We are water-based lifeforms. However, we need fats and fat-like molecules such as cholesterol (collectively known as lipids) for a host of important cellular functions – as critical as building cells and providing cellular energy. Consequently, we have evolved a means of transporting fat around our bodies in water-friendly capsules called lipoproteins. Anyone who has had a standard blood test may be familiar with two of these – HDL (high-density lipoprotein) and LDL (low-density lipoprotein). The cholesterol they carry is often referred to as ‘good’ and ‘bad’ respectively. However, this trivialises a highly-evolved and sophisticated lipid-trafficking network.
A number of acronyms and abbreviations will be used in this post. For reference: Apo apo-lipoprotein; PL phospholipid; FA fatty acid; TG triglyceride; MG monoglyceride; HDL high-density lipoprotein; LDL low-density lipoprotein; IDL intermediate-density lipoprotein; VLDL very low-density lipoprotein; ULDL ultra low-density lipoprotein; CM chylomicron; EC esterified cholesterol; FC free cholesterol; Pr protein.
Some preliminary definitions: Fatty acids (FAs) are building blocks for fat, and are chains of carbon atoms of various lengths (~16-22) that can be straight (saturated) or bent (unsaturated). Fat refers to triglyceride (TG), which is a packet of 3 FAs attached to a glycerol molecule. This is how FAs are pre-packaged for transport in lipoproteins.
Another kind of lipid, a phospholipid (PL), is a TG with one FA removed and replaced by a molecule that is water-friendly. The importance of PLs is that they have one end (the glycerol end) that is water-friendly while the other end (where the FAs are) is fat-friendly. Cholesterol shares this property. PLs and cholesterol make good borders between fats and water, and are therefore good building blocks for the shells of lipoprotein capsules.
1 Lipoprotein capsules
A lipoprotein is made up of an outer shell carrying an inner cargo. It is not a cell, but a structure. The size (and density) of the lipoprotein will depend on the makeup and size of the cargo. The outer shell will enlarge or shrink to match the cargo. The outer shell is built from PLs with their water-friendly ends pointing out and their FA tails pointing in. This enables the shell to carry lipids inside, while flowing through a watery medium outside. Cholesterol can insert itself in the PL shell and be transported in that way. Cholesterol adds structural flexibility and functionality to the shell.
2 Lipoproteins by density
Apart from HDL and LDL, there are also intermediate-density lipoproteins (IDLs), very low-density lipoproteins (VLDLs), and chylomicrons (CMs, an ultra-low-density lipoprotein, ULDL). The first 4 lipoproteins were identified by ultracentrifuge, separating in layers depending on their density, and named accordingly. Their density is determined in part by how much TG they contain – more means a lower density. The CM is the largest, and lowest-density, lipoprotein, but it had been discovered and named previously, so the acronym ULDL is not commonly used (but that’s where it sits in the hierarchy).
While it is convenient to refer to these 5 classes of lipoprotein, there are also various subclasses (3 in the case of HDL, for example), and there is also a range (or even continuum) of densities across classes or subclasses.
3 Which lipoprotein is that?
While we classify lipoproteins by their density, cells of our bodies are not densitometers and they do not. Instead, the lipoproteins are ‘labelled’ by another family of proteins (Apo-lipoproteins, abbreviated Apo). These are embedded in, or course over, the surface of the PL shell. As well as identifying the lipoprotein to other cells, they give the shell added structural integrity,
They come in various forms and sizes, indicated by lettering – the main forms relevant to this topic are ApoA, ApoB, ApoC and ApoE. There are also numbered subtypes within each Apo (e.g. ApoA-1, ApoA-2 etc), and ApoB comes in two lengths, ApoB-100 and ApoB-48. ApoB-100 is one of the largest proteins in our body.
Depending on the lipoprotein capsule, it will have one or more Apos on its surface. For example, HDL carries multiple copies of ApoA-1 (its primary Apo), however, it also carries ApoC and E. LDL, IDL and VLDL have a single copy of ApoB-100, while CMs carry ApoB-48 and minor amounts of ApoA-1 and C. All except the ApoB form can be dynamically exchanged between classes of lipoproteins to alter their functional behaviour and identity as they mature and expire. The Apos are the lipoprotein’s functional code.
