HDLs and Reverse Cholesterol Transport

Question 1

What role do HDLs play in lipid transport?

Answer 1

The HDL metabolic pathway, often termed reverse cholesterol transport, is the conceptual opposite of the chylomicron and VLDL degradation pathways. Rather than transporting cholesterol and fats from the gut or liver to somatic tissues, these return excess cholesterol from extrahepatic tissues to the liver for the production of bile acids and subsequent secretion. HDLs are also the primary source of cholesterol for steroidogenic tissues.

Question 2

What is the origin of the atheroprotecctive capabilities of HDLs?

Answer 2

As well as providing reverse cholesterol transport, they are antioxidant, including restriction of LDL oxidation; anti-inflammatory, inhibiting activation of endothelium; anti-platelet, preventing thrombosis and cytoprotective.

As such, therapies that raise the level of HDLs in the plasma are now a huge priority for pharma research – low HDL levels are associated with increased risk of CVD, though the ‘quality’ of the HDLs is also a significant factor.

Question 3

How many HDLs and what categories are found in circulation?

Answer 3

There are 5-10x more HDLs in the blood than V/LDLs, but are far smaller so only represent 20-25% of plasma cholesterol.

They are highly heterogenous, changing through a variety of states during their lifespan, starting as Preβ-HDLs before cycling through HDL3, HDL2 and HDL1 states.

This pathway represents a steady increase in size and decrease in density of the HDL, once again contrasting with the VLDL pathway.

Question 4

How are immature HDLs produced?

Answer 4

HDLs are made by secretion of their primary apolipoprotein – ApoA1 – by the liver or intestine. Lipid poor ApoA1 is secreted directly and forms discoidal Preβ-HDLs, which contain minimal TG or cholesteryl esters – being formed primarily just of cholesterol.

Question 5

Describe what happens to Preβ-HDLs as they mature.

Answer 5

HDLs are loaded with cholesteryl esters in the peripheral cells by LCAT (Lecithin Cholesterol Acyltransferase), which transfers the 2-position fatty acid chain from lecithin (AKA phosphatidylcholine) to the hydroxyl of the cholesterol in the HDL surface, allowing them to enter the core of the HDL by hiding the hydrophilic ends. This increases the size of the core and decreases the membrane rigidity, forming HDL3 particles.

HDL3 is converted on to HDL2 and HDL1 through continued action of LCAT as well as that of CETP and PLTP.

Question 6

What receptor is involved in HDL uptake?

Answer 6

The HDL receptor was for a long time elusive, but has now been identified as scavenger receptor class B type 1 (SR-B1), a CD36 related receptor that is part of the family involved in atherosclerotic uptake of oxLDL (indeed B1 is capable of both).

SR-B1 is also implicated to be involved in several other processes, including signalling and cell infiltration by hepatitis C, plasmodium and mycobacteria.

Question 7

What is the mechanism of HDL uptake?

Answer 7

The mechanism of uptake is still contentious, but most evidence appears to point towards a retroendocytosis action, in which the SR-B1, perhaps localised to caveolae (non-clathrin coated invaginations), bind the CE rich HDL2¬ and stimulated endocytosis, allowing the cholesteryl esters to be extracted while remaining bound to SR-B1.

The resulting HDL3-like particle is then recycled back out into the plasma, where it can continue to sequester excess cholesterol, and the cholesteryl esters extracted from it are used by the cell.

Question 8

What mutations have been found in the SR-B1 gene?

Answer 8

Three human SR-BI mutations have been identified, all occurring in the large extracellular loop that is characteristic of the CD36 superfamily (Cluster of Differentiation – genes commonly involved in the immune system), indicating the critical function of this in the mechanism.

Two of these mutations, S112F and T175A, are at residues highly conserved across all vertebrates – the 175 residue only varying in zebrafish (Danio rerio).

SR-B1 is also glycosylated at N-109 and N-173, loss of which also impairs function and trafficking to the cell-surface.

Question 9

What is the structure of the extracellular loop implicated in SR-B1 mutations?

Answer 9

The loop in question possesses six cysteine residues – the middle four (280, 231, 323 and 334) forming two disulphide bridges while the outer ones (251, 384) remain reduced. Site-directed mutagenesis of the cysteine 384 reduces activity greatly, implying an important role.

Question 10

What are the symptoms of Tangier Disease?

Answer 10

Tiny Tangier island in Chesapeake Bay is home to a founder effect, with a high concentration of an incredibly rare HDL deficiency disease (100 cases reported worldwide). Tangier Disease is characterised by orange tonsils due to the CE deposits, peripheral neuropathy, enlarged spleen (hepatosplenomegaly) and negligible HDL levels.

Question 11

What causes Tangier Disease?

Answer 11

Despite the low ApoA1 levels associated with Tangiers, this is not the cause (the low levels are a result of increased clearance). TD is caused by a failure to efflux cholesterol to supply the lipid-poor ApoA1, as a result of homozygous mutations in the ABC transporter ABCA1 (AKA CERP, cholesterol efflux regulatory protein).

