Lipoproteins are macromolecular complexes, which are basically vessels used to transport cholesterol, fat, and other fat-soluble nutrients around the body in our circulation.
Firstly, I’ll identify some of the enzymes involved in lipoprotein metabolism and also some of the lipoprotein molecule components, which will help in understanding the processes.
LipoProtein Lipase (LPL) - The enzyme in many peripheral tissues which catalyses the release of their fatty acids.
Firstly, I’ll identify some of the enzymes involved in lipoprotein metabolism and also some of the lipoprotein molecule components, which will help in understanding the processes.
LipoProtein Lipase (LPL) - The enzyme in many peripheral tissues which catalyses the release of their fatty acids.
Hepatic Lipase (HL) - This enzyme is the liver’s version of LPL. It does not require apoC2 to catalyse the removal of fats from lipoproteins, it also tends to have a greater affinity for smaller particles.
Cholesteryl Ester Transfer Protein (CETP) - A plasma protein that facilitates the transport of cholesteryl esters and triglycerides between the lipoproteins.
Lecithin-cholesterol acyltransferase (LCAT) - An enzyme that converts free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol), which is then sequestered into the core of a lipoprotein particle. The enzyme is bound to HDL and LDL particles.
Apoproteins (apo) - are like identification badges of the lipoprotein vessels. They tell specific receptors on the surface of cells what the lipoprotein is and what its carrying, therefore determining where the lipoproteins go, where they can dock, and where they can off-load their cargo.
apoA – the main identification apoprotein of HDL and activator of enzyme LCAT .
apoB – main identification apoprotein found in chylomicrons, VLDL, and LDL. ApoB48 is specific to chylomicrons, apoB100 is common to all other apoB particles originating from the liver (VLDL, LDL).
apoC1 - a small apoprotein which inhibits the uptake of TG-rich lipoproteins via hepatic receptors.
apoC2 – a small apoprotein which activates LPL. Without apoC2 triglycerides cannot be delivered as LPL cannot be activated. apoC2 is reported to inhibit LCAT activity.
apoC3 – a small apoprotein which is instrumental in the inhibition of triglyceride clearance. When apoC3 is dominant over apoC2 in a particle it also inhibits the removal of that particle from circulation via apoE interference. apoC3 is also reported to inhibit LCAT activity.
apoE – this small apoprotein enables particles to dock with LDL receptors and deliver cholesterol.
apoE – this small apoprotein enables particles to dock with LDL receptors and deliver cholesterol.
The Chylomicron
Chylomicrons exist to deliver dietary fats (mostly triglycerides) to peripheral tissues, and to deliver dietary cholesterol to the liver. This is known as the exogenous pathway of lipoprotein metabolism.
Chylomicrons are assembled by the small intestines as a transport mechanism for dietary fats and cholesterol. The predominant fats in chylomicrons are triglycerides (making up 90% of their volume, only 5% is cholesterol). The predominant apolipoproteins in chylomicrons at this stage prior to circulation are apoB48 and apoA1.
Chylomicrons leave the intestine via the lymphatic system and enter the bloodstream at the subclavian vein. The subclavian vein is located under the collar bone on both sides of the body. It’s a major vein which drains blood from the upper extremities and returns it to the heart.
In the bloodstream the chylomicrons interact with other particles exchanging the smaller apolipoproteins, probably passively via gradient diffusion. They rapidly acquire apoC2 and apoE from plasma HDLs. These donated proteins, apoC2 and apoE, are essential for the chylomicrons to deliver its fatty cargo and cholesterol to peripheral tissues respectively.
In the capillaries of peripheral tissues, the fatty acids of chylomicrons are removed from the triglycerides by the action of the enzyme lipoprotein lipase (LPL). The apoC2 in the chylomicrons activates the LPL in the presence of phospholipid. The fatty acids are then absorbed by the tissues and the glycerol backbone of the triglyceride is returned, in the blood to the liver and kidneys, where it is converted to a glycolytic intermediate.
As the chylomicron shrinks from the loss of its triglyceride core it needs to shed some of its shell. Once the chylomicron has delivered 80% of its triglyceride contents it discards its apoC2 and becomes a chylomicron remnant. This shrinking provides unesterified cholesterol, phospholipids, and apoC2 to HDL particles for recycling. The chylomicron remnant is now ready to be recycled by the liver.
The chylomicron remnant uses its apoE molecule which it got from HDL earlier, to dock at the LDL receptors, or the chylomicron remnant receptors (a member of the LDL receptor family), at the liver where it is absorbed and recycled.
Very Low Density Lipoproteins (VLDL)
This is the starting point of what is known as the endogenous pathway of lipoprotein metabolism.