4 Lipoprotein cargo
The primary cargo of lipoproteins is TG and cholesterol (although they can ferry any lipid, and lipid-soluble vitamins – AEDK). However, remember that cholesterol has a water-friendly end and so, while it can travel in the PL shell, it is not going to travel well with fat (TG) in the lipoprotein’s core. Consequently, a FA is added to this end to make the assembly fat-compatible. This combination is referred to as esterified cholesterol (EC). This is in contrast to the cholesterol in the PL shell, which is ‘free’ cholesterol (FC) without anything else attached. These are the only two forms of cholesterol that there are, there is no such thing as good cholesterol or bad cholesterol.
5 The story
This is a picture of the main players in our lipid trafficking system, albeit grossly simplified. We can now turn to the complex story of how they interact with each other.
We need to traffic TG and cholesterol that is already in our bodies (endogenous), as well as that which we acquire through our diet (exogenous). I will start with diet.
The process of absorbing dietary fat occurs in the small intestine. Fat has already been chewed, and and then broken up in the stomach. As it empties into the small intestine, the gallbladder is triggered to contract and release bile, which is a kind of biological detergent synthesised by the liver (from cholesterol). This emulsifies the fat into tiny droplets (micelles) that are acted upon by enzymes released into the intestine from the pancreas. These enzymes break down TG, freeing two of its FAs and leaving just one attached to the glycerol (forming a monoglyceride, MG). They also break down EC into FC and FA.
These break-down products cross into the cells lining the intestinal wall (enterocytes), where they are reformed back into TG and EC. This is how the body gets these molecules across the intestinal wall and into our body, they cannot cross over as they are. This applies universally – all cells need to decompose TG and EC on one side, and recompose them on the other side, for them to cross any cell membrane.
The majority of the cholesterol in the intestine actually comes from the gallbladder, and not from the stomach. As well as synthesising bile, the liver sends surplus cholesterol (in the FC form) to the gallbladder to be emptied into the intestine with the bile. Some of this will be reabsorbed, while the remainder will pass through in faeces. This is the mechanism for eliminating cholesterol, as our body cannot break it down into something else if levels get too high. At the end of the small intestine, bile is re-absorbed and sent back to the liver to be recycled, while the remaining cholesterol will pass into the colon. About 95% of bile is recovered in this way, and during digestion this recycling can occur ~10 times.
We don’t absorb cholesterol (FC) very well. First, some gets re-esterified to EC and cannot be absorbed. Further, enterocytes monitor their FC, and if they already have too much it is transported back out into the intestine to be discharged in faeces. Estimates vary, but ~50% absorption is a round figure. It will also depend on co-nutrients. For example, plants don’t contain cholesterol, but they contain other sterols (phytosterols) that we don’t absorb but that will inhibit the absorption of cholesterol as well. At least one brand of ‘cholesterol-reducing’ margarine has phytosterols added for this purpose.
Now that we have absorbed TG and cholesterol, we can look at how they are trafficked in the circulation. The principal lipoprotein carrier for dietary lipids is the CM.
Intestinal cells are triggered to synthesise (more correctly, translate from the genetic code) ApoB-48, which begins the process of forming a CM (ApoA-1 and C are also incorporated as minor proteins). The CM gets progressively engorged with TG making it large and ultra low-density. The surface shell of these CMs is expansive and incorporates cholesterol in its FC form, and depending on dietary levels the CM will also store EC with its TG. The pie chart (and those to follow) indicates percentage composition of this lipoprotein.
The CM becomes so enlarged that it cannot cross the capillary bed and be released into the blood circulation in the usual way. Instead, CM is released into the lymphatic system and travels to a region below the left side of the neck (thoracic duct) to drain into the blood circulation from there. The lymphatic system has a number of important roles, one of which is to drain any blood trapped in the transition from the arterial to venous systems back into the circulation. The CMs join in with that.
This is an interesting adaptation because normally, anything absorbed in the small intestine gets sent straight to the liver to be managed from there. The CMs uniquely bypass this and deliver their TG cargo directly into the blood circulation. It is clearly a preferred evolutionary adaptation, since a greater number of smaller CMs, that could travel to the liver, would presumably have been an option for natural selection.
The CMs unload their TG cargo quickly – the half-life of a CM in the blood can be as short as ~5 minutes. However, it takes only about one minute for the heart to pump the entire blood circulation (7-8 litres of it) around the body, so CMs exist for ~5 repeats of the whole-body circuit. Furthermore, digestion is a slow process, so there is a steady supply of CMs entering the circulation over a period of some hours.
