Endogenous Lipoprotein Pathway

SEQ: Describe the endogenous lipoprotein pathway. (10 minute/10 marks)
(Every sentence will receive a mark. Can also add a diagram and label it.)

Model Answer:

1. Endogenous pathway happens all the time.

2. It includes VLDL-LDL metabolism, LDL cascade and LDL receptor-mediated uptake.

3. The liver synthesizes VLDL. Hepatocytes releases it into the Space of Disse --> sinusoid --> enters systemic circulation.

4. VLDL has several fates.

a. VLDL carries triglycerides in liver to peripheral tissues for utilization.

b. When VLDL enters vascular beds, it undergoes hydrolysis (lipolysis) where its contents of triglycerides are hydrolysed to free fatty acids (FFA).

c. VLDL remnants are taken up into liver by LDL receptors and LDL receptor -related lipoproteins.

d. The VLDL is reduced in size and is converted into VLDL remnants and IDL, as a result of lipoprotein lipase (LPL) activity.

5. Unesterified FFA has several fates.

a. During lipolysis, some FFA can be carried by plasma albumin and dispersed in plasma for delivery to other cells. Not much of this happens.

b. Normally, FFA can enter underlying adipocytes by simple diffusion. Inside the adipocytes, the FFA are re-esterified to form triglycerides (TG) once more. Adipocytes store TG until required (as an energy souce during fasting or starvation). A majority of FFA are stored in adipocytes following lipolysis.

c. In times of starvation, when blood glucose is low and glycogen reserves are low, triglycerides stored in adipose tissues are hydrolysed by hormone sensitive lipase (HSL) and the FFA are released from adipose tissue. FFA then attach to circulating albumin and brought to liver for beta-oxidation, for continued energy supply.

6. IDL has several fates.

a. IDL can be converted into LDL by LPL in blood (intravascular).

b. IDL can be taken up by hepatic receptors.

c. IDL can be converted into LDL by hepatic lipase (HTGL) in liver.

7. LDL has several fates.

a. Normally, LDL is taken up by hepatic LDL (B,E) receptors. The contents of LDL are broken down into FC, CE, PL, TG/DG/MG/FFA and amino acids. These are kept within the hepatocytes or recycled for use by other cells. Hepatic contents of FC and CE are regulated by ACAT. Few things can happen if cholesterol is low, specifically in the cell or in the blood. If there is low cellular FC, stored cholesterol (as CE) is broken down to free cholesterol (FC) by ACAT. If there is low plasma FC, the liver cell makes more FC via HMG CoA reductase. The 2 enzymes, ACAT and HMG-CoA reductase, are sensitive to cellular and blood cholesterol levels, and combined, they regulate cholesterol levels in cells and blood.

b. LDL can also deliver its contents directly to cells, eg adrenal glands, for synthesis of steroid hormones.

c. Under abnormal levels of LDL in the blood (eg hyperlipidaemia), LDL can be taken up by the scavenger receptors present at extrahepatic tissues (EHT).

i. LDL apoB will be oxidized due to the prolonged presence of LDL in the blood. Also, the LDL particle is now smaller and apoB is not stable at this stage. ApoB becomes easily oxidised. Oxidised apoB has higher affinity for macrophages. Thus, oxidised LDL will be taken up by macrophages in EHT.

ii. Macrophages contain scavenger receptors on their surface. Macrophages are present on blood vessel walls. Oxidised LDL will attach and be internalised.

iii. The oxidised LDL are hydrolysed into component cholesterol, fatty acids, glycerol and amino acids, which are stored within the macrophages.

iv. This uptake of oxidised LDL is unregulated (ie, not controlled). The macrophages take up as much LDL until they stop functioning and die off, becoming foam cells.

LDL (apoB-100,E) receptor mediated uptake

Taken from: http://ethesis.helsinki.fi/

2.2.2. LDL and receptor mediated metabolism

LDL is the most abundant cholesterol-carrying lipoprotein in plasma. CE, located in the hydrophobic core of LDL, is the main form of cholesterol carried in LDL. CE is supposed to be too hydrophobic to pass through cell membranes. The question is how can esterified cholesterol be delivered into cells for their use? The delivery problem is solved by the LDL receptors. The LDL receptors bind LDL and CE packed into LDL particles is delivered into the cell by receptor-mediated endocytosis. The receptor-mediated removal of LDL cholesterol occurs mostly via classical LDL receptors that have been observed in all mammalian cells tested except erythrocytes (Brown and Goldstein 1986) (Fig. 1). The liver plays a crucial role in receptor mediated uptake of LDL: about 75% of the LDL particles removed from the circulation are mediated by the liver. Of these, 75% of the clearance is LDL receptor-mediated, the remainder is by a nonspecific, receptor-independent low affinity process (Pittman et al. 1982, Billheimer et al. 1984). Also SR-BI mediates LDL binding but only CE is selectively delivered to the cell especially in non-placental steroidogenic tissues (van Berkel et al. 1995).

Figure 1. Steps in the LDL pathway in cultured human fibroblasts.  

HMG CoA reductase, 3-hydroxy-3-methylglutaryl CoA reductase; ACAT, acyl CoA:cholesterol acyltransferase (Brown and Goldstein, 1986).

The LDL receptor is a cell surface glycoprotein with a molecular weight of 164 kDa, with a coding gene on chromosome 19 (Francke et al. 1984). It is present on both hepatic and extrahepatic cells. The high binding interaction between LDL apoB and the LDL receptor is responsible for the receptor-mediated uptake and clearance of LDL from the circulation. The ApoE on apoE-containing lipoproteins (VLDL, IDL) is also capable of interacting with the LDL receptors and regulating the metabolism of these lipoproteins (Mahley 1990). Following the binding of LDL to its receptors, the lipoprotein is internalized and delivered into lysosomes where its CE is hydrolyzed. The liberated cholesterol is then used by the cell for the synthesis of plasma membranes, bile acids, and steroid hormones, or stored in the ester form. The increased intracellular cholesterol level will, in return, down regulate the LDL receptor activity, i.e., receptor synthesis. Subsequently, the number of the LDL receptors synthesized decreases when the cellular cholesterol content increases, and vice versa (Brown and Goldstein 1986). Therefore, cellular cholesterol content is the major LDL receptor regulator. ApoB-100, one of the largest monomeric proteins known, is the major protein component of LDL and acts as a ligand for the LDL receptor. 

ApoB-100 is a large (513 kDa), single chain glycoprotein composed of 4536 amino acid residues with a coding gene residing on the short arm of chromosome 2 (Knott et al. 1986, Yang et al. 1986). There is only one apoB-100 molecule in each LDL particle (Tikkanen and Schonfeld 1985, Cladaras et al 1986). ApoB-100 also is not transfered between lipoprotein particles during the metabolic conversion of VLDL into LDL. It is presumed that the apoB-100 binding site resides in the carboxyterminal portion of the molecule. However, the region assumed to be involved in LDL binding is not yet clear. So far three apoB mutations, called familial defective apolipoprotein B-100 (FDB) (Arg3500®Gln, Arg3500®Trp, Arg3531®Cys) have been reported to be related to hypercholesterolemia (Soria et al. 1989, Gaffney et al. 1995, Pullinger et al. 1995). However, none of these mutations have been found in Finland (Hämäläinen et al. 1990).