VLDL is the primary lipoprotein responsible for delivery of triglycerides from the liver to peripheral tissues. It is released from the liver equipped with the apoB100 protein; it then picks up apoE and apoC2 in circulation from HDL. The apoC2 enables VLDL to dock at peripheral tissues and release its triglyceride cargo. The VLDL particle may undergo several cycles of lipolysis as it passes through various tissues and interacts repeatedly with LPL. As it shrinks it sheds its shell, the components of which are taken up by HDL. Once 50% of its contents are deposited; it discards its apoC2 protein giving it back to HDL, becoming a VLDL remnant (also known as an Intermediate Density Lipoprotein [IDL]).
About 50% of the VLDL remnant particles then return to the liver to be absorbed (endocytosed) and recycled, the remainder stay in circulation. While in circulation the remaining VLDL remnants return to the liver and interact with hepatic lipase, which virtually empties the VLDL remnant of its triglyceride contents into the liver. In the process apoE is lost and returned to HDL in the circulation, creating an LDL particle consisting of an apoB100 protein and next to zero triglycerides. The remaining apoB100 protein allows docking to peripheral tissue LDL receptor sites for delivery of essential cholesterol stores. (VLDL particles can also be converted to LDL away from the liver via interaction with HDL particles, as detailed below)
In the well-fed state, when the liver has a plentiful supply of calories, especially from carbohydrates, if the liver cannot store or oxidise incoming calories due to shear rate of delivery, or quantity in relation to available storage space, it will up-regulate hepatic glucose output (regardless of increasing plasma insulin levels) pushing the excess glucose into the blood for peripheral tissues to deal with. Simultaneously, it will initiate de novo lipogenesis (DNL) – the creation of new fat – which is then packaged into VLDL particles for export.
This hepatic DNL has been found to only amount to about 5-10grams per day in response to carbohydrate over-feeding. This is not a significant amount of fat when viewed purely from a body fat regulation perspective, however, from a health perspective 5-10 grams corresponds to a huge number of VLDL particles.
Low Density Lipoproteins (LDL)
The vital supply of cholesterol to the body’s cells is achieved either by self-synthesis within the cell, or via extra-cellular supplies.
The cholesterol derived from the diet is first supplied to the liver by chylomicron remnants. The supply of cholesterol from the liver out to the peripheral tissues is primarily the job of LDL. They have a relatively long half-life of about 3 days, during which time they are relatively stable metabolically.
Cells in need of cholesterol recognise the passing LDL particles by their apoB100 protein. The LDL then docks at the LDL receptor site and is completely taken up by the cell, via LDL receptor-mediated endocytosis. The cholesterol is then incorporated into the cell membrane where needed and any excess cholesterol is re-esterified and stored in the cell for later.
Most cells requiring cholesterol will express LDL receptors at their membrane and consume the LDL particle as a whole, only taking what they need via a self-limiting system, called the SCAP-SREBP2 system.
Macrophages are slightly different as they have special scavenger receptors which are not subject to down-regulation like normal LDL receptors. These macrophage scavenger receptors don’t have a high affinity for normal LDL particles, but do bind enthusiastically to damaged LDL particles.
This uptake of damaged LDL particles by these scavenger receptors appears to be a normal protective mechanism; however when the numbers of damaged LDL exceeds a certain level the process becomes pathological.
High Density Lipoprotein (HDL)
The metabolism of HDL is very complex, and there are many different types of HDL.
HDL are made by the small intestine and the liver, with the primary function of acquiring cholesterol from peripheral tissues and returning it to the liver for recycling, or excretion after conversion to bile acids. This process is referred to as reverse cholesterol transport.
As newly created (nascent) particles, the HDLs are like deflated disc-shaped balls. At this stage they are empty, containing no cholesterol. The primary apoproteins of nascent HDLs are apoA, various apoC, and apoE. As detailed earlier, a major function of HDL is to act as a lending library of apoE and apoC proteins when needed by other particles.
When taking cholesterol from peripheral tissue cells, HDL can extract it directly from the cell membranes, intracellular cholesterol stores are then mobilised to replace the cholesterol removed from the membrane, thus lowering the level of intracellular cholesterol. However, this only accounts for 20% of the reverse cholesterol transport.
The primary source of HDL reverse transport is via the interaction between HDL and immune cells. These macrophages which have been cleaning up the aforementioned damaged LDL, interact via their scavenger receptor sites which bind with HDL’s apoA identification protein. The nascent HDL, in effect, suck up the LDL-derived cholesterol within these macrophages and progressively change in shape from their disc appearance to larger cholesterol filled spheres, becoming HDL2 and HDL3 particles.
