7.) Lipoprotein Transport

30. Which membrane transporter is involved with the release of free cholesterol from plasma membranes into the blood? What happens to this free cholesterol?

Cholesterol ABC transporter are found in the plasma membrane and they transport free cholesterol to the outside of the plasma membrane. This free cholesterol can be "rubbed" into the phospholipid monolayer of HDL. This can handle only limited amounts, and the free cholesterol is used by LCAT to form cholesteryl esters in blood. The cholesteryl esters are totally nonpolar and move immediately from the blood into the core of HDL.

38. Which common diseases can lead to acquired (secondary) hypercholesterolemia? Name two.

Diabetes and hypertension

25. Which lipoproteins are mainly involved with this transfer?

HDL and mainly VLDL are interacting. CETP allows the transfer of TAGs into HDL, and in return cholesteryl esters from HDL are transferred into VLDL.

Which lipoprotein has the highest percentage of proteins? Which lipoprotein has the highest percentage of cholesterol plus cholesterol esters? Which two lipoproteins are TAG-rich? Which lipoprotein contains dietary lipids?

HDL has the highest percentage of proteins. LDL has the highest percentage of cholesterol plus cholesteryl esters Chylomicrons and VLDL are TAG-rich lipoproteins. Chylomicrons are the lipoproteins that contain dietary lipids, mostly TAGs (~90%)

27. Which three major apoproteins are found in the phospholipid monolayer of HDL? Which two are activating enzymes, which one is needed for receptor recognition?

1. apo A-1 activates LCAT 2. apo C-II activates lipoprotein lipase 3. apo E is needed for remnant receptor recognition (chylomicron remnants and IDL which are remnants of VLDL)

24. What is the function of the cholesteryl ester transfer protein (CETP)?

CETP allows the exchange of cholesteryl esters for TAGs

Which two apoproteins are needed for release of lipoproteins and are also needed for uptake into respective cells and are never interchanged with other lipoproteins?

Apo B-48 found in chylomicrons and apo B-100 found in VLDL, IDL and LDL.

19. Compare the size of apo B-48 to apo B-100!

Apo B-48 represents 48% of apo B-100 (mRNA editing)

40. Why are bile acid sequestering drugs sometimes used to treat Type IIa hyperlipidemia?

Bile acid sequestrants (resins) bind and trap bile acids/salts in the intestines and lead to their excretion via the feces. The enterohepatic uptake of primary and secondary bile acids/salts is reduced and free cholesterol in the liver will be used to synthesize primary bile acids. This reduces cytosolic free cholesterol levels in the liver and we find more LDL-receptor synthesis than without treatment.

33. Compare lipoprotein lipase to hepatic lipase!

Bound in capillaries around Change in lipoproteins Lipoprotein lipase [needs apo C-II] Heart, adipose tissue, skeletal muscle VLDL to IDL Chylomicrons to CM remnants Hepatic lipase Liver IDL to LDL

21. What is formed first from the discoidal HDL, is it HDL-2 or is it HDL- 3 in the so called reverse cholesterol transport?

Firstly, HDL-3 is formed from it after uptake of cholesterol esters. Once HDL-3 is filled more with cholesteryl esters and after uptake of phospholipids, the larger HDL-2 is formed. It is HDL-2 that delivers cholesteryl esters to the liver.

28. What is meant by reverse cholesterol transport?

Free cholesterol is found mainly in plasma membranes. Cholesterol itself cannot be degraded by cells and it needs to be brought to the liver, which can use it itself, like for bile acid synthesis, or the liver can release it as free cholesterol into bile. The transport of cholesterol from extra hepatic cells to the liver is named reverse cholesterol transport. This is mainly performed in form of cholesteryl esters inside of HDL. This is different than the uptake of IDL or LDL into the liver, these lipoproteins contain mainly the cholesteryl esters that were released in VLDL by the liver.

Describe the transport of free fatty acids in blood. How are TAGs and cholesteryl esters transported in blood? How are dietary lipid-soluble vitamins and dietary essential fatty acids in TAGs transported to the liver?

