pathway, VLDL will pick up C-2 and E apo-lipoproteins from HDL in the circulation
- LPL is responsible for degradation of VLDLs just like chylomicron pathway
- Degradation product here is LDL
- LDL is 45% cholesterol (enriched for cholesterol)
- LDL particles are taken out of circulaiton mostly by the liver though all tissues have LDL receptors
- Problems in LDL receptor means high LDL levels and high risk for atherogenesis
- LDL uptake in liver is what regulates cholesterol synthesis pathway
What are the important proteins in the VLDL pathway?
- B100 is the all-important apoprotein here
- Full length gene product, B48 in chylomicron pathway is a PTM shortened version
- Like the chylomicron pathway, VLDL will pick up C-2 and E apo-lipoproteins from HDL in the circulation
- LPL is responsible for degradation of VLDLs just like chylomicron pathway
Describe in general the HDL pathway
• This is the more complicated pathway because it interacts with so many other particles
• There are also several functions of the HDL in which molecules are transferred
• Think of it as a trash truck that collects cholesterol and Tg from the periphery and takes back to the liver
• Nascent HDL contains the main structural apo-protein apo-A1
○ Key protein = apo-A1
• These are synthesized in the liver and secreted into blood stream
• In circulation they pick up free cholesterol from peripheral tissues via diffusion and faciliated transport
○ ABC-A1 cassette is what is actively transporting cholesterol in circulation into the HDL particle
• Free cholesterol is “trapped” into the particle by conversion to cholesterol ester, which is performed by LCAT
○ Key enzyme = LCAT = lecithin cholesterol acyltransferase
○ Transfers a fatty acid from phospholipid onto free cholesterol to trap it in the HDL particle
• Maturing (as opposed to nacent or mature) HDL = HDL3
○ In circulation and involved in the transfer of cholesterol esters to VLDL in exchange for triglycerides through CETP
○ CETP = cholesterol ester transfer protein
○ KEY PROTEIN = CETP
• Mature HDL = HDL2
○ Taken up by liver
What are the important proteins and enzymes in the HDL particle?
*Apo-A1 is the scaffold protein made by the liver on which the whole complex is built
○ ABC-A1 cassette is what is actively transporting cholesterol in circulation into the HDL particle
○ Key enzyme = LCAT = lecithin cholesterol acyltransferase
○ Transfers a fatty acid from phospholipid onto free cholesterol to trap it in the HDL particle
*CETP = cholesterol ester transfer protein
○ used in the maturing HDL3 lipoprotein
*In circulation and involved in the transfer of cholesterol esters to VLDL in exchange for triglycerides through CETP
What is important about ABC A1?
• ATP binding cassette (ABC) transporter
• Very important in the transport of cholesterol from periphery to apo-A1
• Tangiers disese = deficiency in ABC A1
○ Unable to remove cholesterol from peripheral tissues
○ Low levels of HDL and have premature atherosclerosis
○ ORANGE TONSILS - classic finding
○ Accumulation of cholesterol in lymphatic tissues
What is • Tangiers disease?
• Tangiers disease = deficiency in ABC A1
○ Unable to remove cholesterol from peripheral tissues
○ Low levels of HDL and have premature atherosclerosis
○ ORANGE TONSILS - classic finding
○ Accumulation of cholesterol in lymphatic tissues
What is important to know about the LCAT enzyme?
• LCAT = lecithin cholesterol acyl transferase
• AKA = PCAT
• Catalyses the transfer of a fatty acid from the phospholipid lecithin to un-esterified cholesterol
• The product of this reaction is cholesterol ester, more non-polar and more tightly bound to HDL particle
○ “trapped” for transport to the liver
• Deficiency in this enzyme means low HDL levels
• COMPLICATIONS = corneal opacities, reanal insufficiency and hemolytic anemia due to the accumulation of un-esterified cholesterol in tissues
• Renal failure is main problem here
• Atherosclerosis is NOT as big a problem as you would expect
• Look for “fish eye disease” or cholesterol deposition in the eye causing corneal opacities
What is important about CETP?
• Cholesterol ester transfer protein = CETP
• Catalyzes the exchange of the non-polar lipids
○ Cholesterol esters and triglycerides
• The transfer is between lipoprotein particles of different class
• Normally the CE from HDL is exchanged for Tg present in VLDL and remnants
○ This is an alnernative Tg clearance pathway
• As HDL is more Tg enriched it is taken out of circulation (leading to lower HDL levels)
• If you have low amounts of this protein, you have high HDL levels, and patients live longer than average
○ BUT medically interfering with this as of now has unforseen CV risk.
○ Thus, we don’t know all that much about the reverse transport pathway
What does apoC3 do?
- Inhibits LPL and as a result high apoC3 means high triglyceride levels and increased risk for CV disease
- Inhibition of apoC3 for treatment of hypertriglyceridemia is under investigation
What is the ligand for the remnant receptor?
- apoE
* Deficiency predisposes to CV disease because of reduced clearance of remnant particles
What is the ligand for the LDL receptor?
• apoB100
what marker is best for cardiovascular disease risk stratification among patients?
- there is a substantial body of data that risk stratifying patients based on their LDL-C level and treating this marker (with statins) has beneficial effects on CVD risk.
- For this reason, LDL-C levels are the focus of most lipid evaluation and treatment strategies.
- The level of LDL cholesterol is closely related to CVD risk and is amenable to treatment. Thus, LDL is the most important lipid parameter to address clinically.