Cholesterol filled HDL particles then return mainly to the liver, or to the steroidogenic organs, and off-load their cholesterol cargo via specific docking sites called the “Scavenger Receptor BI” (SR-BI) sites. Unlike LDL particles they are not fully consumed and internalized.
When triglyceride levels are elevated, especially when this reflects large numbers of VLDL particles, HDL transfer their cholesterol esters to VLDL, whilst triglycerides are transferred in the opposite direction from VLDL to HDL. This is achieved using a plasma protein called CETP (Cholesterol Ester Transfer Protein). In effect the HDLs get fatter with less cholesterol, whilst the VLDL lose fats and gain cholesterol. This process appears to operate along a diffusion gradient and is dependent upon the number of particles in circulation – the more particles the greater the incidence of interaction. This process converts these VLDLs into cholesterol rich and triglyceride deficient LDL particles, whilst HDL particles become triglyceride rich.
This is not an optimal scenario. Unfortunately when HDL particles become enriched with triglycerides they become better targets for hepatic lipase. As hepatic lipase acts upon the over-fat HDL they become more and more unstable, eventually resulting in the release of their apoA identification protein. This makes the HDL particle unable to participate in the reverse cholesterol transport process. This inverse relationship between plasma triglycerides and HDL has been observed in many studies.
Lipoprotein(a)
Lipoprotein(a) binds with a percentage of LDL particles. Whilst all LDL particles contain a single molecule of ApoB100, only some of them have Lp(a).
The presence of the Lp(a) protein appears to indicate oxidative damage, and increases lipoprotein binding to macrophages via a high-affinity receptor that promotes foam cell formation and the deposition of cholesterol in atherosclerotic plaques.
It is therefore no surprise to learn that high levels of LDL with Lp(a) is a risk marker for atherosclerosis, cerebrovascular disease, thrombosis, and stroke.
Lp(a) concentrations vary over one thousand fold between individuals, from < 0.2 to > 200 mg/dL. This range of concentrations is observed in all populations studied so far.
Lp(a) levels appear to be genetically predetermined and only slightly affected by diet, exercise, and other environmental factors. Commonly prescribed lipid-reducing drugs have little or no effect on Lp(a) concentration, although niacin (vitamin B3) and aspirin are two relatively safe, easily available and inexpensive interventions known to significantly reduce the levels of Lp(a) in some individuals.
(EDIT: There is an emerging school of thought that the presence Lp(a) may be an attempted healing reaction to oxidised tissue damage - a mechanism designed to mop up oxidised lipids from damaged tissues - the smoke rather than the fire! It therefore makes perfect sense that dietary saturated fat would lower the fire causing the Lp(a) smoke relative to PUFA, since it's less easily oxidized. )
(EDIT: There is an emerging school of thought that the presence Lp(a) may be an attempted healing reaction to oxidised tissue damage - a mechanism designed to mop up oxidised lipids from damaged tissues - the smoke rather than the fire! It therefore makes perfect sense that dietary saturated fat would lower the fire causing the Lp(a) smoke relative to PUFA, since it's less easily oxidized. )
The Regulation of Lipoprotein Metabolism
The pathways of lipoprotein metabolism are regulated at many stages. Not surprisingly insulin plays a major role. The main enzyme responsible for triglyceride absorption into adipose tissue is LPL, which is very sensitive to activation from insulin.
The removal of triglycerides from chylomicrons is a saturable process, reflecting the limited activity of LPL. After a mixed carbohydrate and fat meal, chylomicrons and VLDL compete for hydrolysis by LPL. This process is known as the common saturable removal mechanism.
LPL is known to act preferentially upon the larger chylomicron particles, therefore leaving smaller LDL particles out in the circulation until most of the larger particles have finished their business. The rate and pattern of triglyceride clearance following a meal therefore depends upon the origin of the triglycerides. Triglycerides which are derived from the liver, packaged into VLDL particles, are cleared slowly; whilst triglycerides derived from the duodenum and packaged into chylomicrons are processed and cleared from the circulation first.
All of the symptoms of dyslipidaemia are common among conditions in which insulin is not as effective as it should be. When carbohydrates are not well tolerated, glucose management and disposal is inefficient, and insulin sensitivity is poor; hepatic VLDL production increases proportionately.
As can be appreciated from the above, the regulation of healthy lipoprotein metabolism depends upon an optimal balance of the various circulating lipoprotein particles.
The following provides a good CGI perspective of lipoprotein physiology:
References
Metabolic Regulation: A Human Perspective. Keith N. Frayn