Free fatty acids are transported in blood complexed to albumin. TAGs and cholesteryl esters are transported inside of the lipoproteins. Dietary lipid-soluble vitamins and dietary essential fatty acids in TAGs reach the liver via chylomicron remnants. Dietary essential fatty acids are also stored after a meal in TAGs in fat cells and can reach the liver during fasting as free fatty acids via transport by albumin

29. Which apoprotein is used for the HDL function of reverse cholesterol transport?

HDL need apo A-1 for reverse cholesterol transport. This is the main apoprotein found in HDL Apo A-1 is needed to signal LCAT that the lipoprotein is HDL and that the phosphatidyl choline of this lipoprotein should be used for formation of cholesteryl esters in blood.

34. What is special about hepatic lipase? Does it cleave TAG, or phospholipids, or even both? Does it contain a ligand-binding action with SR-B1?

Hepatic lipase cleaves both, TAG and phospholipids of lipoproteins which is very unusual. Generally, a lipase cleaves TAG and a different enzyme, a phospholipase cleaves phospholipids. Hepatic lipase also contains a ligand-binding action with SR-B1 in liver

36. Does hyperlipidemia Type III lead to hypercholesterolemia? Is this disease common or rare? Which lipoprotein is increased in plasma and contains cholesteryl esters? Which apoprotein is less functional?

Hyperlipidemia Type III leads to hypercholesterolemia. This is a rare disease. [note: in most cases hypercholesterolemia is due to hyperlipidemia Type II which is common] Plasma levels of mainly IDL (or chylomicron remnants) are increased. The patients are homozygous for apo E-2 which binds poorly to receptors. This reduces the uptake of lipoprotein remnants into the liver.

17. What is found in high concentration in blood in hyperlipidemia Type IIa? Is it LDL, IDL or HDL? Compare it to hyperlipidemia Type IIb! Are these defects common or are they rare?

Hyperlipidemia Type IIa is characterized by high LDL. Hyperlipidemia Type IIb is characterized by high LDL and high VLDL. Both, hyperlipidemia Type IIa and IIb are common.

37. Which Type of Hyperlipidemia is mostly related to defective LDL-receptors? What could be defective in Type IIb? Which lipoproteins are elevated in Type IIa and which are elevated in Type IIb?

Hyperlipidemia Type IIa is mostly related to a genetic defect related to the LDL- receptors and leads to high blood levels of LDL as they are less taken up into cells Type IIb is more complex and could be due to overproduction of apo B-100 and of VLDL and in addition could also be due to defective LDL receptors. In the common Type IIb hyperlipidemia, both LDL and VLDL are elevated in the blood. Type IIa and IIb are also referred to a hypercholesterolemias. [note: LDL have the highest percentage of cholesterol and cholesteryl esters ]

23. Compare LCAT and ACAT related to location (extracellular or intracellular) and substrates!

LCAT Extracellular in blood Synthesized by the liver Phosphatidyl choline of the HDL monolayer and free cholesterol ACAT Cytosol of cells Fatty acyl-CoA and free cholesterol

Where in the human body is LCAT synthesized and where is its site of action?

LCAT is synthesized in the liver and it is released into the blood stream. The site of action is in the blood close to cells that need to get rid of free cholesterol from their plasma membranes. This enzyme is needed to fill HDL with cholesteryl esters. It needs apo A-1 for recognition of HDL.

22. How is LCAT (PCAT) able to form cholesteryl esters from free cholesterol in blood? What is the function of apo A-1?

LCAT uses a fatty acid from lecithin (phosphatidyl choline) of the monolayer of HDL and links it to free cholesterol in order to form cholesteryl esters in blood. This is a special process, as in the blood fatty acyl-CoA is not available. Apo A-1 signals that the small lipoprotein is actually HDL and that HDL should be filled with cholesteryl esters.

11. How can LDL be modified, and how is this normally prevented to a tolerable amount?

LDL is modified by oxidants, especially when it is trapped in the blood with less access to antioxidants, like vitamin E and vitamin C or uric acid and others. The modification or oxidation often changes apo B-100 which is wrapped around the phospholipids of LDL

16. Outline the receptor-mediated process involving LDL receptors.

LDL receptors are negatively charged glycoproteins that are aligned in the coated pit. They recognize apo B-100 in LDL. The uptake is performed via endocytosis. The cholesterol esters of LDL are cleaved to free cholesterol which stimulates the synthesis of fatty acyl-CoA cholesterol acyltransferase (ACAT). Cholesteryl esters are formed in liver cytosol and are stored in the liver. In case that there is still more free cholesterol in the liver, then the synthesis of LDL receptors and the synthesis of HMG-CoA reductase are inhibited. This is performed by prevention of SREBP to initiate transcription.

