Cardiovascular disease (CVD) is the leading cause of death globally. For close to four decades, we have known that high density lipoprotein (HDL) levels are inversely correlated with the risk of CVD. HDL is a complex particle that consists of proteins, phospholipids, and cholesterol and has the ability to carry micro-RNAs. HDL is constantly undergoing remodelling throughout its life-span and carries out many functions. This review summarizes many of the different aspects of HDL from its assembly, the receptors it interacts with, along with the functions it performs and how it can be altered in disease. While HDL is a key cholesterol efflux particle, this review highlights the many other important functions of HDL in the innate immune system and details the potential therapeutic uses of HDL outside of CVD.
Cardiovascular disease (CVD) remains the dominant cause of death globally [
Atherosclerosis is a complex, progressive disease predominantly occurring in the large arteries of the body. Early research suggested that lipids were a major contributing factor to the onset and progression of the atherosclerotic plaque. This was largely attributed to the strong correlation between hypercholesterolemia and atherosclerosis, along with the dominant role LDL appeared to play [
Increasing the levels and improving the function of HDL are attractive targets to alleviate CVD as HDL possesses a vast array of antiatherosclerotic effects. This is largely due to HDL functioning as a cholesterol acceptor, where HDL promotes cholesterol efflux from lipid-laden macrophages and takes this cholesterol back to the liver for processing. HDL also decreases the activation of leukocytes, platelets, and endothelial cells, functions as an antioxidant, and the recent discovery in regulating haematopoiesis (discussed in detail below). Unfortunately, a number of clinical evaluations of HDL raising therapies have been halted due to either off-target effects or futility, but by no means have these trials disproven the HDL hypothesis. Certainly there is still much interest and hope for HDL targeted therapies to treat people with not only CVD but other inflammatory diseases and some myeloid leukaemias. This review discusses the formation and functions of HDL and how HDL is altered in disease states.
High Density Lipoprotein is a complex particle in terms of size, structure, and the molecules that are associated with HDL. The complexity lies in that these forementioned parameters constantly change throughout the life of the HDL molecule. Apolipoprotein A-I (apoA-I) is the main protein of HDL. ApoA-I accepts lipid to form what is known as pre-
Circulating HDLs are quite diverse and are therefore separated into a number of subpopulations based on shape, size, density, electrophoretic mobility, and apolipoprotein composition (summarised in Figure
HDL subpopulations characterised by size, shape, and apolipoproteins. (a) The two shapes of HDL and their migratory characteristics (
Particle shape
Apolipoprotein composition
Particle size
ApoA-I is a 28 kDa protein synthesized in the liver and intestine where it is released in a lipid free state [
There are a number of naturally occurring variants of apoA-I. Four of the most well defined are
There are a number of proteins/enzymes that are found on HDL and participate in remodelling HDL as it progresses from lipid-free apoA-I to mature HDL. These include the following.
LCAT is synthesised in the liver and secreted into the circulatory system [
Cholesterol Ester Transfer Protein (CETP) is expressed in the liver and transfers CEs between lipoproteins [
A member of the paraoxonase (PON) family PON-1 is synthesised in the liver where a portion is secreted into the plasma and is found at varying concentrations [
While apoA-I is the major protein by far, comprising of approximately 70% of all protein found in HDL [
Pioneering studies from the Heinecke laboratory have shown that HDL is an extremely dynamic molecule and carries a cargo of proteins [
Recently, HDL has been shown to carry micro-RNAs (miRNAs), which are short noncoding regulatory RNAs that modulate biological processes by controlling gene expression through mRNA targeting and translational repression [
It was shown that HDL not only carries endogenous miRNAs, but can also deliver these to cells resulting in functional gene regulatory consequences. The delivery of miRNAs from HDL to hepatocytes was mediated in an scavenger receptor class B member 1 (SR-BI) dependent fashion [
Thus, the recent advances in HDL biology suggest that there is still much to understand of how this complex particle acts
Cholesterol efflux is the first step of reverse cholesterol transport (RCT) and is described as the ability of HDL to remove cholesterol from extrahepatic tissues (specifically the vasculature) for clearance in the liver. This event is carried out by HDL through a number of pathways utilising a variety of receptors and HDL particles.
