User:Johnsy/Lipoprotein Modelling/Lipoprotein Structure and Function
Lipids are transported in lipoproteins throughout the circulation due to the fact that lipids are insoluble in the aqueous medium of blood. All lipoproteins contain the basic five components.
- Phospholipid monolayer - serves as a barrier between the hydrophobic interior containing triglycerides and cholesterol esters and the hydrophilic exterior defined by the extracellular fluid or blood plasma.
- Apoproteins - proteins which span the phospholipid monolayer and act as signalling molecules to identify the contents and type of lipoprotein. Each type of lipoprotein has a different apoprotein coat distinguishing them from each other. Apoproteins also serve to interact with the outside aqueous environment as receptor ligands. HDL lipoproteins are characterized by the A-I apoprotein while VLDL, IDL, and LDL are chacterized by having the B-100 apoprotein. Chylomicrons have the B-48 apoporotein.
- Free cholesterol embedded on the phospholipid monolayer whose polar alcohol groups on the unesterified cholesterol project into the aqueous environment.
- Triglycerides - one method by which fatty acids and lipids are transported, usually contained in the core of the lipoprotein. Different lipoprotein classes contain different proportions of triglycerides and cholesterol esters.
- Cholesterol Esters - esterified cholesterol attached to fatty acids (e.g. cholesterol linoleate) utilizing the hydroxyl group on cholesterol to form the ester bond. Cholesterol is transported mainly as cholesterol esters.
There are five different classes of lipoproteins, separated by composition, diameter, density and sources.
- Chylomicrons - Primarily responsible for transporting triglycerides
- VLDL (very low density lipoprotein) - Produced by the liver and also responsible for transporting triglycerides and cholesterol esters
- IDL (intermediate density lipoprotein) - Produced from the catabolism of VLDLs, contains a higher proportion of cholesterol esters than VLDL.
- LDL (low density lipoprotein) - Produced from the catabolism of VLDL, contains the highest proportion of cholesterol ester of all lipoproteins, up to two-thirds of the cholesterol in the human plasma is found in LDL particles. 
- HDL (high density lipoprotein) - Produced in the liver and intensine from the catabolism of chylomicrons and VLDL. Has the lowest proportion of cholesterol ester and triglyceride than any other lipoprotein.
Key Enzymes Associated with Lipoproteins
Several enzymes are required to maintain homeostasis of blood cholesterol levels and ensure that cells receive enough cholesterol to survive. The following are key enzymes that are help to maintain the level of cholesterol in the plamsa.
- Lipoprotein Lipase (LPL) - an extracellular enzyme attached to the cell via heparin sulphate to the endothelial lining of the capillaries perfusing muscle and adipose tissue. LPL hydrolyzes the the triglyceride in the lipoproteins allowing the fatty acids to be transferred to adipose tissue for stroage or to the muscle and liver for metabolic requirements. When VLDL is subjected to the action of LPL, the triglycerides are removed, increasing the percentage of cholesterol esters in the lipoprotein as well as shrinking it. Furthermore, surface remnants (lamellar complexes, unesterified cholesterol, phospholipids, and Apo A and C) are removed leaving behing an IDL. The surface remnants are recycled and join the HDL pool and are an important source of HDL precursors. Chylomicrons undergo similar removal of triglycerides when acted on by LPL, also resulting in an IDL.
- Adipose tissue mobilizing lipase (ATML) - an intracellular enzyme that couters the effect of LPL by hydrolyzing adipose tissue to release free fatty acids into the circulation, especially in times of fasting.
- Hepatic Lipase (HL) - a similar enzyme to LPL secreted by hepatic parenchymal cells and functions to hydrolyze IDL glycerides to produce chylomicron remnants (from chylomicrons) or LDL (from VLDLs). HL is also used to further hydrolyze HDL and LDL glycerides to promote receptor mediated uptake of LDLs and reverse cholesterol transport in HDL.
- Lecithin cholesterol acyl transferase (LCAT) - an enzyme synthesized in the liver and responsible for esterfying free cholesterol acquired by HDL particles by transfer of a fatty acid from lecithin. The esterified cholesterol is then transferred to the core of the particle forming a cholesterol ester-rich HDL2.
- Cholesterol ester transfer protein (CETP) - an enzyme that catalyses the transfer of cholesterol esters from HDL and LDL particles to triglyceride rich lipoproteins
- LDL Receptor - The LDL receptors binds lipoproteins with Apo B-100 and/or Apo E apoproteins (LDL or IDL). The main role of this receptors is to provide cholesterol to the cells throughout the body and deliver excess cholesterol to the liver for recycling or excretion as bile acids. As stated before, the LDL receptor is a major control point in the control of cholesterol levels. The synthesis of receptors is in response to a fall in cellular free cholesterol concentration. The LDL particles bind to the receptor, are internalized, and are subject to lysosomal degradation. Following the LDL particle degradation, the receptors themselves are recycled and either stored if the cellular concentration of cholesterol is high enough or returned to the cell surface if cellular cholesterol levels remain lower than desired.
- Chylomicron remnant (Apo E) receptor - This receptor is mainly in the liver where chylomicron remnants are quickly removed from circulatioin. This receptor is not down-regulated by cholesterol levels, but delivery of cholesterol via this route will down-regulate the synthesis of LDL receptors.
- HDL receptors - HDL receptors are up-regulated by cholesterol loading and allow HDL particles to bind to extra-hepatic (non liver) tissues resulting in a net efflux of cholesterol from the cell.