To unload their cargo, CMs have to be recognised as TG-carriers by cells of the body, and ApoC signals that. The CM picks up additional ApoC as it circulates, mainly from HDL particles (donating its ApoA-1 to them in exchange). As long as a CM holds on to ApoC, it will continue to unload TG, becoming progressively smaller and denser.
When about 2/3 of the TG has been unloaded, the CM undergoes a transformation. It returns its ApoC to HDL, swapping it for one or more ApoEs. The CM is now a CM-remnant. The change from ApoC to ApoE enables the remnant to bind to receptors on liver cells that will absorb it, and recycle its TG and cholesterol, as well as other dietary lipids, proteins and vitamins that the remnant might be carrying or have been built with. This completes a CM cycle.
Slowed clearance of CM-remnants may be a factor in atherosclerosis, and their clearance is delayed in type 2 diabetes. Grazing eating patterns may prolong remnant processing. However, this CM system for trafficking dietary TG and cholesterol will not be detected by a standard blood test carried out after an overnight fast, because the circulation is cleared of any CMs or CM-remnants.
Dietary TG and cholesterol have now combined with that already in our body, and the other lipoproteins take over. I will explain their roles in order of density, starting with VLDL.
VLDLs can be thought of as complementary to CMs, supplying TG to cells between meals (or during an overnight fast). The main site for VLDL synthesis is the liver (although the intestine synthesises it too). They carry a single ApoB-100, which is a longer version of the ApoB-48 on a CM (the difference modifies how CM-remnants are taken up by the liver), as well as ApoA-1, C and E. Physiologically normal VLDL will be released with a payload of TG and EC in a ratio of about 5:1. This assumption is used to estimate VLDL-cholesterol from the measurement of TG in a standard blood test.
What unfolds is a similar story to CM. As long as ApoC remains attached to VLDL, it will unload TG to cells. However, VLDL and HDL also swap TG and EC between them. The HDL acquires TG from VLDL, and the VLDL acquires EC from HDL. This partly explains why HDL cholesterol and TG are often inversely-related in blood tests, other factors notwithstanding.
Over time (~4-6 hours), VLDL becomes depleted of TG and enriched in EC, thereby increasing its density. It has become a VLDL-remnant (like a CM-remnant), and called an IDL. IDL gives its ApoC back to HDL, and accepts a variable number of ApoEs in exchange. ApoE can also be picked up from the circulation (it is water compatible). The IDL retains the original ApoB-100.
IDLs bind to the surface of liver cells (via. ApoB-100 and/or ApoE), where it has two possible fates. If the IDL is carrying sufficient numbers of ApoE, it will be taken up into the liver and disassembled like a CM-remnant. Some EC has thus been transported from HDL to the liver, via. an intermediate VLDL/IDL phase.
If however, there are only a few ApoEs on the IDL, it will linger on the surface of the liver cell. While there, ApoEs are removed (ApoB-100 remains), and the liver takes in any remaining TG from the IDL while adding EC to its cargo. This process remodels IDL into a new species – LDL, low in TG and enriched in EC. It is released back into the circulation.
The primary function of LDL is to offer cholesterol to cells that might need it. Cells in our body have specialised receptors on their surface that the ApoB-100 carried by LDL binds to (provided other Apos, such as C or E, are absent). Cells regulate their uptake of cholesterol by adjusting the number of these receptors on their surface.
Unlike VLDL, which docks with cells to release TG, and then undocks and moves on, the LDL particles are taken into the cell in their entirety through the receptor, and their total cholesterol load released. The LDL has now been cleared from the circulation and the cell has been supplied with cholesterol. Liver cells will also take in LDL, further reducing circulating cholesterol.
LDLs have a continuum of density and size. Early on they are referred to as large buoyant LDL, and over time they become smaller and denser (sdLDL) as their contents are unloaded.
This is a steady-state, ongoing phenomenon – the half-life of LDL is measured in a few days, compared to hours for VLDL (and HDL) and minutes for CM. In such a steady-state system, presumably, there is a balance between LDL half-life and EC uptake that limits sdLDL numbers by disposing of LDL before they get to that late stage. The sdLDL particles are small enough to insert into the arterial wall, and attach via. ApoB-100. They are readily oxidised, and they are particles of interest in contemporary atherosclerosis research. In other words, LDL particle size and number may be more important than their cholesterol load.
The liver and intestines are the main sources of HDL. HDL starts with the generation and release of ApoA-1 into the circulation. Cells of the body release FC if their cholesterol levels are in surplus. These combine, together with circulating PL, to form a nascent HDL. A circulating enzyme begins to form EC out of FC, and the HDL matures. A mature HDL remains in the circulation for 12-24 hours.