The formation of
The formation of
Mature
ABCA1 is a member of the ABC transporter superfamily which is divided into seven classes [
As mentioned earlier, ABCA1 facilitates efflux of cholesterol and PL to lipid-poor apoA-I [
There are a number of intracellular phosphorylation sites in ABCA1 that appear to be largely involved in the stabilisation/degradation of ABCA1. These include the following.
ABCA1 cholesterol efflux and stabilising/destabilising modifications. JAK-2 (red) is activated upon apoA-I binding, resulting in autophosphorylation of JAK-2, stabilisation of ABCA1, increased apoA-I binding, and cholesterol efflux. Phosphorylation of the tyrosine residues in the PEST motif (indicated by the black triangles) results in ABCA1 degradation. Protein Kinase A phosphorylates serines (indicated by the red diamonds) required for lipid transport. Protein Kinase CK2 (indicated by the black circles) phosphorylates ABCA1 and down-regulates the transporters activity.
ABCG1 is also a member of the ABC transporters, but unlike ABCA1, ABCG1 is a half transporter and requires homodimerization to function. ABCG1 promotes cholesterol efflux to mature HDL particles and is under the transcriptional control of LXR/RXR [
As described above,
SR-B1 is a member of the B class scavenger receptors with high homology to CD36; however, these receptors play quite distinct roles in lipid metabolism and atherosclerosis [
HDL signaling to eNOS through SR-B1 and S1P3. ApoA-I provokes a signalling cascade through SR-BI (green arrows) stimulating PI3K via Src. The lysophospholipids bind the S1P3 receptor and through the G proteins also stimulate PI3K (blue arrows). Cross-talk between the two pathways is depicted in red arrows implementing the involvement of Akt and MAPK amplifying the phosphorylation of eNOS.
It is well established that increased levels of HDL protect against atherosclerosis [
Endothelial function is essential for the regulation of blood flow and pressure via the release of soluble factors which regulate vascular tone. HDL has been implicated in the regulation of key molecules in this process on both the endothelial and circulating cells. In fact, HDL has been shown to protect against endothelial damage in a number of
One of the most potent roles of HDL on endothelial cells is to inhibit leukocyte adhesion. This was evident when endothelial cells were simulated with oxLDL in the presence or absence of HDL. Pretreatment of HDL resulted in a significant reduction in the adhesion of U937s (monocytic cells) [
C-reactive protein (CRP) has been demonstrated to have inflammatory actions on both leukocytes and endothelial cells [
It is debatable as to which constituents of HDL play the key role in inhibiting the expression of adhesion molecules. As stated above, there is convincing evidence that lipid-free apoA-I can mimic some of the actions of HDL. However, several studies suggest that HDL-associated lysosphingolipids are capable of inhibiting adhesion molecule expression on endothelial cells [
Adhesion of leukocytes to the vascular endothelium is mediated not only by endothelial adhesion molecules, but also by adhesion molecules expressed on leukocytes. We have shown that HDL and apoA-I decrease the activation of human primary monocytes and neutrophils [
HDL and apoA-I can also indirectly protect leukocytes from activation during infection. This is thought to occur through the ability of both to bind and sequester LPS [
Leukocytes are attracted to sites of vascular inflammation by chemokines. Bursill and co-workers discovered that administration of apoA-I in WTD fed
Once monocytes leave the circulation and migrate into the atherosclerotic lesion, they can polarize into two main populations: M1—classically activated, or M2—alternatively activated. Studies in mice where atherosclerotic lesion regression was assessed using various methods to increase HDL levels have shown that inflammatory genes associated with M1 macrophages, such as TNF-
Both the
We have also recently discovered a role for HDL in regulating the production of platelets. Deletion of the cholesterol transporter
We further explored the therapeutic potential of rHDL in reducing platelet production by examining the effects of rHDL infusion in a mouse model of myelofibrosis and essential thrombocytosis expressing a mutant form of c-MPL (
HDL has also been shown to have antithrombotic effects. Platelets express the full repertoire of HDL/apoA-I receptors (including ABCA1 [
The protective roles of HDL/apoA-I may also extend to preventing viral and bacterial infections. Srinivas and co-workers demonstrated the antiviral capabilities of apoA-I included the reduction of viral yield and cell penetration [
A number of studies using both mouse and rabbit models demonstrating the
A separate development, briefly mentioned above, in the study of the antiatherogenic effect of HDL was the exciting discovery of the naturally occurring variant of apoA-I,
In addition to the regulation of adhesion molecules, there is an emerging role for HDL in endothelial repair where HDL has been shown to dose dependently facilitate endothelial cell migration in an
In summary, these findings suggest that HDL is a potent anti-inflammatory agent, regulating cholesterol transport, NO, adhesion molecules, EPCs, and growth factors. The beneficial effects of HDL include promoting plaque stability to plaque regression, and extend to suppressing myeloid leukaemias, suggesting that raising HDL may therapeutically benefit multiple targets.