Compare LDL-A to LDL-B, Lp(a) and oxLDL to each other. Which of the mentioned lipoproteins are considered to be risk factors for atherosclerosis?

LDL-A is larger than LDL-B. Risk factors for atherosclerosis are LDL-B and Lp(a) and oxLDL Lp(a) is an abnormal LDL with the protein apo (a) linked to apo B-100 via a disulfide bond. A special structure of "kringles" is found. Ox-LDL is formed from LDL, especially at high serum LDL levels. LDL have a half-life of about 4 days in blood circulation. LDL-B is a risk factor as it is smaller and can be trapped in the ECM

10. Which apoprotein is recognized in LDL for the uptake via the LDL-receptor?

LDL-receptors need only apo B-100 for LDLrecognition. Uptake is via endocytosis of both, LDL and the LDL-receptor.

9. What is LRP? Does it recognize apo C-II or apo E? Which lipoprotein is taken up via this receptor into the liver? Which apoprotein is needed in general for recognition by remnant receptors?

LRP is the abbreviation for LDL receptor related protein. It needs apo B-100 and also apo E for uptake of IDL into the liver. Remnant receptors in general need apo E for recognition. They can bind chylomicron remnant or IDL which is the remnant of VLDL.

Which apoprotein is needed for the activation of lipoprotein lipase and which apoprotein is needed for the activation of lecithin:cholesterol acyltransferase (LCAT)?

Lipoprotein lipase cleaves TAGs in lipoproteins. It needs apo C-II for activation. apo C-II is found in the TAG- rich lipoproteins chylomicrons and VLDL. LCAT transfers and links a fatty acid of phosphatidyl choline to free cholesterol. This enzyme is free in the blood and needs apo A-1 for activation. Apo A-1 is found in HDL and allows the reverse cholesterol transport performed by HDL. [note: apo A-1 is also found in chylomicrons, but its function there is not clear]

Where in the human body is lipoprotein lipase (LPL) synthesized and which hormone favors synthesis and release? What is the advantage of anchoring LPL to the walls of capillaries of specific tissues?

Lipoprotein lipase is mainly synthesized in myocardial cells, fat cells and skeletal muscle cells. [Synthesis and release are favored by insulin] The enzyme is then exported and bound in capillaries. LPL is anchored via GAGs to the endothelial surface of these cells. The enzyme does not travel with the blood stream. The site of action of lipoprotein lipase is in the blood, bound in capillaries of adipose tissue, heart and skeletal muscle. The enzyme needs apo C-II for activation which is a signal that the respective lipoproteins are TAG-rich. The fixed location of extracellular LPL ensures that the released fatty acids from TAGs are directly available to heart, skeletal muscle and fat cells. After uptake into the heart and skeletal muscle, these fatty acids are used for β-oxidation and energy metabolism. In fat cells they are used to synthesize TAGs for storage.

13. How are macrophages changed to foam cells? What is released by foam cells and stimulates the migration of smooth muscle cells?

Macrophages that take up excess oxLDL become foam cells. oxLDL are mostly filled with cholesteryl esters Foam cells release growth factors and cytokines

18. Is apo B-100 a small peptide? Explain.

No, it is not a small peptide. Apo B-100 is one of the largest single peptide (protein) chains in humans with more than 4,500 amino acid residues

12. Are oxLDL recognized by the LDL-receptor? How are ox LDL taken up in macrophages? What is the name for this receptor?

Ox LDL are not recognized by the LDL-receptor and cannot be taken up via the LDL-receptor. OxLDL enter macrophages via an unspecific, uncontrolled scavenger receptor. The name of this receptor is SR-A.