This prevents the lipidation of lipid-poor ApoA1, leading to rapid catabolism of the apolipoprotein by the renal cubulin receptor, amongst other degradation pathways.

Question 12

Describe the ABC family

Answer 12

ABC transporters are a colossal superfamily, accounting for 2% of all human genes, including the CF gene. They are active transport pumps that translocate various substrates across extra- and intracellular membranes, involved in cancer therapeutic and bacterial antibiotic resistance (by pumping out the agent) as well as a whole host of inherited diseases.

Question 13

What is the main model for the mechanism of lipid transfer?

Answer 13

Removing cholesterol or lipids from the membrane to lipidate ApoA1 (and for other things) is obviously an unfavourable action, hence the ATP requirement.

The most widely accepted model for how this occurs is the rotational helix model, where the membrane helices form a pore blocked on the cytosolic side by the ATP binding domains.

The helices that form the pore bind the membrane cholesterol, and then are spun on their axis by the ATPase action, releasing the cholesterol into the extracellular space. This is more thermodynamically efficient than an open/close gate mechanism.

Question 14

What are the proposed models for ABCA1 activity?

Answer 14

While the rotational helix model is fairly accepted for general phospholipid/cholesterol removal from a membrane, the exact mechanism of ABCA1 is more contentious – with three primary models; cholesterol pump, PL Translocase and microsolubilisation. More recent work postulates a combined model.

Question 15

What is the cholesterol pump model of ABCA1 activity?

Answer 15

The Cholesterol Pump Model assumes that the cholesterol in the outer leaflet of the membrane is directly taken up by the ApoA1, and that ABCA1 serves to transfer it from the undepleted inner leaflet. However, this is a process that happens naturally at a reasonable rate regardless; despite the hydroxyl group cholesterol is able to flip within membranes fairly easily.

Question 16

What is the phospholipid translocase model of ABCA1 activity?

Answer 16

The 2nd possibility is that ABCA1 functions as a phospholipid translocase – a theory backed by the fact that the transporter was first recognized to flip phosphatidylserine from the inner to outer membrane leaflet as a trigger for apoptosis. In this model, ABCA1 transfers phospholipid molecules from the inner to the outer leaflet, an active process due to the bulky water-soluble phosphate-base grouping.

This effectively lipidates ApoAI while it is bound by ABCA1. Here the preβ-HDL is assumed to take up cholesterol independently, which it is known to do.

Question 17

What is the microsolubilisation model of ABCA1 activity?

Answer 17

In the microsolubilisation model ABCA1 causes local perturbation to the cell-surface membrane making it easier for ApoA1 to sequester cholesterol and phospholipids from the outer leaflet.

Question 18

What is the combined model of ABCA1 activity?

Answer 18

Recent theories combine the last two, with the idea that lipid imbalance between the two leaflets creates strain upon the membrane, leading to creation of curved membrane buds in the outer leaflet that are spontaneously solubilised by ApoA1, a process stimulated by binding of the apolipoprotein to ABCA1.

Question 19

What is caused by heterozygous mutation of ABCA1?

Answer 19

Those heterozygous for a ABCA1 mutation are thought to account for 3 in every 1000 individuals, leading to familial hypoalphalipoproteinaemia (FHA), a prevalence similar to the incidence of FH (LDL-R) and FDB (ApoB100 R3500Q) mutations.

Question 20

What mutations of ABCA1 exist?

Answer 20

While many of the mutations may disable the mechanism of ABCA1, V1704D and L1379F mutations cause HDL deficiency by preventing normal trafficking of ABCA1.

The R219K gain-of-function mutation is very common (25-50% in different populations) and in many cases confer atheroprotection.

However, the link between ABCA1 mutations, low HDL and CVD risk is not always easy to predict due to the complexity of genetic components contributing to lipoprotein metabolism and atherosclerosis.

Question 21

What is ABCA1 primarily regulated in response to?

Answer 21

ABCA1 transcription is upregulated when cells have a high cholesterol content, since an increase in ABCA1 enables cells to efflux excess cholesterol, and downregulated when intracellular CH is low.

Question 22

How is ABCA1 regulated on a transcriptional level in response to CH levels?

Answer 22

A rise in intracellular cholesterol is accompanied by increased levels of oxysterols, which are ligands for LXR, a key regulator of several genes involved in lipid and cholesterol metabolism. Activation of LXR allows it to couple with its obligatory binding partner RXR (which can be further activated by 9-cis retinoic acid), which form a TF that binds to the DR-4 (direct repeat) site and upregulates the gene.

Question 23

What complicated transcriptional regulation of ABCA1?

Answer 23

However, the regulation of this gene is made complex by a second promoter within an intron of the gene to produce an alternative protein; each promoter having its own set of TF binding sites.