Whilst HDL is regarded as an anti-inflammatory molecule, protecting against CVD, it can undergo modification altering its beneficial effects. There are a number of environmental factors which can contribute to the modification of HDL resulting in a defective/impaired molecule [
While disruption of the cholesterol efflux pathway leads to increased inflammatory responses [
It is now becoming apparent that the HDL particle itself can undergo a number of modifications in disease/inflammatory settings that impair its ability to facilitate RCT and carry out its known anti-inflammatory functions. The major culprit in modifying HDL appears to involve activated phagocytes secreting myeloperoxidase (MPO), which through chemical interactions modifies apoA-I. The most damaging modification is the addition of a chlorine to the tyrosine residue (Y192) [
HDL does not only undergo oxidative modifications, but can also undergo nonenzymatic glycation in patients with type 2 diabetes mellitus [
Not only is the RCT system inhibited in diabetes but also so are the anti-inflammatory properties. Glycation of apoA-I results in a significant impairment of the ability of this lipoprotein to attenuate human monocyte activation as assessed by CD11b levels [
While there is a growing body of literature describing the cardioprotective effects of HDL, we still have much to learn about this complex particle. HDL has generally been thought of as the “great hope” in treating CVD. Given the wide-ranging effects of HDL and the difficulty in observing clear improvement in cardiovascular outcomes in clinical evaluations of HDL raising therapies, it remains to be seen in which disease setting increasing HDL levels will be most efficient in reducing outcomes. However, it is important to note that clinical trials assessing HDL raising therapies to date have not disproven the HDL hypothesis, and by no means should HDL raising therapies be abandoned. As CVD is associated with an imbalance in plasma lipids, and HDL can promote reverse cholesterol transport, HDL has long been branded as a therapy for this disease. However, we are learning more about the functions of HDL and beginning to understand that regulating cholesterol levels can control the innate immune system. Evolutionary, this may be the primary role for HDL. There is certainly a strong rationale for HDL administration as an anti-inflammatory agent in diseases other than atherosclerosis. Moreover, the recent discoveries that HDL can inhibit hematopoietic progenitor cell proliferation and mobilization in models of leukaemia, which has opened up an exciting new avenue for potential diseases that may benefit from HDL. Perhaps now we need to extend our view on what type of patients we should be targeting with HDL therapy. It may turn out that people with particular types of CVD, diabetes, obesity, and so forth, are the ideal candidates for HDL raising therapies. However, it is entirely possible that HDL raising may be most effective in treating people with chronic inflammatory conditions where resolution of inflammation fails or diseases that are dependent on maintaining cellular cholesterol in order for cells to proliferate such as leukaemia. None the less, there is still much to be learnt about how to effectively raise HDL, what diseases to treat, and how this will impact on outcomes.
Andrew J. Murphy was supported by a grant from the Viertel Foundation, managed by ANZ Trustees and administered by the Diabetes Australia Research Trust. Dr. Amanda Sampson is thanked for her critical reading of the paper.