32. Compare the SR-B1 found in liver to the SR-A found in macrophages!

SR-B1 in liver binds to HDL-2 and very likely acts together with the phospholipase activity of hepatic lipase. The phospholipid monolayer of HDL-2 may be opened and cholesteryl esters are released into the liver. HDL-2 is very likely not taken up into the liver but becomes the smaller HDL-3 after degradation of some of its phospholipids and the release of some cholesteryl esters. SR-A is a scavenger receptor for oxLDL found in macrophages which takes up the total oxLDL. This uptake is not regulated and leads to foam cells

39. Why are statin drugs used to treat hypercholesterolemia? What is more harmful, is it dietary cholesterol or is dietary uptake of trans-fatty acids? Explain.

Statins inhibit the regulated enzyme of cholesterol synthesis (HMG-CoA reductase). When less free cholesterol is found in the liver, then more LDL-receptors are synthesized. More LDL-receptors in the liver can take up more LDL from the blood and reduce hypercholesterolemia. Dietary cholesterol uptake leads to temporary reduction of LDL-receptor synthesis in the liver. It gets back to normal when the diet contains less cholesterol. The dietary uptake of trans-fatty acids is more harmful as these fatty acids can lead to smaller LDL of the LDL-B type. TAGs and cholesteryl esters with trans- fatty acids are stacked more closely inside of lipoproteins.

31. What is deficient in the rare Tangier disease? Are HDL or are VLDL low? Why does it lead to atherosclerosis?

Tangier disease is related to the deficiency of the cholesterol ABC transporter that normally provides free cholesterol for LCAT apo A-1 is degraded when the nascent HDL cannot be filled with cholesteryl esters. This leads to low levels of HDL in these patients and the reverse cholesterol transport is reduced which can lead to atherosclerosis. This disease allowed the recognition of the importance for apo A-1 and the filling with cholesteryl esters for HDL performing reverse cholesterol transport.

26. How can some cholesteryl esters of HDL reach the liver via IDL and LDL?

The cholesteryl esters that were exchanged for TAGs with VLDL will be found in IDL and in LDL. Both, IDL and LDL can be taken up by the liver. Using CETP, the HDL have achieved that some of their cholesteryl esters are transported inside of IDL and LDL to the liver.

35. Which enzyme is most likely deficient in Type I and in Type IV Hyperlipidemia? Which lipoproteins are high in plasma, respectively? Which lipoproteins accumulate in Type V hyperlipidemia?

The enzyme that is most likely deficient is lipoprotein lipase, (it could be due a defect in the enzyme or it can also be related to deficiency of apo C-II) High plasma levels of chylomicrons are found in Type I which is rare. High plasma levels of VLDL are found in Type IV which is common. In the rare Type V hyperlipidemia, both, chylomicrons and VLDL accumulate in blood. The hyperlipidemias Types I, IV and V are referred to as hypertriacylglycerolemias [note, the TAGs are not free in blood but they are found inside of respective lipoproteins]

20. What is a discoidal nascent HDL?

The newly formed HDL that is not filled with cholesteryl esters has a discoidal shape. It consists mainly of its phospholipid monolayer and apoproteins. Apo C-II and apo E will be given to chylomicrons and VLDL, the apoprotein apo A-1 is kept for HDL reverse cholesterol transport. The formation of nascent HDL is not totally clear, it could be released into the blood by the liver, or it could assemble itself in blood around apo A-1.

8. Which apoprotein is needed for the release of VLDL from the liver into the blood? Which two apoproteins are needed for the uptake of IDL into the liver?

The release of VLDL into the blood from the liver needs apo B-100 The uptake of IDL into the liver needs apo E and apo B-100.

7. Which apoprotein is needed for the release of chylomicrons from intestinal cells into the lymph? Which two apoproteins are needed for chylomicron remnant uptake into the liver?

The release of chylomicrons into the lymph needs apo B-48. The uptake of chylomicron remnants into the liver needs apo E and apo B-48

15. Describe the general percentages for the uptake of IDLs and also for LDLs into the normal liver! Under which conditions are liver LDL-receptors reduced?

~50 % of IDLs are taken up into the liver. The other half is used to form LDL by hepatic lipase which is bound in the capillaries of the liver. ~70 % of LDLs are taken up into the liver via LDL-receptors the rest is distributed via the blood to cells who have LDL-receptors. A temporary reduced amount of LDL-receptors in liver results after a cholesterol-rich meal when the levels of free cholesterol increase in liver cytosol. Genetic deficiency of LDL-receptors is found in hyperlipidemia Type II.