LDL Binding, Internalization, and Degradation (Brown & Goldstein 1979)
Through experiments done on fibroblasts and patients with hypercholesterolemia, the mechanism of receptor-mediated endocytosis was elucidated. Cholesterol is an essential component of the plasma membrane and is necessary for cell survival. The cell has devised unique ways of maintaining the homeostasis of intracellular cholesterol by the two ways mentioned earlier, the de novo pathway where cholesterol is produced from the ubiquitous precursor molecule acetyl-CoA, and via receptor-mediated endocytosis of LDL particles. LDL particles contain around two-thirds of the cholesterol found in human plasma and to use this cholesterol, cells must be able to internalize the particles, take them apart, and hydrolyze the cholesterol esters found inside the lipoprotein.
The LDL receptor itself is synthesized on membrane-bound polyribosomes and glycosylated in the Golgi apparatus. The receptor is inserted into the plasma membrane at random, but localized to areas known as coated pits where the concentration of a protein known as clathrin is found to be high. Although coated pits cover only 2% of the cell surface of the cell, they contain 50-80% of the LDL receptors that are produced.
Once LDL particles attach to the LDL receptor, they are internalized into the cell for processing and degradation. Only LDL particles attached to LDL receptors in clathrin coated pits are internalized. Experiments on a manifestation of familial hypercholesterolemia showed that in one patient with a 6-fold increase in plamsa cholesterol levels compared to normal, receptors were being produced and displayed on the membrane, but no coated pits were being formed and thus endocytosis was not able to occur. Hence, LDL internalization is dependent on the presence of LDL receptors as well as the formation of a clathrin coated pit.
After the LDL particles are internalized, they are moved to the lysosomes where the cholesterol esters are hydrolyzed and the apoproteins are degraded. From experiments blocking the production of LDL receptors at a certain time, it was seen that the cell continued to internalize LDL particles for a very long time after LDL receptor production had been disabled leading to the conclusion that the receptors were recycled from the lysosome to the membrane where they could internalize more cholesterol.
The incoming cholesterol from the internalization and degradation of LDL particles from the blood plasma are also seen to turn off synthesis of the LDL receptor, preventing further entry of LDL and protecting the cells against overaccumulation of cholesterol. However, different types of cells produce lower or higher affinity LDL receptors depending upon the cholesterol need of the cell or organ. For example, the adrenal glands were shown to have a very high number high affinity of LDL receptors, especially in the adrenal cortex, correlating with the fact that the adrenal cortex is responsible for the production of steroid hormones. The other binding site with similarly high affinity was the ovarian corpus luteum, which is also responsible for a high production of steroid hormones.
Furthermore, it was seen that unesterified cholesterol was the main intracellular messenger and regulator for both the de novo and endocytotic pathway for cholesterol accumulation. Patient with genetic blocks in LDL degradation were seen to continually bind and internalize LDL particles. When exogenous unesterified cholesterol was added to the cell, the genetic blocks were bypassed and the cell reduced the intake of LDL particles confirming that unesterified cholesterol was indeed the messenger inhibiting the further intake of cholesterol.
HDL Particles are secreted by the intestine and liver as a pre-B1 HDL nascent particle and acquires the ApoA-I apoprotein when first produced.
Reverse cholesterol transport (RCT), or the action of by which excess cholesterol from the extrahepatic cell is transported to the liver and other organs, consists of 5 steps:
- Uptake of cholesterol from cells by specific receptors (cholesterol efflux)
- Esterification of cholesterol within HDL by LCAT
- Transfer of cholesterol to the apoB-containing lipoproteins (choelsterol transfer)
- Remodelling of HDL
- Uptake of HDL cholesterol by the liver and possibly also by the kidney and smal intestine through lipoprotein receptors (cholesterol uptake)
The small lipid-poor particles are the initial acceptors of cellular choelsterol. Once they accumulate these cholesterol, they become known as pre-B2 HDL and are a substrate for LCAT which esterifies the cholesterol within the lipoprotein and acquires additional apoA-I molecules leading to the formation of α3 HDL particles. These α3 obtain more cholesterol from the cells and other HDL particles transforming them into much larger α2 and α1 HDL particles. These larger particles undergo exchange of cholesterol esters for triglycerides through CETP, transfer of phospholipid and hydrolysis of triglyceride and phospholipid to by hepatic lipase (HL). These particles are remodelled into smaller α3 HDL and lipid-free apoA-I, with the particle becoming relipidated by cellular phospholipid and cholesterol to form pre-B1 HDL particles again. From the model above, it is thought that the rate of formation pf pre-B1 HDL might be critical for the functioning of RCT.
HDL-uptake by cells is thought to occur through receptor-mediated endocytosis similar to LDL. The receptors implicated in the uptake and degradation of HDL are most likely scavenger receptor type BI (SRBI) in the liver and cubilin-megalin pair in the kidney. Modest overexpression of SRBI in the liver is known to increase cholesterol secretion in the liver and very low HDL levels conferring atheroprotection. From this clinical observatio, it was inferred that there was an increased uptake of HDL by the liver corresponding to a decreased concentration in the plasma. A higher cholesterol flux through the RCT was also inferred. However, high overexpression of SRBI was seen to be pro-atherogenic reflecting a complex relatinoship between SRBI, atherogenesis, and HDL levels. It is further thought that SRBI may be involved in cholesterol efflux, the first step of RCT complicating the pathway further.