HDL shuttles its EC back to the liver for uptake, or transfers it to VLDL/LDL as previously described. Ultimately, the HDL is taken up by specific HDL receptors on liver cells and broken down. ApoA and small HDL particles can also be eliminated via. the kidneys.
The liver has various options for the EC cargo, including de-esterifying it to FC that can be used to build bile, or sending it to the gallbladder to empty into the small intestine with bile and either be partly re-absorbed or eliminated in faeces as previously described.
This is the ‘way out’ for cholesterol, and it is for this reason that HDL cholesterol is often referred to as ‘good’, because of the possibility of elimination (we cannot break cholesterol down, it must be physically eliminated). The assumption is that elimination of cholesterol is good. This process is referred to as ‘reverse cholesterol transport’.
However, we have seen that HDL can pass some of its ‘good’ cholesterol to VLDL/IDL and LDL. Does, that mean it is now ‘bad’ cholesterol? What if the LDL delivers some of it to the liver – is it now ‘good’ cholesterol? The process of HDL delivering EC to the liver via lower-density lipoprotein intermediaries is known as ‘indirect reverse cholesterol transport’.
Attempts to increase HDL cholesterol pharmacologically have failed to show clinical benefits, arguing against the good cholesterol story. In reality, HDL carries very little cholesterol anyway (~15% EC by weight). Instead, attention is now turning to the HDL particles themselves. They have complex properties, including anti-inflammatory, anti-oxidant, and vasoprotective properties. Some consider HDL to be part of our innate immune system. The circulating pool of HDL carries multiple forms of Apo, beyond what I’ve described (e.g. A-1, A-2,A-4, C-1, C-2, C-3, D, E, F, H, J, L and M) and over 100 types of lipid. It is thought that remodelling HDL with various combinations of Apo may give it a functional flexibility that would be expected of a component of the immune system.
12 Some things I left out
A whole class of lipoproteins as it happens, referred to as Lipoprotein(a) – Lp(a). While LP(a) carries cholesterol, this does not seem to be a primary function. It may contribute to atherogenesiss, then again it may contribute to tissue repair. It is a form of LDL (at the smaller end of the size spectrum) in which the ApoB-100 has been modified. It doesn’t readily fit in with the other lipoproteins, but that only shows how I have simplified this story, and how much more there is to understand.
Furthermore, the subtypes of Apo have an important influence. For example, remember that VLDL needs to be carrying ApoC in order to unload its TG cargo. However, it is the ApoC-2 subtype that enables this process, while the ApoC-3 subtype opposes it. Another level of checks and balances.
The FAs that are transported around in TG and lipoproteins are long-chain FAs. Short-chain FAs (less than 6 carbons long) can flow in the blood circulation without the need for lipoprotein carriers. We don’t get short-chain FAs from dietary fat, they come from bacterial fermentation in the gut.
Long-chain ‘free’ FAs can also travel without being incorporated in TG and without lipoprotein carriers. They are attached to a large water-soluble protein called albumin, and get carried along by that. This mechanism is important for the distribution of FAs released by adipose tissue to fuel cells, or that spillover when TG (from VLDLs for example) is broken down for transport into cells.
In reality, it doesn’t matter how much cholesterol there is in the diet since virtually all cells in our body synthesise it anyway. If dietary sources increase, cellular synthesis decreases to match. That’s why there is no relationship between dietary cholesterol and the amount of cholesterol in the blood, a fact that was established beyond reasonable doubt in the 1950s and ignored by dietary guidelines for about 60 years thereafter.
The need to traffic lipids around a water-based cellular environment has led to the development of an intricate interconnected network of lipoproteins that I have grossly oversimplified (and probably got wrong in places). All life has had to solve this problem, and this solution was an early evolutionary development – birds, amphibians, fishes and even invertebrates (such as worms) have lipoprotein-based systems like we do for lipid trafficking.
This is a fundamental aspect of our evolutionary biology, not widely appreciated in parts, and greatly (and wrongly) oversimplified by good/bad cholesterol. Even adding only one more layer of complexity (the Apos) has shown how profoundly complex our lipoprotein system is.
However, cholesterol remains in the public perception and influences clinical decision-making. Therefore, in the post to follow, I will look more deeply into cholesterol and cholesterol-lowering medications.
I have mostly avoided clinical implications of lipid trafficking in this post, and nothing here should be taken as medical advice. It may inform understanding instead.
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