Question 24

On what levels is ABC1 regulated?

Answer 24

Transcriptional, turnover and activity.

Question 25

How is ABCA1 turnover regulated?

Answer 25

By PEST-sequence mediated calpain proteolysis, which is prevented by ABCA1 binding to ApoA1.

Poly-Unsaturated Fatty Acids (PUFAs) also destabilise ABCA1, through activation of PKCδ which phosphorylates ABCA1 serine residues, promoting decay.

This is useful to prevent the loss of membrane stabilising cholesterol when membrane fluidity is increased by high PUFA concentration.

Question 26

How is ABCA1 activity regulated?

Answer 26

Curiously, LXR-RXR dimers inacivated by low cholesterol are able to bind the membrane-bound ABCA1, inhibiting it by preventing ATP binding.

Thus the heterodimer is able to activate transcription of the cholesterol efflux transporter when cholesterol levels are high, and inhibit its activity when it is low – a rare example of a protein being both a direct regulator and a TF for the same thing.

Question 27

What enzymes are involved in maturation and remodelling of HDLs?

Answer 27

ABCA1
ABCG1
LCAT
CETP
PLTP (phospholipid transfer protein) 
EL (endothelial lipase).

Question 28

What is the role of ABCG1?

Answer 28

ABCA1 is efficient at lipidating lipid-poor ApoA1, but is not at adding lipids to the core of HDLs to form mature, spherical HDL2 and HDL3 particles; the main mechanism of cholesterol efflux. ABCG1 is responsible for this.

ABCG1 is not defective in Tangier Disease, but without ABCA1 there are no Pre-β HDLs to mature, so its functionality is irrelevant. The transporter is thought to exist as a homodimer, though multiple transcripts have been identified suggesting the possibility of G1-G1 heterodimers. It is also highly expressed in macrophages.

Question 29

What is the role and mechanism of LCAT?

Answer 29

HDLs are loaded with cholesteryl esters in the peripheral cells by LCAT (Lecithin Cholesterol Acyltransferase), which transfers the 2-position fatty acid chain from lecithin (AKA phosphatidylcholine) to the hydroxyl of the cholesterol in the HDL surface, allowing them to enter the core of the HDL by hiding the hydrophilic ends. This increases the size of the core and decreases the membrane rigidity, forming HDL3 particles.

The LCAT is activated upon binding to ApoA1 helix 6 residues (143-164). After the reaction the lysolecithin is removed by albumin.

Question 30

What is the role and mechanism of CETP?

Answer 30

This remodels the lipoprotein core of HDLs, by exchanging the cholesteryl esters within it for TG from VLDLs.

CETP has a flexible concave shape, allowing it to interact with a variety of lipoproteins, extracting CE from HDLs and inserting them into VLDLs, from which is removes TG to continue the cycle. Inhibiting CETP is a major pharmaceutical goal to increase HDL levels by preventing conversion to LDLs.

Question 31

What are the broad approaches to raising HDL levels as therapy?

Answer 31

CETP inhibition
ApoA1 stabilisation (RVX-208)
Niacin treatment (discontinued)

Question 32

What evidence is there that CETP inhibition is atheroprotective?

Answer 32

This is a major area of investigation for pharma companies, and has been for years, since a loss-of-function mutation in CETP common in Japan (25% of population) was associated with longevity.

This is corroborated by model organisms; mice and rats are CVD resistant and lack CETP, while inhibition of it in rabbits was atheroprotective.

Question 33

What drugs are available as CETP inhibitors?

Answer 33

Small molecule drugs have been designed to interfere with CETP lipid binding, with uhhh…. mixed results.

Torcetrapib got to phase III trials before being responsible for around 100 deaths due to the resulting changes in plasma chemistry due to an off-target effect.

Two other drugs, ancetrapid and evacetrapib have since been developed without these consequences, and are entering phase III trials now.

Question 34

What benefits of CETP inhibitors are there other than raising HDL levels?

Answer 34

Another beneficial consequence of inhibiting CETP is to reduce the number of small dense LDL particles generated, which are more prone to oxidation and hence are more atherogenic.

Also, the retention (and accumulation) of CE in HDL reduces the action of hepatic lipase so most particles will remain as the larger and more atheroprotective HDL2 rather than HDL3.

Question 35

What is RVX-208?

Answer 35

This is a drug in trials that stabilises ApoA1 in G1 hepatocytes, both at the transcript and protein level somehow. This has been shown to increase HDL levels in mice, as well as reducing inflammation. Trials in African Green Monkeys continue, as do human phase II trials.

Question 36

Describe niacin treatment.

Answer 36

This was once a very widely used treatment for dyslipidaemia, as it has a huge variety of effects that result in decreased production of all ApoB utilising lipoproteins and increases HDL levels through both direct and indirect pathways.

However, further use of it has been discontinued since a study found no decrease in CVD mortality, and did find an increased stroke risk as well as the previously known serious side effects.